Download The Power Electronics Handbook Part-01 - PakWebINFO

Download The Power Electronics Handbook Part-01 - PakWebINFO
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POWER
ELECTRONICS
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HANDBOOK
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PART I
Overview Kaushik Rajashekara
Diodes Sohail Anwar
Schottky Diodes Sohail Anwar
Thyristors Sohail Anwar
Sohail Anwar
Power Bipolar Junction Transistors
MOSFETs Vrej Barkhordarian
Alex Q. Huang
General Power Semiconductor Switch Requirements
Gate Turn-Off Thyristors Alex Q. Huang
Alex Q. Huang
Insulated Gate Bipolar Transistors
Alex Q. Huang
Gate-Commutated Thyristors and Other Hard-Driven GTOs
Alex Q. Huang
Comparison Testing of Switches
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1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
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Power Electronics
PART II
DC-DC Converters
2.1
2.2
2.3
2.4
2.5
2.6
3
Overview Richard Wies, Bipin Satavalekar, and Ashish Agrawal
Choppers Javad Mahdavi, Ali Agah, and Ali Emadi
Buck Converters Richard Wies, Bipin Satavalekar, and Ashish Agrawal
Boost Converters Richard Wies, Bipin Satavalekar, and Ashish Agrawal
CГєk Converter Richard Wies, Bipin Satavalekar, and Ashish Agrawal
Buck–Boost Converters Daniel Jeffrey Shortt
AC-AC Conversion
3.1
3.2
3.3
4
Power Electronic Circuits and Controls
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Power Electronic Devices
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Contents
SГЎndor HalГЎsz
Introduction
Cycloconverters
Matrix Converters
Rectifiers
4.1
4.2
4.3
Uncontrolled Single-Phase Rectifiers
Sam Guccione
Mahesh M. Swamy
Uncontrolled and Controlled Rectifiers
Three-Phase Pulse-Width-Modulated Boost-Type Rectifiers
Ana Stankovic
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7
Modulation Strategies
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
8
Keith Corzine
Introduction
Multilevel Voltage Source Modulation
Fundamental Multilevel Converter Topologies
Cascaded Multilevel Converter Topologies
Multilevel Converter Laboratory Examples
Conclusion
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6.1
6.2
6.3
6.4
6.5
6.6
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Multilevel Converters
Introduction Michael Giesselmann
Six-Step Modulation Michael Giesselmann
Pulse Width Modulation Michael Giesselmann
Third Harmonic Injection for Voltage Boost of SPWM Signals
Michael Giesselmann
Generation of PWM Signals Using Microcontrollers and DSPs
Michael Giesselmann
Voltage-Source-Based Current Regulation
Michael Giesselmann
Hysteresis Feedback Control
Hossein Salehfar
Space-Vector Pulse Width Modulation
Hamid A. Toliyat and Tahmid Ur Rahman
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Overview Michael Giesselmann
DC-AC Conversion Attila Karpati
Resonant Converters IstvГЎn Nagy
Series-Resonant Inverters
Dariusz Czarkowski
Resonant DC-Link Inverters
Michael B. Ropp
Auxiliary Resonant Commutated Pole Inverters
Eric Walters and Oleg Wasynczuk
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5.1
5.2
5.3
5.4
5.5
5.6
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Inverters
Sliding-Mode Control of Switched-Mode Power Supplies
Giorgio Spiazzi and Paolo Mattavelli
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
Introduction
Introduction to Sliding-Mode Control
Basics of Sliding-Mode Theory
Application of Sliding-Mode Control to DC-DC Converters—Basic Principle
Sliding-Mode Control of Buck DC-DC Converters
Extension to Boost and Buck–Boost DC-DC Converters
Extension to CГєk and SEPIC DC-DC Converters
General-Purpose Sliding-Mode Control Implementation
Conclusions
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DC Motor Drives
11
Control of Induction Machine Drives
Daniel Logue and Philip T. Krein
11.1
11.2
11.3
11.4
12
Introduction
Scalar Induction Machine Control
Vector Control of Induction Machines
Summary
Permanent-Magnet Synchronous Machine Drives
12.1
12.2
12.3
12.4
12.5
13
Introduction
Machine Construction
Motor Characteristics
Power Electronic Converter
Position Sensing
Pulsating Torque Components
Torque-Speed Characteristics
Applications
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10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
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AC Machines Controlled as DC Machines
(Brushless DC Machines/Electronics) Hamid A. Toliyat
and Tilak Gopalarathnam
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Ralph Staus
DC Motor Basics
DC Speed Control
DC Drive Basics
Transistor PWM DC Drives
SCR DC Drives
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9.1
9.2
9.3
9.4
9.5
Switched Reluctance Machines
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
Patrick L. Chapman
Introduction
Construction of PMSM Drive Systems
Simulation and Model
Controlling the PMSM
Advanced Topics in PMSM Drives
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Applications and Systems Considerations
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Part III
Iqbal Husain
Introduction
SRM Configuration
Basic Principle of Operation
Design
Converter Topologies
Control Strategies
Sensorless Control
Applications
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Step Motor Drives
14.1
14.2
14.3
14.4
15
Ronald H. Brown
Introduction
Types and Operation of Step Motors
Step Motor Models
Control of Step Motors
Servo Drives
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SГЎndor HalГЎsz
16.1
16.2
16.3
16.4
Roger Messenger
Introduction
Solar Cell Fundamentals
Utility Interactive PV Applications
Stand-Alone PV Systems
Flexible, Reliable, and Intelligent Electrical Energy Delivery Systems
Alexander Domijan, Jr. and Zhidong Song
19.1
19.2
19.3
19.4
19.5
19.6
19.7
20
Overview Wayne Galli
Power Quality Considerations
Timothy L. Skvarenina
Passive Harmonic Filters Badrul H. Chowdhury
Active Filters for Power Conditioning
Hirofumi Akagi
Unity Power Factor Rectification
Rajapandian Ayyanar and Amit Kumar Jain
Photovoltaic Cells and Systems
18.1
18.2
18.3
18.4
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Power Quality and Utility Interface Issues
17.1
17.2
17.3
17.4
17.5
18
UPS Functions
Static UPS Topologies
Rotary UPSs
Alternate AC and DC Sources
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Laura Steffek, John Hacklesmiller,
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Uninterruptible Power Supplies
Dave Layden, and Brian Young
Introduction
The Concept of FRIENDS
Development of FRIENDS
The Advanced Power Electronic Technologies within QCCs
Significance of FRIENDS
Realization of FRIENDS
Conclusions
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15.1 DC Drives
15.2 Induction Motor Drives
Unified Power Flow Controllers
Ali Feliachi, Azra Hasanovic, and Karl Schoder
20.1 Introduction
20.2 Power Flow on a Transmission Line
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More-Electric Vehicles
Ali Emadi and Mehrdad Ehsani
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20.3 UPFC Description and Operation
20.4 UPFC Modeling
20.5 Control Design
20.6 Case Study
20.7 Conclusion
Acknowledgment
21.1 Aircraft Ali Emadi and Mehrdad Ehsani
21.2 Terrestrial Vehicles Ali Emadi and Mehrdad Ehsani
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Introduction
Nature of a Magnetic Field
Electromagnetism
Magnetic Flux Density
Magnetic Circuits
Magnetic Field Intensity
Maxwell’s Equations
Inductance
Practical Considerations
Roman Stemprok
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22.1
22.2
22.3
22.4
22.5
22.6
22.7
22.8
22.9
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Principles of Magnetics
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Computer Simulation of Power Electronics
Michael Giesselmann
Introduction
Code Qualification and Model Validation
Basic Concepts—Simulation of a Buck Converter
Advanced Techniques—Simulation of a Full-Bridge (H-Bridge) Converter
Conclusions
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23.1
23.2
23.3
23.4
23.5
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1
1.1
Sohail Anwar
Pennsylvania State University
1.2
International Rectifier
Diodes
Characteristics • Principal Ratings for Diodes • Rectifier
Circuits • Testing a Power Diode • Protection of Power
Diodes
Alex Q. Huang
Virginia Polytechnic Institute
and State University
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Thyristor and Triac • Gate Turn-Off Thyristor • ReverseConducting Thyristor (RCT) and Asymmetrical SiliconControlled Rectifier (ASCR) • Power Transistor • Power
MOSFET • Insulated-Gate Bipolar Transistor (IGBT) •
MOS-Controlled Thyristor (MCT)
Delphi Automotive Systems
Vrej Barkhordarian
Overview
1.3
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Kaushik Rajashekara
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Power Electronics
Schottky Diodes
Characteristics • Data Specifications • Testing of Schottky
Diodes
Thyristors
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The Basics of Silicon-Controlled Rectifiers (SCR) •
Characteristics • SCR Turn-Off Circuits • SCR
Ratings • The DIAC • The Triac • The Silicon-Controlled
Switch • The Gate Turn-Off Thyristor • Data Sheet for a
Typical Thyristor
1.5
Power Bipolar Junction Transistors
The Volt-Ampere Characteristics of a BJT • BJT Biasing • BJT
Power Losses • BJT Testing • BJT Protection
1.6
MOSFETs
Static Characteristics • Dynamic
Characteristics • Applications
1.7
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1.8
General Power Semiconductor Switch
Requirements
Gate Turn-Off Thyristors
GTO Forward Conduction • GTO Turn-Off and Forward
Blocking • Practical GTO Turn-Off Operation • Dynamic
Avalanche • Non-Uniform Turn-Off Process among GTO
Cells • Summary
1.9
Insulated Gate Bipolar Transistors
IGBT Structure and Operation
1.10 Gate-Commutated Thyristors and Other
Hard-Driven GTOs
Unity Gain Turn-Off Operation • Hard-Driven GTOs
1.11 Comparison Testing of Switches
Pulse Tester Used for Characterization • Devices Used for
Comparison • Unity Gain Verification • Gate Drive
Circuits • Forward Conduction Loss Characterization •
Switching Tests • Discussion • Comparison Conclusions
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1.1 Overview
Kaushik Rajashekara
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The modern age of power electronics began with the introduction of thyristors in the late 1950s. Now there
are several types of power devices available for high-power and high-frequency applications. The most
notable power devices are gate turn-off thyristors, power Darlington transistors, power MOSFETs, and
insulated-gate bipolar transistors (IGBTs). Power semiconductor devices are the most important functional
elements in all power conversion applications. The power devices are mainly used as switches to convert
power from one form to another. They are used in motor control systems, uninterrupted power supplies,
high-voltage DC transmission, power supplies, induction heating, and in many other power conversion
applications. A review of the basic characteristics of these power devices is presented in this section.
Thyristor and Triac
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The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpn
device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse
across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device.
The turn-off is achieved by applying a reverse voltage across the anode and cathode. The thyristor symbol
and its volt–ampere characteristics are shown in Fig. 1.1. There are basically two classifications of
thyristors: converter grade and inverter grade. The difference between a converter-grade and an invertergrade thyristor is the low turn-off time (on the order of a few microseconds) for the latter. The convertergrade thyristors are slow type and are used in natural commutation (or phase-controlled) applications.
FIGURE 1.1 (a) Thyristor symbol and (b) volt–ampere characteristics. (From Bose, B.K., Modern Power Electronics:
Evaluation, Technology, and Applications, p. 5. В© 1992 IEEE. With permission.)
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FIGURE 1.2 (a) Triac symbol and (b) volt–ampere characteristics. (From Bose, B.K., Modern Power Electronics:
Evaluation, Technology, and Applications, p. 5. В© 1992 IEEE. With permission.)
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Inverter-grade thyristors are used in forced commutation applications such as DC-DC choppers and
DC-AC inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an
external commutation circuit. This requires additional commutating components, thus resulting in
additional losses in the inverter.
Thyristors are highly rugged devices in terms of transient currents, di/dt, and dv/dt capability. The
forward voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 1000 A,
it seldom exceeds 3 V. While the forward voltage determines the on-state power loss of the device at any
given current, the switching power loss becomes a dominating factor affecting the device junction
temperature at high operating frequencies. Because of this, the maximum switching frequencies possible
using thyristors are limited in comparison with other power devices considered in this section.
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Thyristors have I t withstand capability and can be protected by fuses. The nonrepetitive surge current
capability for thyristors is about 10 times their rated root mean square (rms) current. They must be protected
by snubber networks for dv/dt and di/dt effects. If the specified dv/dt is exceeded, thyristors may start
conducting without applying a gate pulse. In DC-to-AC conversion applications, it is necessary to use an
antiparallel diode of similar rating across each main thyristor. Thyristors are available up to 6000 V, 3500 A.
A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol
and volt–ampere characteristics are shown in Fig. 1.2. Because of the integration, the triac has poor reapplied
dv/dt, poor gate current sensitivity at turn-on, and longer turn-off time. Triacs are mainly used in phase
control applications such as in AC regulators for lighting and fan control and in solid-state AC relays.
Gate Turn-Off Thyristor
The GTO is a power switching device that can be turned on by a short pulse of gate current and turned
off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be
turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off
is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more
capability for high-frequency operation than thyristors. The GTO symbol and turn-off characteristics
are shown in Fig. 1.3.
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GTOs have the I t withstand capability and hence can be protected by semiconductor fuses. For reliable
operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber
circuit. A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current
GTO may require as high as 500 A of reverse gate current. Also, a GTO has the tendency to latch at
temperatures above 125В°C. GTOs are available up to about 4500 V, 2500 A.
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FIGURE 1.3 (a) GTO symbol and (b) turn-off characteristics. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, p. 5. В© 1992 IEEE. With permission.)
Reverse-Conducting Thyristor (RCT) and Asymmetrical
Silicon-Controlled Rectifier (ASCR)
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Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commutation/freewheeling purposes. In RCTs, the diode is integrated with a fast switching thyristor in a
single silicon chip. Thus, the number of power devices could be reduced. This integration brings
forth a substantial improvement of the static and dynamic characteristics as well as its overall circuit
performance.
The RCTs are designed mainly for specific applications such as traction drives. The antiparallel
diode limits the reverse voltage across the thyristor to 1 to 2 V. Also, because of the reverse recovery
behavior of the diodes, the thyristor may see very high reapplied dv/dt when the diode recovers from its
reverse voltage. This necessitates use of large RC snubber networks to suppress voltage transients. As the
range of application of thyristors and diodes extends into higher frequencies, their reverse recovery charge
becomes increasingly important. High reverse recovery charge results in high power dissipation during
switching.
The ASCR has similar forward blocking capability to an inverter-grade thyristor, but it has a limited
reverse blocking (about 20 to 30 V) capability. It has an on-state voltage drop of about 25% less than an
inverter-grade thyristor of a similar rating. The ASCR features a fast turn-off time; thus it can work at
a higher frequency than an SCR. Since the turn-off time is down by a factor of nearly 2, the size of the
commutating components can be halved. Because of this, the switching losses will also be low.
Gate-assisted turn-off techniques are used to even further reduce the turn-off time of an ASCR. The
application of a negative voltage to the gate during turn-off helps to evacuate stored charge in the device
and aids the recovery mechanisms. This will, in effect, reduce the turn-off time by a factor of up to 2
over the conventional device.
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FIGURE 1.4 A two-stage Darlington transistor with bypass diode. (From Bose, B.K., Modern Power Electronics:
Evaluation, Technology, and Applications, p. 6. В© 1992 IEEE. With permission.)
Power Transistor
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Power transistors are used in applications ranging from a few to several hundred kilowatts and switching
frequencies up to about 10 kHz. Power transistors used in power conversion applications are generally
npn type. The power transistor is turned on by supplying sufficient base current, and this base drive has
to be maintained throughout its conduction period. It is turned off by removing the base drive and
making the base voltage slightly negative (within –VBE(max)). The saturation voltage of the device is
normally 0.5 to 2.5 V and increases as the current increases. Hence, the on-state losses increase more
than proportionately with current. The transistor off-state losses are much lower than the on-state losses
because the leakage current of the device is of the order of a few milliamperes. Because of relatively larger
switching times, the switching loss significantly increases with switching frequency. Power transistors can
block only forward voltages. The reverse peak voltage rating of these devices is as low as 5 to 10 V.
2
Power transistors do not have I t withstand capability. In other words, they can absorb only very little
energy before breakdown. Therefore, they cannot be protected by semiconductor fuses, and thus an
electronic protection method has to be used.
To eliminate high base current requirements, Darlington configurations are commonly used. They are
available in monolithic or in isolated packages. The basic Darlington configuration is shown schematically
in Fig. 1.4. The Darlington configuration presents a specific advantage in that it can considerably increase
the current switched by the transistor for a given base drive. The VCE(sat) for the Darlington is generally
more than that of a single transistor of similar rating with corresponding increase in on-state power loss.
During switching, the reverse-biased collector junction may show hot-spot breakdown effects that are
specified by reverse-bias safe operating area (RBSOA) and forward-bias safe operating area (FBSOA).
Modern devices with highly interdigited emitter base geometry force more uniform current distribution
and therefore considerably improve secondary breakdown effects. Normally, a well-designed switching
aid network constrains the device operation well within the SOAs.
Power MOSFET
Power MOSFETs are marketed by different manufacturers with differences in internal geometry and with
different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make
them potentially attractive for switching applications. They are essentially voltage-driven rather than
current-driven devices, unlike bipolar transistors.
The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate
draws only a minute leakage current on the order of nanoamperes. Hence, the gate drive circuit is simple
and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws
virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain
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FIGURE 1.5 Power MOSFET circuit symbol. (From Bose, B.K., Modern Power Electronics: Evaluation, Technology,
and Applications, p. 7. В© 1992 IEEE. With permission.)
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capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and
the drive circuit must have a sufficiently low output impedance to supply the required charging and
discharging currents. The circuit symbol of a power MOSFET is shown in Fig. 1.5.
Power MOSFETs are majority carrier devices, and there is no minority carrier storage time. Hence,
they have exceptionally fast rise and fall times. They are essentially resistive devices when turned on,
while bipolar transistors present a more or less constant VCE(sat) over the normal operating range. Power
2
dissipation in MOSFETs is Id RDS(on), and in bipolars it is ICVCE(sat). At low currents, therefore, a power
MOSFET may have a lower conduction loss than a comparable bipolar device, but at higher currents,
the conduction loss will exceed that of bipolars. Also, the RDS(on) increases with temperature.
An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is
present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In
MOSFETs, RDS(on) increases with temperature, and thus the current is automatically diverted away from
the hot spot. The drain body junction appears as an antiparallel diode between source and drain. Thus,
power MOSFETs will not support voltage in the reverse direction. Although this inverse diode is relatively
fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as
100 ns. Since MOSFETs cannot be protected by fuses, an electronic protection technique has to be used.
With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional
MOSFETs. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating
within its specification range at all times, its chances for failing catastrophically are minimal. However,
if its absolute maximum rating is exceeded, failure probability increases dramatically. Under actual
operating conditions, a MOSFET may be subjected to transients—either externally from the power bus
supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the
absolute maximum ratings. Such conditions are likely in almost every application, and in most cases are
beyond a designer’s control. Rugged devices are made to be more tolerant for overvoltage transients.
Ruggedness is the ability of a MOSFET to operate in an environment of dynamic electrical stresses,
without activating any of the parasitic bipolar junction transistors. The rugged device can withstand
higher levels of diode recovery dv/dt and static dv/dt.
Insulated-Gate Bipolar Transistor (IGBT)
The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity
characteristic (low saturation voltage) of a bipolar transistor. The IGBT is turned on by applying a positive
voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal
zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings.
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(a) Nonpunch-through IGBT, (b) punch-through IGBT, (c) IGBT equivalent circuit.
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FIGURE 1.6
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The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of
collector current that will cause a large enough voltage drop to activate the thyristor. Hence, the device
manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There
is also a corresponding gate source voltage that permits this current to flow that should not be exceeded.
Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common
to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and
specified maximum junction temperature of the device under all conditions for guaranteed reliable
operation. The on-state voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low
on-state voltage, a sufficiently high gate voltage must be applied.
In general, IGBTs can be classified as punch-through (PT) and nonpunch-through (NPT) structures, as
+
+
shown in Fig. 1.6. In the PT IGBT, an N buffer layer is normally introduced between the P substrate and
в€’
в€’
the N epitaxial layer, so that the whole N drift region is depleted when the device is blocking the off-state
в€’
voltage, and the electrical field shape inside the N drift region is close to a rectangular shape. Because a
в€’
shorter N region can be used in the punch-through IGBT, a better trade-off between the forward voltage
drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V.
в€’
High-voltage IGBTs are realized through a nonpunch-through process. The devices are built on an N
в€’
wafer substrate which serves as the N base drift region. Experimental NPT IGBTs of up to about 4 kV
have been reported in the literature. NPT IGBTs are more robust than PT IGBTs, particularly under short
circuit conditions. But NPT IGBTs have a higher forward voltage drop than the PT IGBTs.
The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of
parallel-connected IGBTs are (1) on-state current unbalance, caused by VCE(sat) distribution and main
circuit wiring resistance distribution, and (2) current unbalance at turn-on and turn-off, caused by the
switching time difference of the parallel connected devices and circuit wiring inductance distribution.
The NPT IGBTs can be paralleled because of their positive temperature coefficient property.
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MOS-Controlled Thyristor (MCT)
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FIGURE 1.7 Typical cell cross section and circuit schematic for P-MCT. (From Harris Semiconductor, User’s Guide
of MOS Controlled Thyristor. With permission.)
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The MCT is a new type of power semiconductor device that combines the capabilities of thyristor voltage
and current with MOS gated turn-on and turn-off. It is a high-power, high-frequency, low-conduction
drop and a rugged device, which is more likely to be used in the future for medium and high power
applications. A cross-sectional structure of a p-type MCT with its circuit schematic is shown in Fig. 1.7.
The MCT has a thyristor type structure with three junctions and pnpn layers between the anode and
cathode. In a practical MCT, about 100,000 cells similar to the one shown are paralleled to achieve the
desired current rating. MCT is turned on by a negative voltage pulse at the gate with respect to the anode,
and is turned off by a positive voltage pulse.
The MCT was announced by the General Electric R&D Center on November 30, 1988. Harris
Semiconductor Corporation has developed two generations of p-MCTs. Gen-1 p-MCTs are available at
65 A/1000 V and 75 A/600 V with peak controllable current of 120 A. Gen-2 p-MCTs are being developed
at similar current and voltage ratings, with much improved turn-on capability and switching speed.
The reason for developing a p-MCT is the fact that the current density that can be turned off is two
or three times higher than that of an n-MCT; but n-MCTs are the ones needed for many practical
applications.
The advantage of an MCT over IGBT is its low forward voltage drop. n-type MCTs will be expected to
have a similar forward voltage drop, but with an improved reverse bias safe operating area and switching
speed. MCTs have relatively low switching times and storage time. The MCT is capable of high current
densities and blocking voltages in both directions. Since the power gain of an MCT is extremely high, it
could be driven directly from logic gates. An MCT has high di/dt (of the order of 2500 A/Вµs) and high
dv/dt (of the order of 20,000 V/Вµs) capability.
The MCT, because of its superior characteristics, shows a tremendous possibility for applications such
as motor drives, uninterrupted power supplies, static VAR compensators, and high power active power
line conditioners.
The current and future power semiconductor devices developmental direction is shown in Fig. 1.8.
High-temperature operation capability and low forward voltage drop operation can be obtained if silicon
is replaced by silicon carbide material for producing power devices. The silicon carbide has a higher band
gap than silicon. Hence, higher breakdown voltage devices could be developed. Silicon carbide devices
have excellent switching characteristics and stable blocking voltages at higher temperatures. But the silicon
carbide devices are still in the very early stages of development.
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FIGURE 1.8 Current and future power semiconductor devices development direction. (From Huang, A.Q., Recent
developments of power semiconductor devices, VPEC Seminar Proceedings, pp. 1–9. With permission.)
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References
kw
eb
in
fo
Bose, B.K., Modern Power Electronics: Evaluation, Technology, and Applications, IEEE Press, New York, 1992.
Harris Semiconductor, User’s Guide of MOS Controlled Thyristor.
Huang, A.Q., Recent developments of power semiconductor devices, in VPEC Seminar Proceedings,
September 1995, 1–9.
Mohan, N. and T. Undeland, Power Electronics: Converters, Applications, and Design, John Wiley & Sons,
New York, 1995.
Wojslawowicz, J., Ruggedized transistors emerging as power MOSFET standard-bearers, Power Technics
Magazine, January 1988, 29–32.
Further Information
Bird, B.M. and K.G. King, An Introduction to Power Electronics, Wiley-Interscience, New York, 1984.
Sittig, R. and P. Roggwiller, Semiconductor Devices for Power Conditioning, Plenum, New York, 1982.
Temple, V.A.K., Advances in MOS controlled thyristor technology and capability, Power Conversion,
544–554, Oct. 1989.
Williams, B.W., Power Electronics, Devices, Drivers and Applications, John Wiley, New York, 1987.
1.2 Diodes
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Sohail Anwar
Power diodes play an important role in power electronics circuits. They are mainly used as uncontrolled
rectifiers to convert single-phase or three-phase AC voltage to DC. They are also used to provide a path
for the current flow in inductive loads. Typical types of semiconductor materials used to construct diodes
are silicon and germanium. Power diodes are usually constructed using silicon because silicon diodes can
operate at higher current and at higher junction temperatures than germanium diodes. The symbol for a
semiconductor diode is given in Fig. 1.9. The terminal voltage and current are represented as Vd and Id,
respectively. Figure 1.10 shows the structure of a diode. It has an anode (A) terminal and a cathode (K)
terminal. The diode is constructed by joining together two pieces of semiconductor material—a p-type
and an n-type—to form a pn-junction. When the anode terminal is positive with respect to the cathode
terminal, the pn-junction becomes forward-biased and the diode conducts current with a relatively low
voltage drop. When the cathode terminal is positive with respect to the anode terminal, the pn-junction
becomes reverse-biased and the current flow is blocked. The arrow on the diode symbol in Fig. 1.9 shows
the direction of conventional current flow when the diode conducts.
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Characteristics
The voltage-current characteristics of a diode are shown in Fig. 1.11. In
the forward region, the diode starts conducting as the anode voltage is
increased with respect to the cathode. The voltage where the current starts
to increase rapidly is called the knee voltage of the diode. For a silicon
diode, the knee voltage is approximately 0.7 V. Above the knee voltage,
small increases in the diode voltage produce large increases in the diode
current. If the diode current is too large, excessive heat will be generated,
which can destroy the diode. When the diode is reverse-biased, diode
current is very small for all values of reverse voltage less than the diode
breakdown voltage. At breakdown, the diode current increases rapidly
for small increases in diode voltage.
A
Id
om
+
Vd
_
K
tc
FIGURE 1.9
Principal Ratings for Diodes
Diode symbol.
A
Figures 1.12 and 1.13 show typical data sheets for power diodes.
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Maximum Average Forward Current
Peak Inverse Voltage
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The maximum average forward current (If(avg)max) is the current a diode
can safely handle when forward biased. Power diodes are available in
ratings from a few amperes to several hundred amperes. For example,
the power diode D6 described in the data specification sheet (Fig 1.12)
can handle up to 6 A in the forward direction when used as a rectifier.
+
P V
d
N _
K
FIGURE 1.10 Diode structure.
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kw
eb
in
The peak inverse voltage (PIV) of a diode is the maximum reverse voltage
that can be connected across a diode without breakdown. The peak
inverse voltage is also called peak reverse voltage or reverse breakdown
voltage. The PIV ratings of power diodes extend from a few volts to
several thousand volts. For example, the power diode D6 has a PIV rating
of up to 1600 V, as shown in Fig. 1.12.
Id
FIGURE 1.11
Diode voltage-current characteristic.
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FIGURE 1.12
Diode data sheet—ratings. (From USHA, India. With permission.)
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FIGURE 1.13
Diode data sheet—characteristic curves.
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Basic circuit for half-wave rectifier.
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FIGURE 1.14
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V
FIGURE 1.15
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V
Input and output voltage waveforms for the circuit in Fig. 1.14.
Maximum Surge Current
The IFSM (forward surge maximum) rating is the maximum current that the diode can handle as an
occasional transient or from a circuit fault. The IFSM rating for the power diode D6 is up to 190 A, as
shown in Fig 1.12.
Maximum Junction Temperature
This parameter defines the maximum junction temperature that a diode can withstand without failure.
The maximum junction temperature for the power diode D6 is 180В°C.
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Rectifier Circuits
Rectifier circuits produce a DC voltage or current from an AC source. The diode is an essential component
of these circuits. Figure 1.14 shows a half-wave rectifier circuit using a diode. During the positive half
cycle of the source voltage, the diode is forward-biased and conducts for vs(t) > Ef . The value of Ef for
germanium is 0.2 V and for silicon it is 0.7 V. During the negative half cycle of vs(t) , the diode is reversebiased and does not conduct. The voltage vL(t) across the load RL is shown in Fig. 1.15.
The half-wave rectifier circuit produces a pulsating direct current that uses only the positive half cycle
of the source voltage. The full-wave rectifier shown in Fig. 1.16 uses both half cycles of source voltage.
During the positive half cycle of vs(t), diodes D1 and D2 are forward-biased and conduct. Diodes D3 and
D4 are reverse-biased and do not conduct. During the negative half cycle of vs(t), diodes D1 and D2 are
reverse-biased and do not conduct, whereas diodes D3 and D4 are forward-biased and conduct. The
voltage vL(t) across the load RL is shown in Fig. 1.17.
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D4
D1
s
I load
(t)
D3
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Basic circuit for full-wave rectifier.
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FIGURE 1.16
D2
FIGURE 1.17
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V
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Input and output voltage waveforms for the circuit in Fig. 1.16.
Testing a Power Diode
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An ohmmeter can be used to test power diodes. The ohmmeter is connected so that the diode is forwardbiased. This should give a low resistance reading. Reversing the ohmmeter leads should give a very high
resistance or even an infinite reading. A very low resistance reading in both directions indicates a shorted
diode. A high resistance reading in both directions indicates an open diode.
Protection of Power Diodes
A power diode must be protected against over current, over voltage, and transients.
When a diode is reverse-biased, it acts like an open circuit. If the reverse bias voltage exceeds the breakdown
voltage, a large current flow results. With this high voltage and large current, power dissipation at the
diode junction may exceed its maximum value, destroying the diode. For the diode protection, it is a
usual practice to choose a diode with a peak reverse voltage rating that is 1.2 times higher than the
expected voltage during normal operating conditions.
Current ratings for diodes are based on the maximum junction temperatures. As a safety precaution,
it is recommended that the diode current be kept below this rated value. Electrical transients can cause
higher-than-normal voltages across a diode. To protect a diode from the transients, an RC series circuit
may be connected across the diode to reduce the rate of change of voltage.
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1.3 Schottky Diodes
Sohail Anwar
om
Bonding a metal, such as aluminum or platinum, to n-type silicon forms a Schottky diode. The Schottky
diode is often used in integrated circuits for high-speed switching applications. An example of a highspeed switching application is a detector at microwave frequencies. The Schottky diode has a voltagecurrent characteristic similar to that of a silicon pn-junction diode. The Schottky is a subgroup of the TTL
family and is designed to reduce the propagation delay time of the standard TTL IC chips. The construction
of the Schottky diode is shown in Fig. 1.18a, and its symbol is shown in Fig. 1.18b.
Characteristics
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The low-noise characteristics of the Schottky diode make it ideal for application in power monitors of
low-level radio frequency, detectors for high frequency, and Doppler radar mixers. One of the main
advantages of the Schottky barrier diode is its low forward voltage drop compared with that of a silicon
diode. In the reverse direction, both the breakdown voltage and the capacitance of a Schottky barrier diode
behave very much like those of a one-sided step junction. In the one-sided step junction, the doping
level of the semiconductor determines the breakdown voltage. Because of the finite radius at the edges
of the diode and because of its sensitivity to surface cleanliness, the breakdown voltage is always somewhat
lower than theoretical predictions.
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Data Specifications
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The data specification sheet for a DSS 20-0015B power Schottky diode is provided as an example in
Figs. 1.19 and 1.20. Specifications will vary depending on the application and model of Schottky diode.
Testing of Schottky Diodes
Two ways of testing the diodes use either a voltmeter or a digital multimeter. The voltmeter should be
set to the low resistance scale. A single diode or rectifier should read a low resistance, typically, 2/3 scale
from the resistance in the forward direction. In the reverse direction, the resistance should be nearly
infinite. It should not read near 0 Ω in the shorted or open directions. The diode will result in a higher
Metallic
A
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n+
K
A
SiO2
K
n-type
p-type substrate
(a)
FIGURE 1.18
(b)
Diagram (a) and symbol (b) of the Schottky diode.
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IFAV = 20 A
VRRM = 15 V
VF = 0.33 V
Power Schottky Rectifier
VRSM
VRRM
TO-220 AC
Type
V
A
C
V
15
15
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Preliminary Data
C
A
DSS 20-0015B
C (TAB)
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A = Anode, C = Cathode , TAB = Cathode
Conditions
Maximum Ratings
IFRMS
IFAVM
TC = 135 C; rectangular, d = 0.5
IFSM
TVJ = 45ВЎC; t p = 10 ms (50 Hz), sine
EAS
IAS = tbd A; L = 180 H; T VJ = 25ВЎC; non repetitive
IAR
VA =1.5 V RRM typ.; f=10 kHz; repetitive
35
20
A
tbd
mJ
tbd
A
tbd
V/ s
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-55...+150
150
-55...+150
0
C
C
C
Ptot
TC = 25 C
Md
mounting torque
Weight
typical
Symbol
Conditions
IR
TVJ = 25ВЎC VR = VRRM
TVJ = 100ВЎC VR = VRRM
10
200
mA
mA
VF
IF = 20 A;
IF = 20 A;
IF = 40 A;
0.33
0.45
0.43
V
V
V
1.4
K/W
K/W
0.4...0.6
2
W
Nm
g
Features
ВҐ International standard package
Very low VF
Extremely low switching losses
Low IRM-values
Epoxy meets UL 94V-0
Applications
Rectifiers in switch mode power
supplies (SMPS)
Free wheeling diode in low voltage
converters
Advantages
High reliability circuit operation
Low voltage peaks for reduced
protection circuits
Low noise switching
Low losses
Characteristic Values
typ.
max.
TVJ = 125ВЎC
TVJ = 25ВЎC
TVJ = 125 ВЎC
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RthJC
RthCH
9
A
A
350
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(dv/dt)cr
TVJ
TVJM
Tstg
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Symbol
0.5
Dimensions see outlines.pdf
Pulse test:
Pulse Width = 5 ms, Duty Cycle < 2.0 %
Data according to IEC 60747 and per diode unless otherwise specified
IXYS reserves the right to change limits, Conditions and dimensions.
FIGURE 1.19
Data specification sheet for a DSS 20-00105B power Schottky diode (front). (Courtesy of IXYS.)
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100
10000
1000
mA TVJ=150ВЎC
A
IR 100
IF
pF
125ВЎC
CT
100ВЎC
10
TVJ =
150ВЎC
125ВЎC
25ВЎC
1000
75ВЎC
50ВЎC
1
25ВЎC
1
0.0
0.6 V
0.4
0
2
4
6
8
VF
Fig. 2 Typ. value of reverse current IR
versus reverse voltage VR
14
W
12
40
A
DC
10
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d=0.5
IF(AV)
d=
DC
0.5
0.33
0.25
0.17
0.08
8
20
6
2
0
0
0
40
80
120 ВЎC 160
0
Fig. 4 Average forward current IF(AV)
versus case temperature TC
2
1
D=0.5
ZthJC
0.33
0.25
0.17
0.08
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FIGURE 1.20
2
4
6
8
10 12 14 V
VR
Fig. 3 Typ. junction capacitance CT
versus reverse voltage VR
10
15
20 25
IF(AV)
30
A
A
IFSM
1000
100
10
100
1000 s 10000
tP
Fig. 5 Forward power loss
characteristics
Single Pulse
0.1
0.01
0.0001
5
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in
TC
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4
10
0
10000
P(AV)
30
K/W
10 12 14 V
VR
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Fig. 1 Maximum forward voltage
drop characteristics
TVJ= 25ВЎC
100
0.1
0.2
om
10
0.001
0.01
0.1
s 1
DSS 20-0015B
10
t
Data specification sheet for a DSS 20-00105B power Schottky diode (reverse).
scale reading of resistance as a result of its lower voltage drop. What is being measured is the resistance
at a particular low current point; it is not the actual resistance in a power rectifier circuit.
The digital multimeter will usually have a diode test mode. When using this mode, a silicon diode
should read between 0.5 to 0.8 V in the forward direction and open in the reverse direction. A germanium
diode will be in the range of 0.2 to 0.4 V in the forward direction. By using the normal resistance range,
these diodes will usually show open for any semiconductor junction since the voltmeter does not apply
enough voltage to reach the value of the forward drop.
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1.4 Thyristors
Sohail Anwar
The Basics of Silicon-Controlled Rectifiers (SCR)
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Thyristors are four-layer pnpn power semiconductor devices. These devices switch between conducting
and nonconducting states in response to a control signal. Thyristors are used in timing circuits, AC motor
speed control, light dimmers, and switching circuits. Small thyristors are also used as pulse sources for
large thyristors. The thyristor family includes the silicon-controlled rectifier (SCR), the DIAC, the Triac,
the silicon-controlled switch (SCS), and the gate turn-off thyristor (GTO).
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The SCR is the most commonly used electrical power controller. An SCR is sometimes called a pnpn
diode because it conducts electrical current in only one direction. Figure 1.21a shows the SCR symbol.
It has three terminals: the anode (A), the cathode (K), and the gate (G). The anode and the cathode
are the power terminals and the gate is the control terminal. The structure of an SCR is shown in
Fig. 1.21b.
When the SCR is forward-biased, that is, when the anode of an SCR is made more positive with respect
to the cathode, the two outermost pn-junctions are forward-biased. The middle pn-junction is reversebiased and the current cannot flow. If a small gate current is now applied, it forward-biases the middle pnjunction and allows a much larger current to flow through the device. The SCR stays ON even if the gate
current is removed. SCR shutoff occurs only when the anode current becomes less than a level called the
holding current (IH).
Characteristics
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The volt-ampere characteristic of an SCR is shown in Fig. 1.22. If the forward bias is increased to the
forward breakover voltage, VFBO, the SCR turns ON. The value of forward breakover voltage is controlled
by the gate current IG. If the gate-cathode pn-junction is forward-biased, the SCR is turned ON at a lower
breakover voltage than with the gate open. As shown in Fig. 1.22, the breakover voltage decreases with
an increase in the gate current. At a low gate current, the SCR turns ON at a lower forward anode voltage.
At a higher gate current, the SCR turns ON at a still lower value of forward anode voltage.
When the SCR is reverse-biased, there is a small reverse leakage current (IR). If the reverse bias is
increased until the voltage reaches the reverse breakdown voltage (V(BR)R), the reverse current will increase
sharply. If the current is not limited to a safe value, the SCR may be destroyed.
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(anode)
A
G
(gate)
K
(cathode)
(a)
FIGURE 1.21
(b)
(a) The SCR symbol; (b) the SCR structure.
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I G 2 > I G1 > I G 0
Holding current (I H )
I G2
Reverse leakage
current (I R)
- VA K
Reverse blocking
region
Reverse breakdown
+ VAK
Forward breakover
voltage (VFBO)
tc
Forward blocking
region (off state)
I G0
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Maximum reverse
voltage (V(BR)R)
I G1
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+ IA
Forward conduction
region (on state)
- IA
SCR characteristics.
FIGURE 1.23
An SCR turn-off circuit.
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FIGURE 1.22
SCR Turn-Off Circuits
If an SCR is forward-biased and a gate signal is applied, the device turns ON. Once the anode current is
above IH, the gate loses control. The only way to turn OFF the SCR is to make the anode terminal negative
with respect to the cathode or to decrease the anode current below IH. The process of SCR turnoff is called
commutation. Figure 1.23 shows an SCR commutation circuit. This type of commutation method is called
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AC line commutation. The load current IL flows during the positive half cycle of the source voltage. The
SCR is reverse-biased during the negative half cycle of the source voltage. With a zero gate current, the
SCR will turn OFF if the turn-off time of the SCR is less than the duration of the half cycle.
SCR Ratings
A data sheet for a typical thyristor follows this section and includes the following information:
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Surge Current Rating (IFM)—The surge current rating (IFM) of an SCR is the peak anode current an
SCR can handle for a short duration.
Latching Current (IL)—A minimum anode current must flow through the SCR in order for it to stay
ON initially after the gate signal is removed. This current is called the latching current (IL).
Holding Current (IH)—After the SCR is latched on, a certain minimum value of anode current is
needed to maintain conduction. If the anode current is reduced below this minimum value, the
SCR will turn OFF.
Peak Repetitive Reverse Voltage (VRRM)—The maximum instantaneous voltage that an SCR can withstand, without breakdown, in the reverse direction.
Peak Repetitive Forward Blocking Voltage (VDRM)—The maximum instantaneous voltage that the SCR
can block in the forward direction. If the VDRM rating is exceeded, the SCR will conduct without
a gate voltage.
Nonrepetitive Peak Reverse Voltage (VRSM)—The maximum transient reverse voltage that the SCR can
withstand.
Maximum Gate Trigger Current (IGTM)—The maximum DC gate current allowed to turn the SCR ON.
Minimum Gate Trigger Voltage (VGT)—The minimum DC gate-to-cathode voltage required to trigger
the SCR.
Minimum Gate Trigger Current (IGT)—The minimum DC gate current necessary to turn the SCR ON.
The DIAC
A DIAC is a three-layer, low-voltage, low-current semiconductor switch. The DIAC symbol is shown in
Fig. 1.24a. The DIAC structure is shown in Fig. 1.24b. The DIAC can be switched from the OFF to the
ON state for either polarity of applied voltage.
The volt-ampere characteristic of a DIAC is shown in Fig. 1.25. When Anode 1 is made more positive
than Anode 2, a small leakage current flows until the breakover voltage VBO is reached. Beyond VBO , the
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A node 1
N1
P1
N2
P2
A node 2
(a)
FIGURE 1.24
Anode 1
N3
Anode 2
(b)
(a) The DIAC symbol; (b) the DIAC structure.
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I
VBO
I BR
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The DIAC characteristics.
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G
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FIGURE 1.25
N
MT1
N
P
N
N
P
N
M T2
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M T1
(a)
FIGURE 1.26
V
VBO
I BR
Gate
MT2
(b)
(a) The Triac symbol; (b) the Triac structure.
DIAC will conduct. When Anode 2 is made more positive relative to Anode 1, a similar phenomenon
occurs. The breakover voltages for the DIAC are almost the same in magnitude in either direction. DIACs
are commonly used to trigger larger thyristors such as SCRs and Triacs.
The Triac
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The Triac is a three-terminal semiconductor switch. It is triggered into conduction in both the forward
and the reverse directions by a gate signal in a manner similar to the action of an SCR. The Triac symbol
is shown in Fig. 1.26a and the Triac structure is shown in Fig. 1.26b.
The volt-ampere characteristic of the Triac is shown in Fig. 1.27. The breakover voltage of the Triac
can be controlled by the application of a positive or negative signal to the gate. As the magnitude of
the gate signal increases, the breakover voltage decreases. Once the Triac is in the ON state, the gate
signal can be removed and the Triac will remain ON until the main current falls below the holding
current (IH) value.
The Silicon-Controlled Switch
The SCS is a four-layer pnpn device. The SCS symbol is shown in Fig. 1.28a and the SCS structure is
shown in Fig. 1.28b. The SCS has two gates labeled as the anode gate (AG) and the cathode gate (KG).
An SCS can be turned ON by the application of a negative gate pulse at the anode gate. When the SCS
is in the ON state, it can be turned OFF by the application of a positive pulse at the anode gate or a
negative pulse at the cathode gate.
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I
Main Terminal
(positive)
IH
VBR
V
VBR
om
IH
Main Terminal 2
(negative)
The Triac characteristics.
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FIGURE 1.27
A
AG
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K
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in
(a)
Cathode (K)
(b)
(a) The SCS symbol; (b) the SCS structure.
Anode
A
G
Gate
Pa
K
FIGURE 1.29
Anode Gate
(AG)
Cathode Gate
(KG)
KG
FIGURE 1.28
do
Anode (A)
(a)
Cathode
(b)
(a) The GTO symbol; (b) the GTO structure.
The Gate Turn-Off Thyristor
The GTO is a power semiconductor switch that turns ON by a positive gate signal. It can be turned OFF
by a negative gate signal. The GTO symbol is shown in Fig. 1.29a and the GTO structure is shown in
Fig. 1.29b. The GTO voltage and current ratings are lower than those of SCRs. The GTO turn-off time
is lower than that of SCR. The turn-on time is the same as that of an SCR.
Data Sheet for a Typical Thyristor
Figures 1.30 to 1.35 are the data sheets for a typical thyristor.
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FIGURE 1.30
Page 1 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
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Page 2 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
Pa
FIGURE 1.31
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FIGURE 1.32
Page 3 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
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FIGURE 1.33
Page 4 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
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FIGURE 1.34
Page 5 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
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FIGURE 1.35
Page 6 of a data sheet for a typical thyristor. (From Philips Semiconductors. With permission.)
1.5 Power Bipolar Junction Transistors
Sohail Anwar
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Power bipolar junction transistors (BJTs) play a vital role in power circuits. Like most other power devices,
power transistors are generally constructed using silicon. The use of silicon allows operation of a BJT at
higher currents and junction temperatures, which leads to the use of power transistors in AC applications
where ranges of up to several hundred kilowatts are essential.
The power transistor is part of a family of three-layer devices. The three layers or terminals of a transistor
are the base, the collector, and the emitter. Effectively, the transistor is equivalent to having two pn-diode
junctions stacked in opposite directions to each other. The two types of a transistor are termed npn and
pnp. The npn-type transistor has a higher current-to-voltage rating than the pnp and is preferred for most
power conversion applications. The easiest way to distinguish an npn-type transistor from a pnp-type is
by virtue of the schematic or circuit symbol. The pnp type has an arrowhead on the emitter that points
toward the base. Figure 1.36 shows the structure and the symbol of a pnp-type transistor. The npn-type
transistor has an arrowhead pointing away from the base. Figure 1.37 shows the structure and the symbol
of an npn-type transistor.
When used as a switch, the transistor controls the power from the source to the load by supplying sufficient
base current. This small current from the driving circuit through the base–emitter, which must be maintained,
turns on the collector—emitter path. Removing the current from the base–emitter path and making the base
voltage slightly negative turns off the switch. Even though the base–emitter path may only utilize a small
amount of current, the collector–emitter path is capable of carrying a much higher current.
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IC
Collector
C
C
P
+
N
P
B
VCE
--
IB
om
B
Base
lE
E
Emitter
(a)
(b)
pnp transistor structure (a) and circuit symbol (b).
fo
Collector
do
FIGURE 1.36
tc
E
kw
eb
in
Base
Emitter
(a)
FIGURE 1.37
(b)
npn transistor structure (a) and circuit symbol (b).
The Volt-Ampere Characteristics of a BJT
Pa
The volt-ampere characteristics of a BJT are shown in Fig. 1.38. Power transistors have exceptional
characteristics as an ideal switch and they are primarily used as switches. In this type of application, they
make use of the common emitter connection shown in Fig. 1.39. The three regions of operation for a
transistor that must be taken into consideration are the cutoff, saturation, and the active region. When the
base current (IB) is zero, the collector current (IC) is insignificant and the transistor is driven into the cutoff
region. The transistor is now in the OFF state. The collector–base and base–emitter junctions are reversebiased in the cutoff region or OFF state, and the transistor behaves as an open switch. The base current
(IB) determines the saturation current. This occurs when the base current is sufficient to drive the
transistor into saturation. During saturation, both junctions are forward-biased and the transistor acts
like a closed switch. The saturation voltage increases with an increase in current and is normally between
0.5 to 2.5 V. The active region of the transistor is mainly used for amplifier applications and should be
avoided for switching operation. In the active region, the collector–base junction is reversed-biased and
the base–emitter junction is forward-biased.
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Saturation
(ON)
Saturation voltage Vce(sat)
Active
Region
BJT V-I characteristic.
FIGURE 1.39
Biasing of a transistor.
Vce
BJT Biasing
kw
eb
in
fo
do
FIGURE 1.38
Leakage
current
tc
Cutoff (OFF)
om
Ic
Pa
When a transistor is used as a switch, the control circuit provides the necessary base current. The current
of the base determines the ON or OFF state of the transistor switch. The collector and the emitter of the
transistor form the power terminals of the switch.
The DC load line represents all of the possible operating points of a transistor and is shown in Fig. 1.40.
The operating point is where the load line and the base current intersect and is determined by the values
of VCC and RC .
In the ON state, the ideal operating point occurs when the collector current IC is equal to VCC /RC and
VCE is zero. The actual operating point occurs when the load line intersects the base current at the saturation
point. This occurs when the base current equals the saturation current or IB = IB(sat). At this point, the
collector current is maximum and the transistor has a small voltage drop across the collector–emitter
terminals called the saturation voltage VCE(sat).
In the OFF state, or cutoff point, the ideal operating point occurs when the collector current IC is zero
and the collector–emitter voltage VCE is equal to the supply voltage VCC. The actual operating point, in
the OFF state, occurs when the load line intersects the base current (IB = 0). At the cutoff point, the
collector current is the leakage current. By applying Kirchoff ’s voltage law around the output loop, the
collector–emitter voltage (VCE) can be found.
The operating points between the saturation and cutoff constitute the active region. When operating
in the active region, high power dissipation occurs due to the relatively high values of collector current
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FIGURE 1.40
tc
om
Saturation
(ON)
DC load line.
BJT Power Losses
do
IC and collector–emitter voltage VCE . For satisfactory operation, a slightly higher than minimum base
current will ensure a saturated ON state and will result in reduced turn-on time and power dissipation.
BJT Testing
kw
eb
in
fo
The four types of transistor power losses are the ON-state and OFF-state losses and turn-ON and turn-OFF
switching loss. OFF-state transistor losses are much lower than ON-state losses since the leakage current
of the device is within a few milliamps. Essentially, when a transistor is in the off state, whatever the
value of collector–emitter voltage, there is no collector current. Switching losses depend on switching
frequency. The highest possible switching frequency of the transistor is limited by the losses due to the
rate of switching. In other words, the higher the switching frequency, the more power loss in the transistor.
Testing of the state of a transistors can be done with a multimeter. When a transistor is forward-biased,
the base–collector and base–emitter regions should have a low resistance. When reverse-biased, the base–
collector and base–emitter regions should have a high resistance. When testing the resistance between
the collector and the emitter, the resistance reading should result in a much higher than forward bias
base–collector and base–emitter resistance. However, faulty power transistors can appear shorted when
measuring resistance across the collector and emitter, but still pass both junction tests.
BJT Protection
Pa
Transistors must be protected against high currents and voltages to prevent damage to the device. Since they
are able to absorb very little energy before breakdown, semiconductor fuses cannot protect them. Thermal
conditions are vitally important and can occur during high-frequency switching. Some of the most
common types of BJT protection are overcurrent and overvoltage protection. Electronic protection
techniques are also frequently used to provide needed protection for transistors.
Overcurrent protection turns the transistor OFF when the collector–emitter voltage and collector
current reach a preset value. When the transistor is in the ON state, an increase in collector–emitter
voltage causes an increase in the collector current and therefore an increase in junction temperature.
Since the BJT has a negative temperature coefficient, the increase in temperature causes a decrease in
resistance and results in an even higher collector current. This condition, called positive feedback, could
eventually lead to thermal runaway and destroy the transistor. One such method of overcurrent protection
limits the base current during an external fault. With the base current limited, the device current will be
limited at the saturation point, with respect to the base current, and the device will hold some value of
the voltage. This feature turns the transistor off without being damaged and is used for providing
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Features
n
n
n
n
n
n
n
n
Internal thermal limiting
Greater than 1.0A output current
3.0 A typical base current
500 ns switching time
2.0V saturation
Base can be driven up to 40V without damage
Directly interfaces with CMOS or TTL
100% electrical burn-in
Pa
kw
eb
in
Simplified Circuit
For low-power applications (under 100 mA), refer to the
LP395 Ultra Reliable Power Transistor.
The LM195/LM395 are available in the standard TO-3, Kovar
TO-5, and TO-220 packages. The LM195 is rated for operation from 55 C to +150 C and the LM395 from 0 C to
+125 C.
fo
The LM195 offers a significant increase in reliability as well
as simplifying power circuitry. In some applications, where
protection is unusually difficult, such as switching regulators,
lamp or solenoid drivers where normal power dissipation is
low, the LM195 is especially advantageous.
The LM195 is easy to use and only a few precautions need
be observed. Excessive collector to emitter voltage can destroy the LM195 as with any power transistor. When the device is used as an emitter follower with low source imped-
ance, it is necessary to insert a 5.0k resistor in series with
the base lead to prevent possible emitter follower oscillations. Although the device is usually stable as an emitter follower, the resistor eliminates the possibility of trouble without
degrading performance. Finally, since it has good high frequency response, supply bypassing is recommended.
tc
The LM195/LM395 are fast, monolithic power integrated circuits with complete overload protection. These devices,
which act as high gain power transistors, have included on
the chip, current limiting, power limiting, and thermal overload protection making them virtually impossible to destroy
from any type of overload. In the standard TO-3 transistor
power package, the LM195 will deliver load currents in excess of 1.0A and can switch 40V in 500 ns.
The inclusion of thermal limiting, a feature not easily available in discrete designs, provides virtually absolute protection against overload. Excessive power dissipation or inadequate heat sinking causes the thermal limiting circuitry to
turn off the device preventing excessive heating.
do
General Description
FIGURE 1.41
om
LM195/LM395
Ultra Reliable Power Transistors
LM195/LM395 Ultra Reliable Power Transistors
July 2000
1.0 Amp Lamp Flasher
DS006009-16
DS006009-1
Typical data sheet for a power transistor (page 1). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
om
tc
Connection Diagrams
TO-3 Metal Can Package
do
TO-220 Plastic Package
DS006009-3
Case is Emitter
Top View
Order Number LM395T
See NS Package Number T03B
fo
DS006009-2
Bottom View
Order Number LM195K/883
See NS Package Number K02A
(Note 5)
DS006009-4
Bottom View
Order Number LM195H/883
See NS Package Number H03B
(Note 5)
Pa
kw
eb
in
TO-5 Metal Can Package
FIGURE 1.42
Typical data sheet for a power transistor (page 2). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Absolute Maximum Ratings (Note 1)
Base to Emitter Voltage (Reverse)
Collector Current
Power Dissipation
Operating Temperature Range
LM195
LM395
Storage Temperature Range
Lead Temperature
(Soldering, 10 sec.)
Collector to Emitter Voltage
LM195
LM395
Collector to Base Voltage
LM195
LM395
Base to Emitter Voltage (Forward)
LM195
LM395
42V
36V
42V
36V
Preconditioning
Parameter
Conditions
do
100% Burn-In In Thermal Limit
(Note 2)
LM195
Min
Collector-Emitter Operating Voltage
IQ ≤ IC ≤ IMAX
(Note 4)
42
TO-3, TO-220
VCE ≤ 15V
1.2
TO-5
VCE ≤ 7.0V
1.2
Collector Current
fo
0 ≤ VCE ≤ VCEMAX
Base to Emitter Breakdown Voltage
eb
in
IC ≤ 1.0A, TA = 25 C
Saturation Voltage
0 ≤ IC ≤ IMAX
Base Current
0 ≤ VCE ≤ VCEMAX
Quiescent Current (IQ)
Vbe = 0
0 ≤ VCE ≤ VCEMAX
Base to Emitter Voltage
IC = 1.0A, TA = +25 C
Switching Time
VCE = 36V, RL = 36Ω,
TA = 25 C
Thermal Resistance Junction to
Case (Note 3)
55 C to +150 C
0 C to +125 C
65 C to +150 C
260 C
tc
42V
36V
Electrical Characteristics
20V
Internally Limited
Internally Limited
om
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Typ
LM395
Max
Min
Typ
42
Units
Max
36
V
36
60
V
2.2
1.0
2.2
A
1.8
1.0
1.8
A
1.8
2.0
1.8
2.2
V
3.0
5.0
3.0
10
A
2.0
5.0
2.0
10
mA
0.9
0.9
V
500
500
ns
TO-3 Package (K)
2.3
3.0
2.3
3.0
C/W
TO-5 Package (H)
12
15
12
15
C/W
4
6
C/W
kw
TO-220 Package (T)
Note 1: »Absolute Maximum Ratings…indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits.
Note 2: Unless otherwise specified, these specifications apply for 55 C ≤ Tj ≤ +150 C for the LM195 and 0 C ≤ +125 C for the LM395.
Note 3: Without a heat sink, the thermal resistance of the TO-5 package is about +150 C/W, while that of the TO-3 package is +35 C/W.
Note 4: Selected devices with higher breakdown available.
FIGURE 1.43
Pa
Note 5: Refer to RETS195H and RETS195K drawings of military LM195H and LM195K versions for specifications.
Typical data sheet for a power transistor (page 3). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Typical Performance Characteristics
Collector Characteristics
(for K and T Packages)
tc
DS006009-34
do
DS006009-33
Base Emitter Voltage
eb
in
Response Time
kw
Saturation Voltage
DS006009-39
DS006009-38
DS006009-37
DS006009-36
DS006009-35
Base Current
fo
Quiescent Current
Response Time
DS006009-40
DS006009-41
Typical data sheet for a power transistor (page 4). (From National Semiconductor. With permission.)
Pa
FIGURE 1.44
Bias Current
om
Short Circuit Current
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m
(for K and T Packages) (Continued)
10V Transfer Function
36V Transfer Function
ot
co
Typical Performance Characteristics
DS006009-8
od
DS006009-7
Small Signal Frequency
Response
inf
Transconductance
DS006009-10
Typical data sheet for a power transistor (page 5). (From National Semiconductor. With permission.)
Pa
kw
FIGURE 1.45
eb
DS006009-9
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В© 2002 by CRC Press LLC
Pa
kw
eb
in
fo
do
tc
om
DS006009-11
Schematic Diagram
FIGURE 1.46
Typical data sheet for a power transistor (page 6). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Typical Applications
do
tc
om
1.0 Amp Voltage Follower
DS006009-12
fo
* Solid Tantalum
Time Delay
eb
in
Power PNP
DS006009-13
DS006009-14
1.0 MHz Oscillator
Pa
kw
* Protects against excessive base drive
** Needed for stability
DS006009-15
FIGURE 1.47
Typical data sheet for a power transistor (page 7). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Typical Applications
(Continued)
do
tc
om
1.0 Amp Negative Regulator
DS006009-17
fo
†Solid Tantalum
FIGURE 1.48
DS006009-18
Pa
†Solid Tantalum
kw
eb
in
1.0 Amp Positive Voltage Regulator
Typical data sheet for a power transistor (page 8). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Typical Applications
(Continued)
Fast Optically Isolated Switch
tc
om
Optically Isolated Power Transistor
do
DS006009-19
Two Terminal Current Limiter
fo
CMOS or TTL Lamp Interface
DS006009-20
40V Switch
DS006009-21
eb
in
DS006009-22
Two Terminal 100 mA Current Regulator
Pa
kw
6.0V Shunt Regulator with Crowbar
DS006009-23
* Drive Voltage 0V to ≥ 10V ≤ 42V
FIGURE 1.49
DS006009-25
DS006009-24
Typical data sheet for a power transistor (page 9). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
Typical Applications
(Continued)
Low Level Power Switch
tc
om
Power One-Shot
DS006009-26
do
Turn ON = 350 mV
Turn OFF = 200 mV
DS006009-27
T = R1C
R2 = 3R1
R2 ≤ 82k
High Input Impedance AC Emitter Follower
eb
in
fo
Emitter Follower
DS006009-28
* Need for Stability
DS006009-29
Pa
kw
Fast Follower
DS006009-30
* Prevents storage with fast fall time square wave drive
FIGURE 1.50
Typical data sheet for a power transistor (page 10). (From National Semiconductor. With permission.)
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Typical Applications
(Continued)
fo
do
tc
om
Power Op Amp
* Adjust for 50 mA quiescent current
†Solid Tantalum
DS006009-31
Pa
kw
eb
in
6.0 Amp Variable Output Switching Regulator
DS006009-32
* Sixty turns wound on Arnold Type A-083081-2 core.
** Four devices in parallel
†Solid tantalum
FIGURE 1.51
Typical data sheet for a power transistor (page 11). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
inches (millimeters) unless otherwise noted
do
tc
om
Physical Dimensions
Pa
kw
eb
in
fo
TO-5 Metal Can Package
Order Number LM195H/883
NS Package Number H03B
FIGURE 1.52
TO-3 Metal Can Package
Order Number LM195K/883
NS Package Number K02A
Typical data sheet for a power transistor (page 12). (From National Semiconductor. With permission.)
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В© 2002 by CRC Press LLC
inches (millimeters) unless otherwise noted (Continued)
do
tc
om
Physical Dimensions
TO-220 Plastic Package
Order Number LM395T
NS Package Number T03B
LIFE SUPPORT POLICY
fo
NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
kw
eb
in
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: [email protected]
www.national.com
National Semiconductor
Europe
Fax: +49 (0) 180-530 85 86
Email: [email protected]
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
FranГ•ais Tel: +33 (0) 1 41 91 8790
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
or effectiveness.
National Semiconductor
Asia Pacific Customer
Response Group
Tel: 65-2544466
safety Fax: 65-2504466
Email: [email protected]
National Semiconductor
Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
FIGURE 1.53
Typical data sheet for a power transistor (page 13). (From National Semiconductor. With permission.)
Pa
protection in low power converters by limiting the current during an external fault. Other methods of
overcurrent protection for more severe faults use a shorting switch, or shunt switch, in parallel with the
transistor. When a fault is detected, an external circuit activates the parallel shorting switch, providing
an alternate path for the fault current.
Overvoltage protection is used to protect a transistor from high voltages. When a transistor is in the
OFF state, high collector-base reverse–bias voltages can cause avalanche breakdown. Avalanche breakdown occurs when the reverse voltage exceeds the reverse voltage limit of the collector–base region. High
collector–base reverse-bias voltages can easily damage the transistor. One simple method to ensure
overcurrent protection of a transistor is to connect an antiparallel diode across the transistor.
Most power transistors are unable to block reverse voltages in excess of 20 V. Reverse voltages can
easily damage the transistor and therefore they should not be used in AC control applications without
a reverse shunting diode connected between the emitter and the collector.
A typical data sheet for a power transistor is provided in Figs. 1.41 through 1.53.
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1.6 MOSFETs
Vrej Barkhordarian
Field
oxide
Gate Gate
Drain
oxide metallization contact
n+ Source
tc
Source
contact
om
The metal-oxide-semiconductor field-effect transistor (MOSFET) is the most commonly used active
device in very large scale integrated (VLSI) circuits. Figure 1.54 shows the device schematic, currentvoltage characteristics, transfer characteristics and device symbol for a MOSFET. It is a lateral device and
though very suitable for integration into integrated circuits, it has severe limitations at high power levels.
The power MOSFET design is based on the original field-effect transistor and, since its invention in the
early 1970s, has gone through several evolutionary steps. The processing of power MOSFETs is very
similar to that of today’s VLSI circuits although the device geometry is significantly different from the
n+ Drain
do
tox
p-Substrate
i
fo
Channel
lD
kw
eb
in
(a)
lD
VGS > VП„
0
0
VDS
VGS = VП„
0
Pa
(b)
0
VП„
VGS
(c)
lD
D
SB
(Channel or
substrate)
G
S
(d)
FIGURE 1.54 (a) Schematic diagram, (b) current-voltage characteristics, (c) transfer characteristics, and (d) device
symbol for an n-channel enhancement mode MOSFET.
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kw
eb
in
fo
do
tc
om
design used in these circuits. Power MOSFETs are commonly used as switches in power electronic
applications.
The invention of the power MOSFET was partly driven by the limitations of bipolar power transistors
which, until recently, were the devices of choice in power electronics applications. Although it is not
possible to define absolutely the operating boundaries of a power device, we will loosely refer to the
power device as any device that is capable of switching at least 1A. The bipolar power transistor is a
current-controlled device and a large base drive current as high as one fifth of the collector current is
required to keep the device in the on state. Also, higher reverse base drive currents are required to obtain
fast turn-off. Despite the very advanced state of manufacturability and lower costs of bipolar power
transistors, these limitations have made the base drive circuit design more complicated and hence more
expensive. There are two further limitations to the bipolar power transistor. First, both electrons and
holes contribute to conduction in BJTs. Presence of holes with their higher carrier lifetime causes the
switching speed to be several orders of magnitude slower than for a power MOSFET of similar size and
voltage rating. Secondly, the BJTs suffer from thermal runaway. The forward voltage drop of a BJT
decreases with increasing temperature causing diversion of current to a single device when several devices
are paralleled. Power MOSFETs, on the other hand, are majority carrier devices with no minority carrier
injection. They are superior to the BJTs in high-frequency applications where switching power losses are
important and can withstand simultaneous application of high current and voltage without undergoing
destructive failure due to second breakdown. Power MOSFETs can also be paralleled easily since the
forward voltage drop increases with increasing temperature, ensuring an even distribution of current
among all components. However, at high breakdown voltages (>в€ј200V) the on-state voltage drop of the
power MOSFET becomes higher than that of a similar size bipolar device with a similar voltage rating,
making it more attractive to use the bipolar power transistor at the expense of worse high-frequency
performance. Figure 1.55 shows the present current-voltage limitations of power MOSFETs and BJTs.
New materials, structures and processing techniques are expected to push these limits out over time. A
relatively new device which combines the high-frequency advantages of the MOSFET with the low onstate voltage drop of high voltage BJTs is the insulated-gate-bipolar transistor (IGBT).
2000
Bipolar
transistors
1000
Pa
Holdoff voltage (V)
1500
MOS
500
0
1
10
100
1000
Maximum current (A)
FIGURE 1.55
Current-voltage limitations of MOSFETs and BJTs.
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Gate
oxide
p+ Body Region
+
n
p
Polysilicon
gate
Channels
Source
metallization
p+
n+
Drift Region
nв€’ epilayer
p+ substrate
(100)
Drain
metallization
S
tc
Drain
G
Schematic diagram for an n-channel power MOSFET and the device symbol.
Metal
do
FIGURE 1.56
D
om
Source
Cgsm
CGS2
nв€’
RCh
CGD
nв€’
JFET
RB
BJT
CDS
fo
pв€’
CGS1
LTO
REPI
kw
eb
in
nв€’ epilayer
nв€’ substrate
FIGURE 1.57
The origin of parasitic components for a power MOSFET.
Pa
MOSFETs used in integrated circuits are lateral devices with gate, source and drain all on the top of
the device and with current flow taking place in a path parallel to the surface. Although this design lends
itself to integration, it is not suitable for discrete power device applications due to large distances required
between source and drain in order to maintain isolation. Having all three terminals as the upper surface
makes the metallization and isolation of terminals more complicated from the processing point of view.
The vertical double diffused MOSFET solves this problem by using the substrate of the device as the
drain terminal. Figure 1.56 shows the schematic diagram and the circuit symbol for an n-channel power
MOSFET. When a positive bias greater than the threshold voltage is applied to the gate, the silicon
surface in the channel region is inverted and a current starts to flow between the source and drain. For
gate voltages of less than V+ no surface inversion occurs in the channel and the device remains in the offstate. The current in this device flows horizontally along the inverted channel first and then vertically
between the drain and source. The term “double-diffused” refers to the two consecutive ion implantation
steps using the poly as a mask. For an n-channel device, the regions formed by double implant and
subsequent diffusion are first p-type to define the channel and then n-type to define the source. The pbody implant is performed in a separate step. The terms “body drift” and “body-drain” diodes are used
interchangeably to denote the p-n junction formed by this p-body implant and the drift region.
Figure 1.57 shows the physical origin of the parasitic components in an n-channel power MOSFET.
The parasitic JFET appearing between the two body implants restricts current flow when the depletion
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S
G
Source
S
Source
Gate
Gate
oxide
Oxide
n+
p
Electron flow
n+ substrate
(100)
D
Channel
om
n- epilayer
Drain
(a)
tc
(b)
do
FIGURE 1.58 Schematic diagram of (a) V-groove trench MOSFET showing the current crowding at the apex and
(b) truncated V-groove design.
Pa
kw
eb
in
fo
widths of the two adjacent body diodes extend into the drift region with increasing drain voltage.
Poly line-width and the epi layer resistivity under the poly are two important design parameters for
minimizing the JFET effect. The parasitic BJT can make the device susceptible to unwanted device turnon and premature breakdown. The base resistance RB has to be minimized through careful design of the
doping and distance under the source region. These two components and the parasitic resistances are
discussed further in the next sections. There are several parasitic capacitances associated with the power
MOSFET as shown in Fig. 1.57. CGS is the capacitance due to the overlap of the source and the channel
regions by the polysilicon gate and is independent of applied voltage. GGD is made up of two parts. The
first part is the capacitance associated with the overlap of the polysilicon gate and the silicon underneath
in the JFET region. The second part is the capacitance associated with the depletion region immediately
under the gate. CGD is a nonlinear function of voltage and is discussed further in the “Dynamic Characteristics” section. Finally, CDS is the capacitance associated with the body-drift diode and varies inversely
with the square root of the drain-source bias.
There are currently two designs of power MOSFETs. These are usually referred to as the planar and
the trench designs. The planar design has already been introduced in the schematics of Figs. 1.56 and
1.57. Two variations of the trench power MOSFET are shown in Fig. 1.58. The V-groove device is
fabricated by etching a groove in the silicon after the double diffusion step. The use of an anisotropic
etch results in the sides of the groove to be at an angle of 54.7В° to the surface of the wafer. Etching stops
when the groove sides, which are planes, reach each other. The gate oxide and gate poly or metallization
are then grown in the groove followed by the source metallization. Current crowding at the apex of the
V-groove reduces current handling capability. In a truncated V-groove design, the anisotropic etch is
stopped before this point is reached. The trench technology has the advantage of higher cell density but
is more difficult to manufacture compared with the planar device.
Static Characteristics
One of the important features of the power MOSFET is the very high input impedance which simplifies
the gate drive circuitry and reduces cost. It is a voltage-controlled device with to gate current flow during
operation. Figure 1.59 shows I–V characteristics of an enhancement mode (normally off) power MOSFET.
Data sheets contain typical graphs which can be used to determine if the device is in the fully on state
or in the constant-current region for a given value of gate bias and drain current. Temperature effect on
threshold voltage (about 6 mV/C reduction) and the difference between typical values of parameters and
the maximums should be taken into account.
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25
7
GATE
VOLTAGE
)
10
tc
SATURATION
REGION
(
15
(LINEAR REGION)
5
IDS vs VDS LOCUS
do
NORMALIZED DRAIN CURRENT
6
om
20
4
5
kw
eb
in
fo
3
0
0
5
10
2
1
15
DRAIN VOLTAGE (VOLTS)
FIGURE 1.59
Current-voltage characteristics of a power MOSFET.
Breakdown Voltage
Pa
This is the drain voltage at which the reverse-biased body-drift diode breaks down and a significant
current starts to flow between the source and drain by the avalanche multiplication process, while the
gate and source are shorted together. Breakdown voltage, BVDSS, is normally measured at a drain current
of 250 ВµA. For drain voltages below BVDSS and with no bias on the gate, no channel is formed under the
gate at the surface and the drain voltage is entirely supported by the reverse-biased body-drift pn junction.
There are two related phenomena which can occur in poorly designed and processed devices. These are
punch-through and reach-through.
Punch-through is observed when the depletion region on the source side of the body-drift pn-junction
reaches the source region at drain voltages below the rated avalanche voltage of the device. This provides
a current path between source and drain and causes a soft breakdown characteristic as shown in Fig. 1.60.
The leakage current flowing between source and drain is denoted by IDSS. Careful selection and optimization
of the doping profile used in the fabrication of a power MOSFET is therefore very important. Figure 1.61
shows a typical diffusion profile for a power MOSFET. The surface concentration of the body diffusion and
the channel length (distance between the two pn-junctions formed by the source diffusion and the channel
diffusion) will determine whether punch-through will occur or not. There are trade-offs to be made between
on-resistance Rdson which requires shorter channel lengths and punch-through avoidance which requires
longer channel lengths. An approximate equation giving the depletion region width as a function of silicon
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om
ID
Soft
tc
Sharp
BVDSS
Breakdown characteristics of a power MOSFET showing the ideal (sharp) and nonideal (soft)
do
FIGURE 1.60
behaviors.
VDS
1026
fo
n+
1024
kw
eb
in
Doping concentration (m-3)
1025
1023
p
Surface region
Along carrier path
1022
1021
20
Pa
10
Source
Channel
0
1
n
(n-)
Epilayer
Drain drift
region
2
Distance along channel (Вµm)
FIGURE 1.61 Typical doping profile of a power MOSFET, in a direction parallel to the device surface. Threshold
voltage is determined by the peak carrier concentration in the channel region.
background doping is given by:
4 в‘Ђ s KT
N
- ln ------A
W ≈ -------------2
ni
q NA
(1.1)
where ⑀s is semiconductor permittivity, K is Boltzmann’s constant, T is temperature in K, q is electronic
charge, NA is background doping, and ni is the intrinsic carrier density.
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GATE
N+
SOURCE
RA
RJ
P-BASE
om
RSOURCE RCH
RD
DRAIN
The origin of the internal resistances in a power MOSFET.
do
FIGURE 1.62
tc
N+ SUBSTRATE
Rsub
On-Resistance
kw
eb
in
fo
Also, higher channel implant dose is beneficial from the punch-through point of view since depletion
width will be smaller, but the Rdson will suffer through reduced carrier mobility. The design of the doping
profile involves choosing channel and source implant doses, diffusion times and temperatures that give
a designed threshold voltage while simultaneously minimizing Rdson and IDSS. Optimizing these performance parameters with manufacturability in mind is one of the challenges of power MOSFET design.
The reach-through phenomenon, on the other hand, occurs when the depletion region on the drift
side of the body-drift pn-junction reaches the epilayer-substrate interface before avalanching takes place
in the epi. Once the depletion edge enters the high carrier concentration substrate, a further increase in
5
drain voltage will cause the electric field to quickly reach the critical value of 2 Г— 10 V/cm at which
avalanching begins.
Other factors that affect the breakdown voltage of power MOSFETs for a given epitaxial layer include
termination design, cell spacing (poly line width) and curvature of the body diode depletion region in
the epi which is a function of diffusion depth. Power MOSFETs are designed such that avalanche
breakdown occurs in the active area first.
The on-state resistance of a power MOSFET is made up of several components as shown in Fig. 1.62.
where
Pa
R dson = R source + R ch + R A + R J + R D + R sub + R wcml
(1.2)
Rsource = source diffusion resistance
Rch = channel resistance
RA = accumulation resistance
= the “JFET” component-resistance of the region between the two body regions
RJ
RD = drift region resistance
Rsub = the substrate resistance; wafers with resistivities of up to 20 mΩ-cm are used for high-voltage
devices and less than 5 mΩ-cm for low-voltage devices
Rwcml = sum of bond wire resistance, contact resistance between the source and drain metallization
and the silicon, metallization resistance, and leadframe contributions; these are normally
negligible in high-voltage devices but can become significant in low-voltage devices
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Voltage Rating:
50 V
100 V
500 V
Packaging
Rwcml
Metallization
RCh
om
Source
Channel
JFET
tc
region
REPI
do
Epitaxial
layer
Substrate
FIGURE 1.63
Relative contributions to Rdson in devices with different voltage ratings.
kw
eb
in
fo
Figure 1.63 shows the relative importance of each of the components to Rdson over the voltage spectrum.
As can be seen, at high voltages the Rdson is dominated by epi resistance and the JFET component. This
component is higher in high-voltage devices due to the higher resistivity or lower background carrier
concentration in the epi. At lower voltages, the Rdson is dominated by the channel resistance and the
contributions from the metal to semiconductor contact, metallization, bond wires, and leadframe. The
substrate contribution becomes more significant for lower breakdown voltage devices.
Transconductance
This parameter is a measure of the sensitivity of drain current to changes in gate-source bias and is
defined as:
∆I D 
g fs =  --------- ∆V gs
V ds constant
(1.3)
Pa
i.e., the gradient of the Id vs. Vgs graph. In the saturation region, g fs is given by:
W
g fs = µ C ox ----- ( V gs – V th )
L
(1.4)
This parameter is normally quoted for a Vgs that gives a drain current equal to about one half of the
maximum current rating value and for a VDS that ensures operation in the constant current region. With
mobility Вµ fixed for a given semiconductor, the design parameters influencing transconductance of a
MOSFET are gate width W, channel length L, and gate oxide thickness tox and hence Cox. Gate width is the
total polysilicon gate perimeter of the cellular structure and increases in proportion to the active area as
the cell density increases. The cell density has increased over the years from around half a million per square
inch in 1980 to around 8 million for planar MOSFETs and around 12 million for the trench technology at
the present time. The limiting factor for even higher cell densities is the photolithography process control
and resolution which allows contacts to be made to the source metallization in the center of the cells.
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Reduced channel length is beneficial to both gfs and on-resistance, with punch-through as a trade-off.
The lower limit of this length is set by the ability to control the double-diffusion process and is around
1 to 2 Вµm today. Finally, reductions in gate oxide thickness give higher Cox and higher gfs. The reduction
in oxide thickness will reduce Vth unless channel implant dose is increased which in turn will cause a higher
Rdson. Ultimately, the lower limit of tox is set by the maximum gate-source voltage rating. This is В±30 V for
high-voltage devices and В±20 V for lower-voltage logic-level devices used in portable electronic applications.
tc
om
Threshold Voltage
This is defined as the minimum gate electrode bias required to strongly invert the surface under the poly
and form a conducting channel between the source and the drain regions. Vth is usually measured at a
drain-source current of 250 ВµA. A value of 2 to 4 V for high-voltage devices with thicker gate oxides and
logic-compatible values of 1 to 2 V for lower-voltage devices with thinner gate oxides are common. With
power MOSFETs finding increasing use in portable electronics and wireless communications where
battery power is at a premium, the trend is toward lower values of Rdson and Vth. Gate oxide quality and
integrity become major issues as the gate oxide thickness is reduced to achieve lower Vth. An approximate expression for Vth is given by:
do
4 в‘Ђ s KTN A ln ( N A /n i ) 2KT
- + ----------- ln ( N A /n i )
V th ≈ ---------------------------------------------------( ⑀ ox /t ox )
q
(1.5)
kw
eb
in
fo
where в‘Ђox and tox are oxide permittivity and thickness and the other parameters are defined in Eq. (1.1).
Processing methods used and their influence on the chemistry of the silicon surface have pronounced
effects on Vth. Fixed and mobile surface and interface charges as well as charges in the gate oxide act to
change the value of Vth from the intended value. Therefore, control of these charges in the process is
necessary for obtaining consistent Vth values in production. Also, the presence of mobile charges away
from the gate oxide and oxide/silicon interface may find their way to the device surface over the lifetime
of the device and cause a gradual shift in Vth. For example, sodium ions in the low-temperature oxide
(LTO) or in the metallization can cause a shift in Vth by changing the charge distribution at the interface.
Accelerated life-tests are used by manufacturers to evaluate new processes and also to monitor Vth shift
in production. Monitoring and control of contamination in the clean room equipment are routinely
carried out by capacitance-voltage measurements of test diodes.
In real devices, Vth is altered by the unequal metal and semiconductor work functions. Denoting the
barrier height between the metal and silicon oxide as П†B, the work function difference is given by:
q φ ms = q φ B + q χ o – ( q χ + Eg /2 + q ψ B )
(1.6)
Pa
where П€B is the potential difference between the intrinsic and Fermi levels in the semiconductor; П‡ and
П‡o are the semiconductor and oxide electron affinities and Eg is the semiconductor band-gap energy.
Taking into account this effect and also the various fixed and mobile charges that may alter the value
of Vth from that given above, the expression for Vth becomes:
Q s + Q ss + Q I + Q FCпЈ¶
V th = φ ms + 2 ψ B –  ---------------------------------------------
пЈё
C ax
(1.7)
where
Qs = surface charge, which is a function of surface potential and determines channel conductivity
10
12
в€’2
Qss = interface state charge (typically 10 to 10 cm ); caused by dangling bonds at the semiconductor
surface, these can charge and discharge with changes in the surface potential
QI = charge due to mobile ions in the oxide
QFC = fixed surface charge at the silicon–oxide interface
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TJ = 150oC
TJ = 25oC
1
VGS = 0V
0.1
0.5
1.0
1.5
2.0
VSD, Source-to-Drain Voltage (V)
2.5
do
0.0
FIGURE 1.64
om
10
tc
ISD, Reverse Drain Current (A)
100
Typical source-drain (body) diode forward voltage characteristics.
fo
It is worth mentioning that the success of silicon devices lies partly in the low density of these interface
states which is due to the existence of native oxide in silicon as opposed to other semiconductors such
as GaAs where such a native oxide does not exist and oxide layers have to be deposited with several orders
of magnitude higher interface state densities.
kw
eb
in
Diode Forward Voltage (VF or VSD)
This is the guaranteed maximum forward drop of the body-drain diode at a specified value of source
current. Figure 1.64 shows a typical I–V characteristic for this diode at two temperatures. p-Channel
devices have higher values of VF due to the higher contact resistance between metal and p-silicon compared
with n-type silicon. Maximum values of 1.6 V for high-voltage devices (>100 V) and values of 1.0 V for
low-voltage devices (<100 V) are common.
Power Dissipation
The maximum allowable power dissipation which will raise the die temperature to the maximum allowable when the case temperature is held at 25В°C is an important parameter and is given by:
(1.8)
Pa
T jmax – 25
P d = пЈ« ---------------------пЈ­ R thJC пЈё
where Tjmax is the maximum allowable temperature of the pn junction in the device (normally 150 or
175В°C) and RthJC is the junction to cause thermal impedance of the device.
Dynamic Characteristics
Switching and Transient Response
When the MOSFET is used as a switch, its basic function is to control the drain current by the gate
voltage. Figure 1.65 shows the transfer characteristics and an equivalent circuit model often used for the
analysis of MOSFET switching performance. For a detailed discussion of this topic see Chapter 4 in Grant
and Gower (1989). The following is a summary of the important points.
The switching performance of a device is determined by the time required to establish voltage changes
across capacitances and current changes in inductances. RG is the distributed resistance of the gate and
2
is approximately inversely proportional to active area. Values of around 20 Ω-mm are common for the
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ID
tc
om
slope = gfs
VGS
do
(a)
D
LD
CGD
RG
G′
ID
CDS
Body-drain
diode
kw
eb
in
G
fo
D′
S′
CGS
LS
S
(b)
FIGURE 1.65 (a) Transfer characteristics and (b) an equivalent circuit diagram showing the MOSFET parasitic
components that have the greatest effect on switching speed.
Pa
product of RG and active area for polysilicon gates. LS and LD are source and drain lead inductances and
are around a few tens of nH. The physical origin of the capacitances CGS , CGD , and CDS were discussed
in the introduction of this chapter regarding the device schematic shown in Fig. 1.57. The typical values
of input (Ciss), output (Coss), and reverse transfer (Crss) capacitances given in the data sheets are used by
circuit designers as a starting point in determining circuit component values. The data sheet capacitances
are defined in terms of the equivalent circuit capacitances as:
C iss = C GS + C GD ,
C DS shorted
C rss = C GD
C oss = C DS + C GD
The gate-to-drain capacitance CGD is a nonlinear function of voltage and is the most important parameter
since it provides a feedback loop between the output and the input of the circuit. CGD is also called the
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RD
VDS
VGS
D.U.T.
RG
om
в€’
+ VDD
в€’10 V
10%
90%
FIGURE 1.66
tf
kw
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in
VDS
td(off)
do
VGS
tr
fo
td(on)
tc
Pulse Width <
_ 1Вµs
Duty Factor <
_ 0.1%
Switching time test circuit and resulting VGS and VDS waveforms.
Miller capacitance since it causes the total dynamic input capacitance to become greater than the sum
of the static capacitances.
Figure 1.66 shows a typical switching time test circuit. Also shown are the components of the rise and
fall times with reference to the VGS and VDS waveforms. Turn-on delay, td(on), is the time taken to charge
the input capacitance of the device before drain current conduction can start. Similarly, turn-off delay
td(off) is the time taken to discharge the capacitance after the gate is switched off.
Gate Charge
Pa
Although input capacitance values are useful, they do not provide accurate results when comparing the
switching performances of two devices from different manufacturers. Effects of device size and transconductance make such comparisons more difficult. A more useful parameter from the circuit design point
of view is the gate charge rather than capacitance. Most manufacturers include both parameters on their
data sheets. Figure 1.67 shows a typical gate charge waveform and the test circuit. When the gate is
connected to the supply voltage, VGS starts to increase until it reaches Vth, at which point the drain current
starts to flow and the CGS starts to charge. During the period t1 to t2, CGS continues to charge, the gate
voltage continues to rise and the drain current rises proportionally. At time t2, CGS is completely charged
and the drain current reaches the predetermined current ID and stays constant while the drain voltage
starts to fall. With reference to the equivalent circuit model of the MOSFET shown in Fig. 1.67, it can
be seen that with CGS fully charged at t2, VGS becomes constant and the drive current starts to charge the
Miller capacitance CGD . This continues until time t3. Note that the charge time for the Miller capacitance is
larger than that for the gate to source capacitance CGS, due to the rapidly changing drain voltage between
t2 ant t3 (current = C dV/dt). Once both of the capacitances CGS and CGD are fully charged, the gate voltage
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VDD
ID
D
om
D
CDG
G
ID
S
CGS
S
tc
TEST CIRCUIT
OGS
OGD
VG
'2
fo
GATE
VOLTAGE
VG(TH)
'0 '1
do
(a)
'3
kw
eb
in
DRAIN
VOLTAGE
'4
DRAIN CURRENT
VDD
ID
WAVEFORM
(b)
FIGURE 1.67
(a) Gate charge test circuit and (b) resulting gate and drain waveforms.
Pa
VGS starts increasing again until it reaches the supply voltage at time t4. The gate charge (QGS + QGD)
corresponding to time t3 is the bare minimum charge required to switch the device on. Good circuit
design practice dictates the use of a higher gate voltage than the bare minimum required for switching
and therefore the gate charge used in the calculations is QG corresponding to t4.
The advantage of using gate charge is that the designer can easily calculate the amount of current required
from the drive circuit to switch the device on in a desired length of time; since Q = CV and I = C dV/dt
then Q = time Г— current. For example, a device with a gate charge of 20 nC can be turned on in 20 Вµs if a
current of 1 mA is supplied to the gate or it can turn on in 20 ns if the gate current is increased to 1 A.
These simple calculations would not have been possible with input capacitance values.
dV/dt Capability
This is also called the peak diode recovery and is defined as the maximum rate of rise of drain-source
voltage allowed. If this rate is exceeded then the voltage across the gate-source terminals may become
higher than the threshold voltage of the device, forcing the device into the current conduction mode and
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DRAIN
I1
D
CGD
CDB
NPN
BIPOLAR
TRANSISTOR
I2
om
G
RB
RG
CGS
tc
S
SOURCE
APPLIED
RAMP
VOLTAGE
do
FIGURE 1.68 Equivalent circuit model of a power MOSFET showing the two possible mechanisms for dV/dtinduced turn-on. (From Baliga, B. J., Modern Power Devices, В© 1987 John Wiley & Sons, Inc., New York. Reprinted
by permission of John Wiley & Sons, Inc.)
fo
under certain conditions a catastrophic failure may occur. There are two possible mechanisms by which a
dV/dt induced turn-on may take place. Figure 1.68 shows the equivalent circuit model of a power MOSFET,
including the parasitic BJT. The first mechanism of dv/dt induced turn-on becomes active through the
feedback action of the gate-drain capacitance CGD . When a voltage ramp appears across the drain and
source terminals of the device, a current I1 flows through the gate resistance RG by means of the gatedrain capacitance CGD . RG is the total gate resistance in the circuit and the voltage drop across it is given by:
kw
eb
in
V GS = I 1 R G
dV
= R G C GD пЈ« --------пЈ¶
пЈ­ dt пЈё
(1.9)
When the gate voltage VGS exceeds the threshold voltage of the device Vth, the device is forced into
conduction. The dV/dt capability for this mechanism is thus set by:
V th
пЈ« dV
--------пЈ¶ = --------------пЈ­ dt пЈё
R G C GD
(1.10)
Pa
It is clear that low Vth devices are more prone to dV/dt turn-on. The negative temperature coefficient of
Vth is of special importance in applications where high-temperature environments are present. Also, gate
circuit impedance has to be chosen carefully in order to avoid this effect. CGD is an internal device
parameter and is determined by the overlap area between poly gate and silicon and gate oxide thickness.
Higher gate oxide thicknesses reduce CGD and also increase Vth, both advantageous to dV/dt rating, as
long as the higher Vth is acceptable in the application.
The second mechanism for the dV/dt turn-on in MOSFETs is through the parasitic BJT as shown in
Fig. 1.69. The capacitance associated with the depletion region of the body diode extending into the drift
region is denoted as CDB and appears between the base of the BJT and the drain of the MOSFET. This
capacitance gives rise to a current I2 which flows through the base resistance RB when a voltage ramp
appears across the drain-source terminals. With analogy to the first mechanism, the dV/dt capability of
this mechanism is given by:
V BE
пЈ« dV
--------пЈ¶ = -------------пЈ­ dt пЈё
R B C DB
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(1.11)
SOURCE
GATE
N+
A
LN+
CDB
tc
DRAIN
om
RS
do
FIGURE 1.69 Physical origin of the parasitic BJT components that may cause dV/dt-induced turn-on in power
MOSFET. (From Baliga, B. J., Modern Power Devices, В© 1987 John Wiley & Sons, Inc., New York. Reprinted by
permission of John Wiley & Sons, Inc.)
Applications
kw
eb
in
fo
If the voltage that develops across RB is greater than about 0.7 V, then the base–emitter junction is forwardbiased and the parasitic BJT is turned on. Under the conditions of high dV/dt and large values of RB, the breakdown voltage of the MOSFET will be limited to that of the open-base breakdown voltage of the BJT. If
the applied drain voltage is greater than the open-base breakdown voltage, then the MOSFET will enter
avalanche and may be destroyed if the current is not limited externally.
Increasing dV/dt capability therefore requires reducing the base resistance RB by increasing the body
region doping and reducing the distance the current I2 has to flow laterally before it is collected by the
source metallization. As in the first mode, the BJT related dV/dt capability becomes worse at higher
temperatures since RB increases and VBE decreases with increasing temperature.
The following are two of the major markets where power MOSFETs are finding increasing applications
as either logic-controlled or analog switches.
Portable Electronics and Wireless Communication
Automotive
Pa
With the recent advances in the portable electronic products, low Rdson, logic level surface mount power
MOSFET are experiencing explosive demand. A portable computer, for example, uses power MOSFETs
in the AC-DC converters, the DC-DC converters and voltage regulators, load management switches,
battery charger circuitry, and reverse battery protection. Required features of MOSFETs in these applications are small size, low power dissipation, and low on-resistance for extended battery life. Reduction
of both conduction and switching losses are important considerations in the design of MOSFETs aimed
at this market.
Mechanical contact breakers have mostly been replaced by semiconductor devices in ignition circuits in
modern cars. A suitable semiconductor device must be capable of blocking high voltages in a severe
environment where line voltage surges are common due to the opening and closing of switches and the
connection and disconnection of inductive loads during maintenance and loose connections. Bipolar
transistors with their susceptibility to secondary breakdown are not suited whereas power MOSFETs with
avalanche capability are ideally suited. Voltage transients are clamped by the avalanching of the MOSFET
without the need to use any external protection circuits.
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om
In 12-V battery vehicles the most commonly used MOSFETs are rated at 50 or 60 V breakdown voltages. The significant guard-banding is necessary in order to avoid device failure due to the alternator
producing high voltages after shedding a heavy load.
The other features of power MOSFETs which make them suitable for the automotive applications are high
dV/dt ratings, high-temperature performance, ruggedness and high reliability. Logic level, surface mount
devices with low Rdson have recently found application in this field. The smaller footprint of surface mounts
offers space savings and the lower Rdson does away with the need to parallel devices to reduce on-resistance.
This in turn translates into fewer device counts and heat-sinks which lowers the overall cost.
In addition to ignition control, power MOSFETs are used in anti-lock brake (ABS) systems, electronic
power steering (EPS) systems, air bags, electronic suspension, and numerous motor control applications
such as power windows, power seats, radiator fan, wipers, fuel pump, etc.
tc
References
do
Baliga, B.J. 1987. Modern Power Devices, John Wiley & Sons, New York.
Grant, D.A. and Gower, I. 1989. Power MOSFETs—Theory and Applications, John Wiley & Sons, New York.
International Rectifier, 1995, HEXFET Power MOSFET Designer’s Manual—Application Notes and Reliability Data, International Rectifier, El Segundo, CA.
Oxner, E.S. 1982. Power FETs and Their Applications, Prentice-Hall, Englewood Cliffs, NJ.
Sze, S.M. 1981. Physics of Semiconductor Devices, John Wiley & Sons, New York.
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1.7 General Power Semiconductor Switch Requirements
Alex Q. Huang
Pa
kw
eb
in
A power semiconductor switch is a component that can either conduct a current when it is commanded
ON or block a voltage when it is commanded OFF through a control. This change of conductivity is
made possible in a semiconductor by specially arranged device structures that control the carrier transportation. The time that it takes to change the conductivity is also reduced to the microsecond level
compared with the millisecond level of a mechanical switch. By employing this kind of switch, a properly
designed electrical system can control the flow of electric energy, shaping the electricity into desired
forms.
Parameters describing the performance of a power conversion system include reliability, efficiency,
size, and cost. The power switch plays an important role in determining these system-level performances
[1]. To facilitate the analysis, a simple buck converter shown in Fig. 1.70a (buck converter) and 1.70b
(its switching waveforms) is used as an example. There are two switches SW and DF in the circuit. The
purpose of this circuit is to deliver energy from a power source with a higher voltage VCC to the load
with a lower voltage VO requirement. When the power switch SW is on, the energy is delivered from the
source VCC through switch SW, inductor L to the load. When the output voltage is high enough, this
energy link will be shut down by turning off SW. Energies stored in L and CO will maintain the load
voltage. The typical circuit waveforms are depicted in Figs. 1.70a and b its switching waveforms. The
circuit has four different operating modes: (1) (t0–t1) SW off and DF on; (2) (t1–t3) SW turn-on and DF
turn-off; (3) (t3–t4) SW on and DF off; (4) (t4–t6) SW turn-off and DF turn-on.
Generally, the following parameters are important for a semiconductor switch designed for power
conversion applications:
1.
2.
3.
4.
5.
Maximum current carrying capability
Maximum voltage blocking capability
Forward voltage drop during ON and its temperature dependency
Leakage current during OFF
Thermal capability
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L
SW
+ VSW в€’
V0
ISW
Vcc
IL
+
VD
CO
DF
RL
om
в€’
ID
(a)
tc
Control
do
IL
IL
I SW
fo
V CC
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in
VSW
V CC
VD
IL
ID
t0
t1
t2
t3
Switch turn-on
Diode turn-off
t4
t5
t6
Switch turn-off
Diode turn-on
(b)
6.
7.
8.
9.
10.
11.
12.
(a) Buck converter and (b) its switching waveforms.
Pa
FIGURE 1.70
Switching transition times during both turn-on and turn-off
Capability to stand dV/dt when the switch is OFF or during turn-off
Capability to stand dI/dt when the switch is ON or during turn-on
Controllable dI/dt or dV/dt capability during switching transition
Ability to withstand both high current and voltage simultaneously
Switching losses
Control power requirement and control circuit complexity
The above items can be further divided into three categories: static, dynamic, and control parameters.
Items 1 to 5 relate to the static performance of a switch. Both current and voltage ratings describe the
power handling capability of a switch. For a certain application, devices with higher current and voltage
ratings are more robust to transient overcurrent and voltage due to switching transitions or circuit faults,
increasing the system level reliability. For the buck converter, the nominal current of SW when it is on
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Pa
kw
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in
fo
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is equal to the current of the output inductor. However, SW will experience higher peak current during
the turn-on period between t2 and t3 due to diode DF reverse recovery. When the load RL is shorted or
DF is fail shorted, SW will observe a much higher fault current.
Lower forward voltage drop and leakage current lead to a lower power loss, which is good from the
energy efficiency and the thermal management point of view. Between t0 and t1, SW is on and its power
dissipation is (ILVF), where VF is the forward voltage drop of SW. Between t3 and t4, SW is off and its power
dissipation is (VCCILKG), where ILKG is the leakage current of SW. Good thermal capability, which refers to
the thermal resistance from the device to ambient and the maximum temperature the device can withstand,
allows the device to operate at its full power rating instead of being limited by the thermal management.
Items 6 to 11 are related to the dynamic performance of a switch. Short transition times are required
to increase the switching frequency and reduce the switching loss. The latter is caused by the overlap of
current and voltage on the switch. For the buck converter, the turn-on transition time of SW is (t3 в€’ t1)
and the turn-off transition time is (t6 в€’ t4). The current/voltage of the switch overlaps; hence, its switching
losses are approximately proportional to the switching times. Item 7 describes the external dV/dt immunity of the device. In a system, the switch is generally exposed to a complex electromagnetic environment.
However, the state and the operation of the switch should only be controlled by its control command
instead of the environment. When the switch is in the OFF state or during turn-off operation, the switch
should stay OFF or continue its turn-off process no matter what the external dV/dt across its anode and
cathode (or collector/emitter) is. Similarly, there is a dI/dt requirement when the switch is ON or during
the turn-on transition. Devices with a large cell size such as the gate turn-off (GTO) thyristor have lower
dI/dt limitations because of the longer time required for uniform current distribution.
While a good switch should be able to withstand severe dynamic voltage and current changes, it should
also be able to provide the system with an acceptable electromagnetic noise. This requires the controllable
dI/dt and dV/dt capabilities from the switch [2]. A typical turn-on operation of a switch in a power
conversion system is associated with a turn-off process of another switch (or diode). The dI/dt is generally
determined by the turn-on switch and shared by the turn-off switch, which may not be able to withstand
the high dI/dt. For example, a diode has a turn-off problem and high turn-off dI/dt may overstress it.
In the buck converter, the turn-off of the diode DF is accompanied with the turn-on of SW starting
from t1. The falling dI/dt of the DF of the is equal to that of rising dI/dt of the SW. After t2, DF enters its
reverse recovery process, experiencing its highest instant power before its current finally goes to zero.
To protect these associated devices effectively, the maximum turn-on dI/dt should be limited. Similarly,
a typical turn-off operation of a switch in a power conversion circuit is associated with a turn-on process
of another switch (or diode). The dV/dt is generally determined by the turn-off switch and shared by the
turn-on switch, which may not be able to withstand the high dV/dt. The maximum dV/dt of the active
switch should be limited to protect the associated switches. Both dV/dt and dI/dt controls normally require
a device to possess a forward-biased safe operation area (FBSOA) [3]. The FBSOA defines a maximum Vв€’
I region in which the device can be commanded to operate with simultaneous high voltage and current.
The device current can be controlled through its gate (or base) and the length of the operation is only
limited by its thermal limitation. Devices with FBSOA normally have an active region in which the device
current is determined by the control signal level, as is shown in Fig. 1.71. It should be noted, however,
that dI/dt control in practice means slowing down the transient process and increasing the turn-on loss.
During a typical inductive turn-off process, the voltage of a switch will rise and its current will decrease.
During the transition, the device observes both high voltage and high current simultaneously. Figure 1.72
depicts the typical voltage–current trajectory of an inductive turn-off process as is the case in the buck
circuit shown in Figs. 1.70a and b, between t4 and t6 in time domain. The current of the device stays
constant while its voltage rises. Its current begins to decrease once its voltage reaches its nominal value.
The voltage spike is caused by the dI/dt and stray inductance in the current commutation loop. On the
I–V plane of the device, the curve that defines the maximum voltage and current boundary within which
the device can turn off safely, is referred to as the reverse-biased safe operation area (RBSOA) [4] of the
device. Obviously, the RBSOA of a device should be larger than all its possible turn-off I–V trajectories.
Devices without a large enough RBSOA need an external circuit (such as an auxiliary soft-switching circuit
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I
IG4 (or VG4 ) or beyond
IG2 (or VG2 )
tc
IG1 (or V G1 )
om
IG3 (or V G3 )
V
do
FIGURE 1.71 Forward I–V characteristic of a device and its FBSOA (shaded area) definition. The control of the
device may be current or voltage.
without snubber
kw
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in
INOM
fo
I
with snubber
FIGURE 1.72
off snubber.
VNOM
V
Turn-off I–V trajectories of a device under typical inductive load condition with and without a turn-
Pa
or a dV/dt snubber) to shape their turn-off I–V trajectories to a smaller one to ensure safe turn-off
operation. Devices with turn-off snubbers can therefore survive with a much smaller RBSOA. However,
a dV/dt snubber increases the component count of the system, hence the system’s size and cost. The turnoff operation conducted without the help of a snubber is called snubberless turn-off or hard turn-off,
whereas a process with the help of a snubber is called snubbered turn-off.
During the turn-on transition, a switch will also observe both high voltage and high current simultaneously. Figure 1.73 depicts the typical voltage–current trajectory of an inductive turn-on process as
is the case in the buck circuit shown in Fig. 1.70 between t1 and t3 in time domain. The voltage of the
device stays constant while its current increases until it hits the nominal current level of the device. The
current overshoot is due to the reverse recovery of an associated diode (or a switch). A device without
a large enough FBSOA needs an external snubber circuit to help its I–V trajectory, as is shown in Fig.
1.73. The stress on the device can be significantly reduced with the turn-on snubber. However, a turnon snubber circuit also increases the component count, size, and cost of a system.
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I
without snubber
om
INOM
tc
with snubber
VNOM
I
do
Turn-on I–V trajectories of a device under a typical inductive load condition.
device without current limiting capability
ILIM
fo
FIGURE 1.73
V
INOM
FIGURE 1.74
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in
device with current limiting capability
V
Forward I–V characteristics of two types of devices with and without self-current limiting capability.
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Item 10 defines the capability of a switch to withstand high instant power. However, this capability
during turn-on and turn-off will be different for a semiconductor device because of the difference in
free carrier distribution. RBSOA is mostly used to describe the turn-off capability of a device, while
FBSOA is used to measure its turn-on capability. FBSOA, as implied by its name, is also used to measure
the ability of a device to withstand high voltage and high current under DC and short-circuit conditions.
A load short circuit is a threat to the device that is ON or is turning on in a typical circuit. A temporary
load short can introduce an extremely high current that generates high instant power dissipation, leading
to the failure of the switch. To effectively protect the switch under a short-circuit condition, the ability
to limit its maximum current at a given DC voltage is required. In this case, the peak instant power is
2
(VCCILIM), whereas for a device without this ability it is (V CC /r), where VCC is the DC voltage, ILIM is the
maximum current limitation of the device, and r is the effective resistance of a device while it is ON.
Since r is normally low in a practical device, the instant power of a device under a load short circuit
without the maximum current limitation is much higher. Figure 1.74 shows the Iв€’V characteristics during
the ON state for devices with or without the self-current limitation capability.
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The ability of a switch to limit its maximum current regardless of the voltage applied is an effective
method to limit its instant power. A device with FBSOA capability normally has self-current limiting
capability and, hence, can survive a short-circuit fault for a short time as determined by its thermal
limitation [5].
References
1.8 Gate Turn-Off Thyristors
Alex Q. Huang
do
tc
om
1. M. Nishihara, Power electronics diversity, presented at IPEC ’90, 1990, 21–28.
2. R. Chokhawala et al., Gate drive considerations for IGBT modules, presented at IAS ’92, 1992,
1186–1195.
3. B.J. Baliga, Trends in power semiconductor devices, Electron Devices IEEE Trans., 43(10) 1717–1731,
1996.
4. D.Y. Chen, G. Carpenter, and F.C. Lee, RBSOA characterization of GTO devices, in PESC ’93 Record,
24th Annual IEEE 1993, 489–495.
5. H.C. Eckel et al., Optimization of the turn-off performance of IGBT at over current and short circuit
current, in 5th European Conference on Power Electronics and Application Rec., 1993, 317–322.
Pa
kw
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in
fo
The first power semiconductor switch that was put in use was the silicon controllable rectifier (SCR) [1]
invented in 1950s. The SCR is a latch-up device with only two stable states: ON and OFF. It does not
have FBSOA. It can be switched from OFF to ON by issuing a command in the form of a small gatetriggering current. This will initiate a positive-feedback process that will eventually turn the device on.
The SCR has a good trade-off between its forward voltage drop and blocking voltage because of the strong
conductivity modulation provided by the injections of both electrons and holes. Moreover, the structure
of an SCR is very simple from a manufacturing point of view because its gate can be placed at one small
region. The size of a single SCR can therefore be easily expanded to increase the current capability of the
device without too many processing problems. There are 8.0 kA/10.0 kV SCRs commercially available
that use a 6-in. silicon wafer for current conduction. However, SCRs cannot be turned off through their
gate controls.
Because of the limitation of the turn-off controllability of the SCR, the gate turn-off (GTO) thyristor
[2] was subsequently developed. As its name denotes, a GTO is a device that can be turned off through
its gate control. Its basic structure is very similar to that of an SCR. However, many gate fingers are placed
in the GTO surrounding its cathode. During a turn-off operation, the latch-up mechanism can be broken
through the gate control. A GTO is thus a device with full gate control and similar high current–voltage
rating of an SCR. To date, the GTO has the highest power rating and the best trade-off between the
blocking voltage and the conduction loss of any fully controllable switch. However, the dynamic performance of GTOs is poor. A GTO is slow in both turn-on and turn-off. It lacks FBSOA and has poor
RBSOA so it requires snubbers to control dV/dt during the turn-off transition and dI/dt during turn-on
transition.
The GTO thyristor was one of the very first power semiconductor switches with full gate control. It
has served many power applications ranging from low power (below 100 W) in its early years to high
power up to hundreds of megawatts. A state-of-the-art GTO can be fabricated on a silicon wafer as big
as 6 in. and can be rated up to 6.0 kA and 6.0 kV [3]. This rating is much higher than the ratings of any
other fully controllable devices.
The GTO static parameters are excellent: low conduction loss due to its double-sided minority carrier
injection, high blocking voltage, and low cost due to its fabrication on a large single wafer. However, its
dynamic performance is poor. The requirements of a dV/dt snubber during turn-off operation, a dI/dt
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om
snubber during turn-on operation, and minimum on and off times make the GTO difficult to use. To
improve the dynamic performance of the GTO while keeping its good static performance, a better
understanding of the mechanism of the GTO is necessary. In this section, the basic operating principle
of the GTO, its advantages and disadvantages, and the mechanism that determines its performance are
summarized and discussed. A new gate-driving concept, namely, unity-gain turn-off, is then introduced.
The advantages of this special driving method are analyzed and discussed. Finally, all known approaches
that make use of this special driving technique are summarized.
GTO Forward Conduction
do
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Figure 1.75a illustrates the cell structure and the doping profile of a typical high power GTO. Figure 1.75b
shows the two-transistor GTO model; and Fig. 1.75c is a photograph of a 4-in. GTO along with its gate
в€’
lead. The structure is a three-terminal, four-layer pnpn structure with a lightly doped n voltage-blocking
+
layer in the center [4]. The electrode on the external p layer is called the anode where the current
+
normally flows into the device. The electrode on the external n layer is called the cathode from where
the current normally flows out. The electrode on the internal p layer (p-base) is called the gate, which
is used for control.
The operating principle of a GTO can be understood through its equivalent circuit model shown in
Fig. 1.75b. The pnp transistor represents the top three layers of the GTO, whereas the npn transistor
J1
(a)
N+ IK
Gate
J2
N+
P
IG
N-
N-
P
kw
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IA
P+
P+
Doping Profile
fo
Anode
J3
Cathode
(b)
Pa
anode
gate
cathode
(c)
FIGURE 1.75 (a) GTO cell structure and its doping profile; (b) The two-transistor GTO model; (c) a photograph
of a 4-in. GTO along with its gate lead.
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FIGURE 1.77
Current flow in a GTO with gate drive current.
tc
Turn-on and current-sustaining process in a GTO.
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FIGURE 1.76
в€’
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represents the bottom three layers of the GTO. Since the n layer serves as the base of the pnp and the
collector of the npn, and the internal p layer serves as the base of the npn and the collector of the pnp,
the two transistors are cross-coupled. This structure has two stable states: ON and OFF, which are
determined by its gate control. When a current is injected into the GTO from its gate to its cathode, the
npn structure is turned on and its collector current flows from the anode of the GTO through J1 junction.
Since J1 is the emitter junction of the pnp structure, the collector current of the pnp is then the base
current of the npn. The two transistors therefore provide base currents to each other, forming a positive
feedback among them until they reach a self-sustaining state commonly known as latch-up or latched.
Under the latched condition, high-level minority carrier injections are available from the anode to the
cathode, with all three pn junctions forward-biased. A high conductivity therefore exists from anode to
cathode, allowing high current to flow from the anode to the cathode. Figure 1.76 illustrates this turnon process.
At the silicon level, the turn-on of junction J3 results in the injection of electrons into the p-base region.
These electrons diffuse across the p-base and are mostly collected by the reverse biased junction J2. To
в€’
maintain the continuity of the current, junction J1 will supply a current by injecting holes into the n
в€’
region. Part of these holes will diffuse across the n region and are collected by junction J2, resulting in
more electron injection from junction J3. When both transistors operate at sufficient current gain, a
positive feedback mechanism is sufficient to result in latch-up.
Let the common base current gain of the pnp and npn be О±pnp and О±npn, respectively. Normally, О±pnpP
is lower than О±npn since the pnp is a wide-base structure. The current flow inside a GTO is illustrated in
Fig. 1.77. At junction J2, the current due to cathode side injection is О±npnIK; the current due to anode side
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injection is αpnpIA; and the leakage current is IL. According to Kirchhoff ’s law,
I A = О± pnp I A + О± npn I K + I L
(1.12)
IA = IK – IG
(1.13)
Combining these equations,
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and
tc
I A = ( α pnp I G + I L )/ (1 – α pnp – α npn )
(1.14)
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fo
do
This equation shows that the thyristor structure can sustain its anode current by itself once the sum
of the common base current gain (О±pnp + О±npn) of both transistors is approaching unity. For a GTO, О±npn
is designed low and is normally depending on IG to ensure its gate turn-off capability. This will be
discussed later. With this self-sustaining capability, the gate of a GTO does not need to supply a lot of
current and does not need to be very close to its cathode as is necessary in a bipolar junction transistor
(BJT) design. The dimension of a typical GTO cell shown in Fig. 1.75 is 100 to 150 Вµm wide. This is
very large compared with the micron and/or even submicron process used for modern MOSFETs and
insulated gate bipolar transistors (IGBTs). The large cell size design is cost-effective and makes it possible
to fabricate large single-die devices to boost their current capability. A state-of-the-art GTO die is as large
as 6-in. in diameter with a turn-off current capability of up to 6.0 kA [3]. Figure 1.75c shows a large
GTO fabricated by ABB. The GTO shown is fabricated on a 4-in. silicon wafer consisting of thousands
of cells like the one shown in Fig. 1.75 and packaged in a so-called press-pack or hockey-puck package.
The large cell structure in the GTO introduces a current spreading problem during the turn-on
transition of a GTO. When a gate current is injected, the turn-on occurs first in the vicinity of the gate
contact. The conduction area then spreads across the rest of the cathode area. This can be characterized
by a propagation velocity called the spreading velocity [5]. Experimental measurements [6] have shown
a typical spreading velocity of 5000 cm/s. This velocity also depends on the GTO design parameters, the
gate turn-on injection current, and its dIG/dt.
Because of this spreading velocity, it takes time for the whole GTO cell to turn on. To avoid overstressing
the part of the cell that is turned on first, the increasing rate of the anode current should be limited. This
sets the maximum turn-on dI/dt limitation for a GTO.
The major advantages of the GTO are its low forward voltage drop and high-voltage blocking capability.
These can be understood as the major benefits of its double-side minority carrier injection mechanism.
For high-voltage GTO, a thick and lightly doped n-base is needed (see Fig. 1.75). The forward voltage
drop in this case is mainly determined by the resistive voltage drop in the voltage-blocking region where
minority carriers play an important role.
в€’
Figure 1.78a shows the minority carrier distribution in the n region of a GTO and Fig. 1.78b shows
в€’
the case of an IGBT (see Section 1.9). For the same blocking voltage design, their n regions should have
similar thickness and doping. Since there is only one transistor in the IGBT structure, minority carriers
в€’
can only be injected from one side; therefore, the conductivity modulation in the n region is weaker
than that of the GTO. In the GTO, since there are two transistors, minority carriers can be injected from
both ends, making a more uniform plasma distribution in the whole area. For a 4.5-kV state-of-the-art
2
GTO, its forward voltage drop at a current density of 50 A/cm can be as low as 2.0 V [7] if a constant
gate current injection presents. Figure 1.79 shows the on-state characteristics of a state-of-the-art GTO
manufactured by ABB [7]. The forward voltage drop at 2000 A is only about 1.5 V for this 4.5-kV GTO.
This result is typical of a low conduction loss GTO.
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anode
anode
gate
om
gate
cathode
cathode
(b)
tc
(a)
gate
fo
p
n-
p
+
emitter
cathode
n+
current
current
On-state minority carrier distribution in the voltage blocking region for (a) GTO and (b) IGBT.
kw
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in
FIGURE 1.78
p
+
collector
Plasma level
anode
gate
p
do
n+
GTO Turn-Off and Forward Blocking
Since
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If the GTO gate is pulling out current from the GTO, the current injection into the npn base will be
reduced. Once this is reduced below a certain level, the collector current of the npn, and hence the base
current of the pnp, will also decrease, leading to the reduced pnp collector current. This will further
reduce the base current of the npn since it is the difference between the collector current of the pnp and
the gate pullout current. This positive-feedback process will eventually turn off the GTO.
Figure 1.80 shows the current flow inside the GTO when its gate is pulling out current to turn off the
device. The base drive current required to maintain current conduction in the npn transistor is (1 в€’ О±npn)IK.
The base drive current available to the npn transistor in this case is (О±pnpIA в€’ IG). Thus, the condition to turn
off the GTO through the gate control is given by:
α pnp I A – I G < (1 – α npn )I K
(1.15)
IK = IA – IG
(1.16)
(α pnp + α npn – 1)
- IA
I G > -------------------------------------О± npn
(1.17)
the condition to turn off the GTO is
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tc
do
fo
On-state characteristics of 5SGT 40L4502, a 4-kA, 4.5-kV GTO from ABB.
IG
Gate
N
IK
IL
P
О±NPNIK
IA
N
P
Anode
О±PNP IA
Cathode
J3
J2
J1
Current flow inside the GTO when its gate is pulling out current.
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FIGURE 1.80
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in
FIGURE 1.79
The ratio of the anode current to the gate current at which level a GTO is turned off is defined as the
turn-off gain. From Eq. (1.17), the maximum turn-off gain [4] can be expressed as:
I
О± npn
β m ≡ ---A- = --------------------------------I G α pnp + α npn – 1
(1.18)
A large turn-off gain is normally desirable to reduce the current requirements of the gate driver. A
lower (О±pnp + О±npn) value is necessary to ensure a reasonable turn-off gain. It is also important to point
out that О±npn in Eq. 1.18 is not a constant; normally it decreases when gate current IG increases.
When a GTO is OFF, its junction J2 is reverse-biased and can support a high voltage applied between
its anode and cathode, as shown in Fig. 1.81a. If the junction J3 is reverse-biased or shorted by the gate
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Gate
Em
E-field
N
P
VAK
om
N
Anode
P
Cathode
J3
J2
∆x
J1
- +
tc
(a)
N
Em
do
Gate
P
N
P
E-field
Anode
J3
J2
fo
Cathode
J1
VAK
kw
eb
in
+ -
∆x
(b)
FIGURE 1.81
Electric field profile when a GTO is blocking forward (a) and reverse voltage (b).
driver, the maximum forward blocking voltage BVAK of the GTO is determined by the avalanche breakdown
capability of the pnp transistor under the open-base condition [8]. This voltage can be expressed as:
BV AK = (1 – α pnp ) BV j2
Pa
1/n
(1.19)
where О±pnp is the common base current gain of the pnp structure at low current levels; n is an empirical
constant, and BVJ2 is the avalanche breakdown voltage of the pn-junction J2. Since this pnp has a wide
base structure, its common base current gain О±pnp is low compared with a normal bipolar transistor.
Thus, the forward voltage blocking capability BVAK of a GTO is very close to the breakdown voltage
of junction J2.
A GTO can also block a reverse voltage by its junction J1, as shown in Fig. 1.81b. When the junction
J3 is gated off, the reverse voltage blocking capability is similarly determined by the avalanche breakdown
of the pnp structure under the open-base condition. A GTO with both forward and reverse blocking
capability is called symmetric blocking GTO. Most GTOs manufactured today, however, are asymmetric
GTOs because the reverse blocking capability is not utilized (J1 junction not designed to support high
reverse voltage) or cannot be utilized because of other design requirements, such as the need to introduce
anode-short at junction J1 to speed the turn-off.
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Practical GTO Turn-Off Operation
RI
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The turn-off capability of a GTO is limited dominantly by a non-uniform current distribution (also called
current filamentation) problem during turn-off transient. This causes current to concentrate to a few GTO
cells and destroy the device with the high power stress. Furthermore, the current filamentation is believed
to be initiated by the dynamic avalanche (see next section) in an inhomogeneous, large-area GTO.
A GTO normally requires a dV/dt snubber circuit to conduct actual turn-off operation under high
voltage and high current condition. This is because a large current GTO turn-off will fail without such
a dV/dt snubber as a result of its small RBSOA. This small RBSOA is caused by a non-uniform current
distribution or current filamentation problem in the GTO.
Figure 1.82 shows a practical setup in which a typical dV/dt snubber formed by DS, RS, and CS is used,
and Fig. 1.83 shows a typical GTO turn-off characteristic under snubbered condition. Before t0, the GTO is
ON, so a current is built up in the load inductor LL and the device under test (DUT). The anode current IA
DI
di/dt snubber
DF
VAK
LI
do
dv/dt snubber
LL
IA
Ds
Rs
Lstray
IG
Cs
V OFF +
kw
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in
VDC
fo
DUT
(a)
Pa
SW
VOFF
IA
P
GTO
N
LG
P
>300nH
N
IG
IK
18V
(b)
FIGURE 1.82 (a) The turn-off circuit of a GTO with a typical RCD dV/dt snubber. (b) Typical GTO gate drive
circuit with a large gate inductance LG.
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tc
FIGURE 1.83
Typical GTO turn-off characteristics under dV/dt snubber condition.
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do
is approximately equal to the cathode current IK because the gate current IG is negligible. Starting from
time t0, a negative voltage VOFF is applied to the gate of the GTO. The gate current IG then decreases
linearly at a rate determined by the applied negative turn-off gate voltage VOFF and the gate lead stray
inductance LG. At t1, the device could not maintain the latch any longer so the anode current begins to
decay. The current from the load inductor is diverted to the dV/dt snubber path. At t2, when the anode
current observes its maximum dI/dt, the anode voltage shows a spike due to the stray inductance LS in
the dV/dt snubber path. At t3, the anode current enters its tail stage. At t4, the anode voltage reaches the DC
link voltage so the freewheeling diode DF will be conducting. The energy in the stray inductance in the
loop of power supply, freewheeling diode, and the dV/dt snubber is released to the snubber capacitor,
causing another voltage peak. The anode voltage dip between t4 and t5 is due to the reverse recovery of
the dV/dt snubber diode DS. The turn-off trajectory of a GTO with a dV/dt snubber is significantly
reduced, as shown in Fig. 1.72 (see Section 1.7 on General Power Semiconductor Switch Requirements).
Dynamic Avalanche
Pa
Under a high electric field, an avalanche process occurs within the silicon. The static critical electric field
is a function of the doping profile. The lower the doping, the lower the critical avalanche electric field.
The static avalanche voltage of a single side abrupt pn-junction is determined by both the critical electric
field and the depletion region width.
While the reverse-biased junction conducts high current, as is the case of a GTO turn-off with or
without a dV/dt snubber, the avalanche voltage decreases significantly because of the existence of carriers
in the depletion region. This process is called dynamic avalanche [9]. Figure 1.84 shows the cross section
of a pnp transistor under both current and voltage stress. A GTO turn-off with a dV/dt snubber enters
the pnp conduction mode between t2 and t3, as shown in Fig. 1.82. Assuming carriers in the depletion
region are moving at their saturation speed, then both the anode current density and the anode–cathode
voltage can be expressed as:
J A = qpv s
(1.20)
V AK = E m W E /2
(1.21)
≈ ( ε s E C )/2qp = ( ε s E C )/(J A /v s )
2
2
(1.22)
where p is hole density in the depletion region, EC is the critical electric field causing avalanche breakdown,
and vS is the saturation velocity of holes. In the depletion region, holes are the only carriers. In the presence
of holes in the depletion region, the charge density in the depletion region is higher compared with the
case without the current, so the peak electric field is also higher at the same width of the depletion region.
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om
Dynamic avalanche in the blocking junction of a pnp structure.
tc
FIGURE 1.84
At the point when dynamic avalanche happens, the power density of the device, which is the product
of both the current and the voltage applied on the device, can thus be expressed as:
J A V AK = Оµ s v s E C /2
2
do
2
(1.23)
kw
eb
in
fo
which is about 200 to 300 kW/cm for silicon.
The onset of dynamic avalanche itself is not a stable condition because the generated carrier is not
sufficient to maintain the current. Hence it is not a failure condition from the point of view of the physics
of the device. The dynamic avalanche is, however, widely regarded as the failure mechanism of the GTO
because it will initiate a non-uniform current distribution among large-wafer-size GTOs. The current
crowding or current filament formed after the onset of dynamic avalanche is enough to destroy the device
at one location in the form of a melted spot [10].
Non-Uniform Turn-Off Process among GTO Cells
For a high-power GTO, the experimentally obtained instant turn-off power it can withstand is far below
the value set by the dynamic avalanche breakdown shown in Eq. (1.21). So a GTO needs help from a
dV/dt snubber to shape its turn-off I–V trajectory, as is shown in Fig. 1.72, and to lower the maximum
average instant power the external circuit can apply. Non-uniform current distribution or current filament
[10] among GTO cells during the turn-off operation accounts for this limitation. The current filament can
be formed at the beginning of the turn-off due to differences in storage times or caused by the onset of
the dynamic avalanche during the turn-off when the voltage and current are both high [11].
Current Filamentation Caused by Storage Time Difference
Pa
The non-uniform turn-off process can be understood by considering two GTO cells in parallel, as is
shown in Figs. 1.85 and 1.87. The two cells are identical except for their storage time. This storage time
difference is considered unavoidable in high-current GTOs because of differences in carrier lifetime,
wafer thickness, and doping. Although only two cells are shown, GTO1 can represent a group of slower
cells whereas GTO2 represents a group of faster cells. The turn-off process starts from t0. Since it has a
shorter storage time, GTO2 turns off earlier at t1. The current originally shared by GTO2 is now
transferred to GTO1. At t2, GTO1 is turned off at twice its previous current. Turn-off failure can happen
if its current at t2 exceeds the maximum turn-off capability of GTO1. This can easily be the case when
the number of faster cells is much larger than the number of slower cells. This type of failure typically
occurs at the very beginning of the GTO turn-off before voltage rises and is caused by a rapid formation
of current filament due to storage time difference.
What makes this type of failure likely is that there is also a positive feedback mechanism that will further
increase the storage time difference, as shown in Fig. 1.86. At higher current density, the common-base
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om
tc
do
fo
Current crowding among two GTO cells as a result of their storage time difference.
kw
eb
in
FIGURE 1.85
ts1>ts2
Ia2
Ia1
emitter injection
Ic1
to GTO1
Positive feedback
ts1
Store more
charges in GTO1
FIGURE 1.86 Positive feedback mechanism enhances the storage time difference and pushes the current filament
into the slowest cell.
Pa
current gains of both transistors in GTO1 increase. Thus, its turn-off gain becomes even lower according
to Eq. (1.16), requiring more gate current for turn-off, hence increasing its storage time. Considering
that the typical storage time of a high-current GTO is in the range of 20 Вµs, there is enough time for the
dangerous current filament to form.
Current Filamentation Caused by Onset of Dynamic Avalanche
Even if the above-discussed failure did not occur because GTO1 is beginning to turn off before the current
filament density is too high, another failure mechanism can exist. At t2, where both current and voltage
are high, GTO1 is subject to much higher instant power stress than that of GTO2. The dynamic avalanche
could then occur first at GTO1 and initiate another positive feedback that will further increase the localized
current density (hence the name current filament) and enable the relatch of GTO1. Dynamic avalanche
in a few cells can be viewed as an effective increase in the conductivity of those cells. If the number of slower
cells is much smaller than that of faster cells, the current density in GTO1 can then become extremely
high. This process can occur very quickly around t2 with the area of the current filament smaller and
smaller and the current density higher and higher (due to positive feedback). The excessive energy
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t0
Ia
Ia1
t1
Ia
Ia2
P
P
P
P
P
P
N
N
N
N
N
N
P
P
N
Ig
N
Ia1=Ia2
Ic
P
P
N
Ig
N
Ia1 ≈ Ia2
t5
Ia
Ig
Ic
P
N
Ia1< Ia2
t4
Ia
P
P
P
N
N
N
N
N
P
P
N
Ic
P
N
Ig
N
Ia1 ≈ Ia2
N
Ig
Ic
P
N
Ia1<<Ia2
N
Ic
do
Ia1 ≈ Ia2
P
P
tc
P
N
Ic
t3
Ia
P
P
N
om
P
Ig
t2
Ia
FIGURE 1.87 Semiconductor level analysis of the non-uniform turn-off process. The shaded region represents
stored charge (plasma) in the GTO.
Pa
Summary
kw
eb
in
fo
dissipated on the stressed cells can cause permanent failure because the temperature can be very high.
After the device failure, a GTO device loses its blocking capability and behaves resistively.
It can therefore be concluded that the combination of storage time differences and the onset of
possible localized dynamic avalanche makes the turn-off capability of a GTO small. Practical RBSOAs
2
of high-power GTOs are below the 50 kW/cm power constant line. This low limitation mandates the
use of a dV/dt snubber. Even with a dV/dt snubber, GTO failure can still occur if the instant power is
too high at t2 and exceeds the RBSOA limit of the GTO at that time instance. The voltage spike at t2
can be reduced by minimizing the stray inductance of the dV/dt snubber. The size of the snubber (Cs
value) is typically between 3 to 6 ВµF. The disadvantages of using a dV/dt snubber are the increased count
and size of the components, its high-energy loss, and its increased thermal management requirement
to cool resistor Rs.
The concerns of non-uniform current distribution also mandate a minimum on-time rating for GTOs
to ensure that conduction current is uniformly distributed in the ON state before a turn-off can be
performed. Minimum off-time is also a commonly used rating for the GTO to guarantee the tail current
of the GTO is completely gone and the GTO cells are all in the OFF state.
Advantages of the GTO include:
1. High current–voltage capability
2. Low conduction loss
3. Low cost
Disadvantages
1.
2.
3.
4.
5.
Non-uniform turn-off—poor RBSOA and dV/dt snubber required
Non-uniform turn-on—dI/dt snubber required
Current control—high gating power
Long switching time—long storage time, minimum on-time and off-time requirements
No current limitation capability (FBSOA)
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References
9.
10.
11.
om
7.
8.
tc
6.
S.K. Gandhi, Semiconductor Power Devices, Wiley, New York, 1977.
E.D. Wolley, Gate Turn-Off in P-N-P-N devices, IEEE Trans. Electron Devices, ED-13, 590–597, 1966.
Mitsubishi GTO FG6000AU-120D data sheet.
B.J. Baliga, Power Semiconductor Devices, PWS Publishing Company, Boston, 1996.
W.H. Dodson and R.L. Longini, Probed determination of turn-on spread of large area thyristors,
IEEE Trans. Electron Devices, ED-13, 478–484, 1966.
H.J. Ruhl, Spreading velocity of the active area boundary in a thyristor, IEEE Trans. Electron Devices,
ED-17, 672–680, 1970.
ABB GTO 5SGT 40L4502 data sheet.
P.L. Hower and V.G.K. Reddi, Avalanche breakdown in transistors, IEEE Trans. Electron Devices, ED17, 320–335, 1970.
I. Takata, T. Hikichi, and M. Inoue, High voltage bipolar transistor with new concepts, presented at
IEEE IAS ’92, 1992, 1126–1134.
Y. Shimizu, S. Kimura, H. Kozaka, N. Matsuura, T. Tanaka, and N. Monma, A study on maximum
turn-off current of a high-power GTO, Electron Devices, IEEE Trans., 462, 413–419, 1999.
K. Lilja and H. Gruning, Onset of current filamentation in GTO devices, in PESC ’90 Record, 21st
Annual IEEE, 398–406, 1990.
do
1.
2.
3.
4.
5.
1.9 Insulated Gate Bipolar Transistors
fo
Alex Q. Huang
Pa
kw
eb
in
When the development of power MOSFETs encountered difficulty in increasing their current-handling
capability, the idea of a MOS-controlled bipolar device was developed to overcome the problem. This
effort led to today’s insulated gate bipolar transistor (IGBT) [1]. The IGBT fundamentally changes the
BJT current control into voltage control while maintaining the advantages of the BJT. In addition, the
use of a wide-base pnp transistor in the IGBT structure results in a much inproved conductivity modulation effect than a conventional BJT, pushing the voltage rating of the IGBT toward the level of GTOs.
The internal pnp structure also does not have the second breakdown problem as a conventional npn
structure because the high voltage is supported by the base region of the pnp transistor instead of by the
collector region as is the case for a conventional npn transistor. IGBTs also have excellent RBSOA and
FRSOA. Having undergone several years’ development, IGBTs have become the best device for applications in the range of 600 to 3000 V.
Although there are a number of other devices that have been developed or are being developed, the
workhorse power semiconductor devices today are SCRs, GTOs, MOSFETs, and IGBTs. Each of these devices
dominates a specialized power arena. The MOSFET has excellent dynamic and static performance. It
dominates low voltage applications below 600 V. The IGBT is slower than the MOSFET but has better
forward voltage drop above 600 V. It dominates applications from 600 to 3000 V. At an even higher voltage
level, the GTO becomes the dominant device with better current-carrying capability but much slower
dynamic response. Without turn-off capability, the SCR has an even better current conduction capability,
so it is suitable for even higher power AC applications where gate-controlled turn-off capability is not
necessary.
For a typical application, the switching frequency is an important index in determining system performance. Generally, the higher the switching frequency, the better the dynamic performance of the
system, the smaller the size of the system due to reduced passive components, and the lower the cost of
the system due to savings on passive components. The practical switching frequency of an application
system is a trade-off of many issues including maximum device switching frequency, maximum magnetic
switching frequency, switching losses of the power switches, overall system efficiency, etc. In the low
power field where the MOSFET plays the major role, the switching frequency is normally subject to
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IGBT Structure and Operation
tc
om
system efficiency and/or magnetic considerations instead of device limitations. In the medium power
field, where the IGBT plays the major role, the situation changes. At the lower end, the limitation of the
device does not dominate since the lower-rating IGBT is normally fast enough. However, when the power
rating is higher, the IGBT switching speed decreases and the switching losses increase significantly. The
practical switching frequency is thus subject to the limitation of the device. When the power level moves
even higher, the GTO is the only available device. Since it has several tens of microseconds switching
time, significant turn-off, and dV/dt snubber loss, the GTO is traditionally the limitation of the switching
frequency of the system.
The above trend shows that when the power level moves higher, power semiconductor devices limit
the maximum system switching frequency, hence the performance of the system, especially at the GTO
level. To meet the increasing demand for better performance in high-power systems, many efforts have
been made to improve the performance of high-power semiconductor devices. Among them, one effort
is to push the IGBT toward higher power ratings based on the module concept. With its good dynamic
performance, high-power systems equipped with IGBTs can operate at a much higher switching frequency
and have many benefits compared with a conventional GTO system. The state-of-the-art IGBT rating is
currently 3.3 kV/1.2 kA [2], which is at the low end of that of the GTO.
kw
eb
in
fo
The name insulated gate bipolar transistor sterns from its operation based on an internal interaction
between an insulated-gate FET (IGFET) and a bipolar transistor. It has previously been called an IGT
(insulated-gate transistor), an IGR (insulated-gate rectifier), a COMFET (conductivity-modulated fieldeffect transistor), a GEMFET (gain-enhanced MOSFET), a BiFET (bipolar FET), and an injector FET.
IGBTs have been successfully used since they were first demonstrated in 1982 and are currently the most
widely used power semiconductor switches with applications from several kilowatts to a few megawatts.
A cross section of the planar junction–based IGBT structure introduced in the 1980s is shown in
Fig. 1.88a. The IGBT structure is similar to that of a planar power MOSFET except the difference in the
substrate doping type. The fabrication of the IGBT therefore is almost the same as a power MOSFET.
This has made its manufacture relatively easy immediately after conception, and its ratings have grown
at a rapid pace as a result of the ability to scale up both the current and the blocking voltage ratings.
Today, the largest single-chip IGBT can carry about 100 A and block more than 3000 V. Larger current
IGBTs are also introduced by paralleling more IGBT chips in a single package. These IGBTs are also
called IGBT modules. Figure 1.89 shows a photograph of a 1200-A, 3300-V IGBT module fabricated by
Mitsubishi.
emitter
gate
emitter
n+
P
+
Pa
J2
P
N drift
J1
P + substrate
collector
a
FIGURE 1.88
R
NPN
gate
PNP
collector
parasitic thyristor
b
(a) Cross section of the IGBT structure and (b) equivalent circuit for the IGBT.
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om
tc
do
2
FIGURE 1.89 Photograph of a 1200-A, 3300-V IGBT module in which 24 1-cm IGBT dies are paralleled together
by wire bonds.
Pa
kw
eb
in
fo
The equivalent circuit for the IGBT, shown in Fig. 1.88b, consists of a wide-base pnp bipolar transistor
driven by a short-channel MOSFET. Notice the main current path for the IGBT is not through the pnp
transistor but through the indicated path. In the IGBT structure, when a positive bias voltage larger than
the threshold voltage of the DMOS channel is applied to the gate electrode, an inversion layer is formed
along the p-base surface of the DMOS, and the DMOS channel is turned ON. Also an accumulation
layer of electrons is formed at the surface of the n region below the gate. When a positive bias is applied
+
to the collector, electrons flow from the n emitter contact via the DMOS channel and the accumulation
в€’
layer into the n drift region. This provides the base drive current for the vertical pnp transistor in the
+
IGBT structure. Since the emitter junction (J1) for this bipolar transistor is forward-biased, the p region
в€’
injects holes into the n base region. When the positive bias on the collector terminal of the IGBT is
в€’
increased, the injected hole concentration increases and reduces the resistance of the n drift region.
Consequently, the IGBT can operate at much higher current densities than the VDMOS even when it is
designed to support high blocking voltages.
As long as the gate bias is sufficiently large to produce a strong inversion layer and an accumulation
в€’
layer of electrons at the n base region surface, the IGBT forward conduction characteristic resembles
that of a pin diode. Therefore, the IGBT can also be considered a pin diode in series with a MOSFET.
Electron injections are provided by the accumulation layer electrons beneath the gate and between the
adjacent p-body regions. However, not all injected holes recombine with these electrons; instead, some
of the holes are collected by the p-body region, which acts as the collector region of the parasitic pnp
transistor. IGBT design for low conduction drop requires minimizing the parasitic pnp transistor current
and maximizing the pin current that maximizes the conductivity modulation.
However, if the DMOS channel becomes pinched off and the electron current saturates, the hole current
also saturates because of the saturation of the base drive current for the pnp transistor. Consequently, the
device operates with current saturation in its active region with a gate-controlled output current. This
current saturation characteristic is useful for applications in which the device is required to sustain a
short-circuit condition.
When the gate voltage is lower than the threshold voltage of the DMOS, the inversion layer cannot
sustain and the electron current via the DMOS channel is terminated. The IGBT then operates in the
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I CE
VG
om
Output characteristics of the IGBT.
tc
FIGURE 1.90
VCE
forward
characteristics
reverse
characteristics
Pa
kw
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in
fo
do
forward blocking mode. A large voltage can then be supported by the reverse-biased p-base/n-drift
junction (J2). Figure 1.90 shows a typical output characteristic of the IGBT.
The IGBT was the first commercially successful device based upon combining the physics of MOS-gate
control with bipolar current conduction. Due to the injection of a high concentration of holes from the
+
в€’
в€’
p substrate into the n drift, the conductivity of the long n region is modulated and the IGBT exhibits
pin diodelike on-state characteristics with a low forward voltage drop. Thus, the IGBT exhibits excellent
current-carrying characteristics with forward conduction current densities 20 times higher than that of
a power MOSFET and five times greater than that of a bipolar transistor operating at a current gain of 10.
Since the input signal for the IGBT is a voltage applied to the MOS-gate, the IGBT has the high input
impedance of the power MOSFET and can be classified as a voltage-controlled device. However, unlike
the power MOSFET, the switching speed of the IGBT is limited by the time taken to remove the stored
charges in the drift region due to the injection of holes during on-state current conduction. The turnoff time for the IGBT is dictated by the conduction modulation of the drift region and the minority
carrier lifetime. The frontier is specified dominantly by the current gain of the wide-base pnp transistor,
and the latter can be controlled by a lifetime control process, such as electron irradiation. Although the
lifetime control process can be successful in reducing the turn-off time, it was found that there was a
trade-off between the on-state voltage drop (conduction loss) and the turn-off time (switching loss). A
shorter minority carrier lifetime makes the switching loss of the IGBT lower, but the shorter minority
carrier lifetime also results in a higher conduction loss.
One of the problems encountered when operating the IGBT at high current levels has been the latch-up
of the parasitic pnpn thyristor structure inherent in the device structure. Latch-up of this thyristor can
occur, causing losses of gate-controlled current conduction. Since the current gains of the npn and pnp
transistors increase with increasing temperature, the latching current decreases with increasing temperature. This effect is also aggravated by an increase in the resistance of the p-base with temperature due
to a decrease in the mobility of holes. Many methods have been explored to suppress the latch-up of the
+
+
parasitic thyristor, such as the use of a deep p diffusion, a shallow p diffusion, or a self-aligned sidewall
+
diffusion of n emitter. State-of-the-art IGBTs have basically solved this problem, and latch-up does not
occur for all gate voltages applied. These IGBTs therefore exhibit close to square FBSOA.
Traditionally, IGBTs are fabricated on a lightly doped epitaxial substrate, such as the one shown in
Fig. 1.88a. Because of the difficulty of growing the lightly doped epitaxial layer, the breakdown voltage
of this type of IGBT is limited to below 1000 V. To benefit from such a design, an n buffer layer is normally
+
в€’
в€’
introduced between the p substrate and the n epitaxial layer, so that the whole n drift region is depleted
в€’
when the device is blocking the off-state voltage, and the electric field shape within the n drift region
is close to rectangular. This type of design is referred to as Punch-Through IGBT (PT IGBT), as shown
in Fig. 1.91a. The PT structure allows it to support the same forward blocking voltage with about half
в€’
the thickness of the n base region of the pnp transistor, resulting in a greatly improved trade-off relationship between the forward voltage drop and the turn-off time. Thus, the PT structure together with
lifetime control is preferred for IGBTs with forward blocking capabilities of up to 1200 V.
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gate
emitter
n+
gate
emitter
+
n+
+
P
emitter
P
P
P+
P
n+
gate
N drift
N drift
(substrate)
om
P base
N drift
N buffer layer
N buffer layer
p+ substrate
collector
p + layer
collector
FIGURE 1.91
b
c
do
a
collector
tc
p + substrate
(a) PT IGBT structure, (b) NPT IGBT structure, and (c) UMOS gate PT IGBT structure.
Pa
kw
eb
in
fo
For higher blocking voltages, the thickness of the drift region becomes too large for cost-effective
epitaxial growth. Another type of design, the Non-Punch-T hrough IGBT (NPT IGBT, as shown in
в€’
Fig. 1.91b), is gaining popularity. In the NPT IGBTs, devices are built on an n wafer substrate that
в€’
serves as the n base drift region. The collector is implanted from the backside of the wafer after proper
wafer thinning, and no field stopping n buffer layer is applied to the NPT IGBT. In this concept, the
в€’
shape of the electric field is triangular in the forward blocking state, which makes a longer n base region
necessary to achieve the same breakdown voltage as compared with the PT IGBT. However, the NPT
IGBT offers some advantages over the PT IGBT. For example, the injection efficiency from the collector
+
side can be controlled (due to the use of implanted p region) and devices with voltage ratings as high
+
as 4 kV can be realized. Further, by optimizing the emitter efficiency of carriers from the p collector
в€’
layer and the transport factor of carriers in the n base, the trade-off between the forward voltage drop
and the turn-off time for the NPT IGBT can be improved to become similar to that of the PT type IGBT
without lifetime control.
Generally speaking, the current tail in the NPT IGBT is longer than the PT IGBT, but the NPT IGBT
is more robust than the PT IGBT, particularly under a short-circuit condition. The trench gate IGBT
(UMOS-gate IGBT) structure is shown in Fig 1.91c. With the UMOS structure in place of the DMOS
gate structure in the IGBT, the channel density is greatly increased and the JFET region is eliminated. In
addition, the electron-hole concentration is enhanced at the bottom of the trench because an accumulation layer forms. This creates a catenary-type carrier distribution profile (see Fig. 1.91) in the IGBT,
which resembles that obtained in a thyristor or pin diode. These improvements lead to a large reduction
in the on-state voltage drop until it approaches that of a pin diode, hence approaching the theoretical
limit of a silicon device. The latching current density of the UMOS IGBT structure is superior to that of
the DMOS structure. This is attributed to the improved hole current flow path in the UMOS structure.
As shown in Fig. 1.90c, the hole current flow can take place along a vertical trajectory in the UMOS
+
structure, whereas in the DMOS structure hole current flow occurs below the n emitter in the lateral
direction. The resistance for the hole current that causes the latch-up is determined only by the depth
+
+
of the n emitter region. A shallow p layer can be used, as shown in the figure, to reduce this resistance.
As a consequence, the RBSOA of the UMOS IGBT structure is superior to that of the DMOS IGBT
structure. Further, because of a very strong percentage of electron current flow in the trench gate IGBT,
the turn-off speed of the trench-based IGBT is generally faster than the DMOS-based IGBT. It can be
anticipated that trench gate IGBTs will replace the DMOS IGBT structures in the future.
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References
om
1. Baliga, B.J., Adler, M.S., Love, R.P., Gray, P.V., and Zommer, N., The insulated gate transistor: a new
three terminal MOS controlled bipolar power device, IEEE Trans. Electron Devices, ED-31, 821–828,
1984.
2. Brunner, H., Hier, T., Porst, A., and Spanke, R., 3300V IGBT module for traction application, in EPE
Conf. Rec., 1056–1059, 1995.
1.10 Gate-Commutated Thyristors
and Other Hard-Driven GTOs
Alex Q. Huang
tc
Unity Gain Turn-Off Operation
Traditional GTO Gate Drive Circuit
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eb
in
fo
do
Traditional GTOs are generally designed with a turn-off gain of 3 to 5. This is the result of trade-offs
between the performances of the GTO and the current (hence power) requirement its gate drive circuit.
Figure 1.92b shows the typical turn-off gate drive circuitry for a traditional GTO. A negative turn-off
voltage source VOFF is connected to the GTO gate–cathode junction J3 through the turn-off control switch
SW. Since both sides of the junction J3 are highly doped, its breakdown voltage BVGC is practically about
20 V and can hardly be increased. The turn-off voltage VOFF is selected below the junction J3 breakdown
voltage to avoid constant breakdown of this junction when the GTO is in the off state. To turn off the GTO,
switch SW is turned on so the negative turn-off voltage VOFF is applied on the GTO gate–cathode junction.
The current originally flowing through the cathode is then diverted to the gate, causing cathode current
IK to decrease and the gate current to increase. Because of the existence of the GTO gate lead stray
inductance LG, which is practically on the order of several hundreds of nanohenry determined by the
lead structure and length, the cathode current will decrease linearly and the gate current will increase
linearly. This current commutation rate is thus given by:
dI G /dt = V OFF /L G
t0
t1 t2 t3
Ia
(1.24)
t4
t5
t6
Va
Pa
Ic
Ig
FIGURE 1.92
Typical turn-off characteristics of a GTO.
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t2
t0 t1’
t1
t4
t3
Ia
t5
t6
Va
om
Ig
GTO turn-off waveform under unity gain.
do
FIGURE 1.93
tc
Ic
kw
eb
in
fo
The higher the turn-off gate current slew rate, the shorter the storage time. To obtain the shortest
storage time, the turn-off voltage is normally selected very close to BVGC to realize highest turn-off gate
dIG /dt. The typical turn-off gate dIG /dt is on the order of several tens of amperes per microsecond, and
the typical storage time of a high current GTO is about 20 Вµs. Figure 1.93 shows the typical current and
voltage waveforms of a GTO turning off with a turn-off gain higher than 1. After the GTO is turned off,
its gate current will drop back to 0 slowly by breaking down the GTO gate–cathode junction due to the
energy stored in LG . The energy required from the gate driver during this turn-off transition is the integration
of the gate current times the turn-off voltage VOFF . This energy is significant because the gate current
lasts for a long period.
Because of the long transient process, storage time differences among GTO cells become bigger and the
non-uniform current redistribution after t1 is significant. The practical RBSOA of a GTO is normally much
2
lower than the 200 kW/cm limit set by the dynamic avalanche because of the non-uniform current turnoff (storage time differences and localized dynamic avalanche).
Unity Turn-Off Gain of the GTO
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If the gate driver of a GTO is very fast so the gate current can increase rapidly to the anode current level
and the cathode current decreases to zero before the anode current begins to decay, then the current and
voltage waveforms of the device are as shown in Fig. 1.93. According to the definition above, the turnoff gain in this case is unity.
The internal turn-off process of the GTO changes significantly under the unity turn-off gain condition.
Most important is that the GTO turn-off is now conducted in the pnp transistor mode after the unity
gain is established. Figure 1.94 shows minority carrier distribution during the turn-off transition. Inside
the p-base, there are two functioning parts of minority carriers (electrons). The first part is the electrons
related to the bias of the gate–cathode pn-junction; the second part is the electrons related to the forward
bias of junction J2. Before the turn-off process at point t0, minority carriers have been accumulated in
в€’
the p-base and n region. Starting from t0, the cathode current decreases rapidly and the gate current
increases rapidly in the reverse direction. By t′1 , the cathode current comes to zero so minority carriers
associated with the gate–cathode junction are removed. Zero cathode current cuts minority carrier
+
injection from the n side into the p-base. From this moment, the GTO is like an open-base pnp-transistor
instead of a pnpn latch-up structure. This difference makes the GTO more rugged during turn-off
transition. Negative gate current continues the extraction of minority carriers out from the p-base until
t1 when they are totally removed.
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t0
t1
Ia
P
Ia
P
J1
N-
NJ2
P
P
N+
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Ic
Ia ≈ Ic
t4
Ic
t3
Ia
Ic=0
Ia
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N-
P
P
N+
Ic
Ig
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FIGURE 1.94
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J3
Ic=0
N+
Ic
Ic=0
Internal process of a GTO under unity gain turn-off.
P
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ANODE
GTO1
t5
Ia1
N
P
t4
GTO2
P
N
t2 t3
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t0 t1
Va
Ia2
P
N
N
GATE
CATHODE
t S1 > t S2
(a)
Turn-off waveforms of GTO cells under unity gain turn-off condition.
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FIGURE 1.95
(b)
Advantages of Unity Gain Turn-Off
With unity turn-off gain, the storage time of a GTO is significantly reduced. The storage time in this case
is the time required to remove minority carriers in the p-base. In the normal GTO case, the gate current
is much less than the anode current so the removal speed is slow. Furthermore, the cathode current is
not reduced to zero so minority carrier injection continues during the whole storage phase. With unity
turn-off gain, the gate current is as high as the anode current, leading to a rapid carrier removing speed.
Also, the cathode current is reduced to zero, hence instantly stopping the minority carrier injection into
the p-base. Generally, the storage time of a GTO under unity turn-off gain is about 1 Вµs compared with
that of about 20 Вµs in a normal GTO case with high turn-off gain.
Another important performance improvement with unity gain turn-off is in the RBSOA. As is
analyzed above, the GTO current tends to crowd toward the cell with a longer storage time. This process
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If Ia1>Ia2
∆Q1>∆Q2
∆ x 1> ∆ x2
negative feedback
∆V1> ∆V2
∆Em1> ∆Em2
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Ia1 , Ia2
FIGURE 1.96 Negative feedback mechanism in the current sharing of two paralleled GTO cells during voltage
increasing phase.
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significantly limits the average instant power a GTO can withstand so a dV/dt snubber circuitry is
normally required to limit the voltage level, hence the instant power stress, during turn-off transition.
GTO cells under unity gain turn-off have a tendency toward uniform current sharing, hence large RBSOA.
First, the current filamentation due to the difference of storage time is greatly reduced because the absolute
storage time is reduced to less than 1 Вµs. During the voltage rising phase after the storage time, if one cell
still shares more current, that cell will have a faster carrier extraction rate and hence will turn off that
cell faster. There is therefore a negative feedback process with current sharing instead of a positive one.
This negative process is shown in Fig. 1.96.
With this uniform current distribution tendency provided by the unity turn-off gain, a GTO as a whole
can be assumed to be more uniform in current sharing and hence can withstand much higher average
instant power during turn-off transition. The RBSOA should now be pushed toward a power constant of
2
200 kW/cm as predicted by Eq. (1.23) (in Section 1.8). This RBSOA is sufficiently large that a GTO should
be able to perform turn-off operation even without the help of a dV/dt snubber. It should also be pointed
out again that the onset of dynamic avalanche may not be the actual RBSOA boundary because if it does
not initiate a runaway current filament, it is not a destructive one. Experimental results [1] on IGCT turnoff, however, suggest that the dynamic avalanche is not uniform and that it does lead to failure of a device.
Unity gain turn-off is therefore effective in removing any current filament problem associated with storagetime differences and the dynamic avalanche soon after the current filament is formed.
Hard-Driven GTOs
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Unity turn-off gain can significantly improve the performance of a GTO in several aspects, including
RBSOA and turn-off storage time. Several innovative approaches have been proposed to realize unity
turn-off gain. In all approaches, achieving unity turn-off gain is critical. This would require that cathode
current be commuted to the gate path very fast. To turn off a 4-kA GTO with unity gain, the commutation
rate should be higher than 6 kA/Вµs. This high commutation rate requirement distinguishes the performance
of each of the devices discussed below. According to their realizations, they can be classified into two
different categories: hard-driven type and MOS-controlled type. Hard-driven type approaches use a
powerful gate driver to realize unity turn-off gain. The gate driver supplies the gate current and the gating
power. Falling in this category is the integrated gate commutated thyristor (IGCT) [2]. The MOScontrolled approaches use MOSFETs to aid the turn-off process of the GTO. Other than the unity turnoff gain, these approaches also save control power for the turn-off process. Falling in this category are
the emitter turn-off (ETO) [3] thyristor and the MOS turn-off (MTO) [4] thyristor.
IGCT
The key to achieving a hard-driven or unity-gain turn-off condition lies in the gate current commutation
rate. A rate as high as 6 kA/Вµs is required for 4-kA turn-off. Two methods have been demonstrated for the
implementation of a hard-driven GTO. The first is to hold the gate loop inductance low enough (3 nH)
that a DC gate voltage less than the breakdown voltage of the gate–cathode junction (18 to 22 V) can
generate a slew rate of 6 kA/Вµs. This approach is used in the IGCT/GCT [2, 5] (IGCT is an ABB product,
GCT is by Mitsubishi, but the concept is the same), where a special low-inductance GTO housing and a
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-
+
LG<3
Driver
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Mitsubishi 4kA / 4.5 kV GCT
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ABB 4 kA / 4.5 kV IGCT
FIGURE 1.97 GCT operation principle and two GCTs developed by Mitsubishi and ABB. (Photographs courtesy
of Mitsubishi (top) and ABB (bottom).)
carefully designed gate driver meet this requirement. The power consumption by the GCT driver is greatly
reduced compared with that of a conventional GTO driver, since the gate current is present for a much
shorter period of time [6]. Figure 1.97 shows the external view of the two commercially available GCTs.
The key disadvantage of the GCT approach is the high cost associated with the low-inductance housing
design for the GTO and the low inductance and high current design for the gate driver.
MTO
Figure 1.98a shows the turn-off principle of a MTOв„ў [4, 7] developed by Silicon Power Corporation.
The MTO device packages a number of low-voltage MOSFETs within a normal GTO device housing to
form a current path that is in parallel with the emitter junction of the GTO. Therefore, the MTO looks
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L
10nH
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(a)
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(b)
FIGURE 1.98 MTO equivalent circuit (a) and 500-A/4.5-kV MTO with gate driver (b) developed by Silicon Power
Corp. (Photograph courtesy of Silicon Power Corp.)
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just like a conventional GTO from the outside. The turn-off is initiated by turning on the MOSFET that
shorts the GTO emitter junction. MTO, like the ETO, is therefore a MOS turn-off device requiring very
little turn-off gate power. To achieve a high gate current commutation rate, very low gate inductance
(<0.1 nH) is required.
Because of the use of the hybrid approach, a prototype 500-A, 4500-V device is available from SPCO.
The major problem for the MTO, however, is still the limitation of the RBSOA [7]. This is because
the gate current commutation rate is determined by the packaged gate inductance, which has to be
reduced to below 0.1 nH. There are three reasons for this. First, in the MTO the commutation rate is
determined by
0.7
пЈ« dI
-------G- ≤ ------ dt  max L G
(1.25)
Second, the resistive voltage in the GTO p-base region and the MOSFET determines the peak gate current
that can be commutated:
0.7
I Gmax ≤ ------------------------------R MOS + R p-base
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(1.26)
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ETO equivalent circuit (a) and a 4-kA/6-kV ETO (b). An ETO gate driver is also shown.
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FIGURE 1.99
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Third, since there is no reverse bias voltage applied to the GTO emitter junction in the MTO, it is very
easy to become latched again. Snubberless turn-off capability of the MTO is therefore lower than the
GCT and ETO.
ETO
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The method to achieve unity gain in the ETO thyristor is to insert an additional switch in series with
the cathode of the GTO. The cathode of the GTO is the emitter of the internal npn transistor, so the
series switch is referred to as the emitter switch and the new device is termed ETO. Turning off the emitter
switch generates a high transient voltage long enough to commutate the emitter current to the gate path
even with a higher parasitic inductance present. Because of this higher tolerance for parasitic inductance,
conventional GTOs can be used in the ETO. An additional switch is connected to the gate of the GTO,
and is complementary to the emitter switch. These switches are implemented with many paralleled lowvoltage, high-current MOSFETs to minimize the additional conduction loss due to the emitter switch.
The typical value for the conduction loss due to the series switch is 0.2 V at the average GTO current
rating. The turn-off driving power for the ETO is negligible, since the turn-off is purely due to the
removal of a MOSFET gate signal. The ETO in many aspects is similar to the IGBT. For example, the
turn-off mechanism used in IGBT is also an emitter turn-off, and the IGBT always turns off in the rugged
pnp transistor mode.
Figure 1.99 shows the equivalent circuit and hardware photograph of the developed 4-kA, 6-kV ETO
by Virginia Tech. Other lower current rating ETOs have also been demonstrated by Virginia Tech. Because
of the use of hybrid approach based on conventional GTO, ETO devices have clear advantages in terms
of cost and gate drive power requirement over GCTs. ETO devices also have two other advantages when
compared with the GCT. One is its feasibility of having a FBSOA [3, 8], and the other is its simplicity
in overcurrent protection [8].
Conclusions
These newly developed GTOs (IGCT, MTO, and ETO) all utilize the unity gain turn-off concept and
have dramatically improved performance compared with conventional GTOs. Quantitative comparisons
of these devices are provided in a separate section on high-power IGBTs (Section 1.9).
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References
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1. I. Takata, M. Bessho, K. Koyanagi, M. Akamatsu, K. Satoh, K. Kurachi, and T. Nakagawa, Snubberless
turn-off capability of four-inch 4.5k V GCT thyristor, presented at IEEE International Symposium
on Power Semiconductor Devices and Ics, 1998, 177–180.
2. P.K. Steimer, H.E. Gruning, J. Werninger, E. Carrol, S. Klaka, and S. Linder, IGCT—a new emerging
technology for high power, low cost inverters, presented at IEEE Industry Applications Society Annual
Meeting, New Orleans, Louisiana, Oct. 5–9, 1997, 1592–1599.
3. Y. Li, A.Q. Huang, and F.C. Lee, Introducing the emitter turn-off thyristor, presented at 1998 IEEE
Industry Applications Society 33rd Annual Meeting, 1998, 860–864.
4. D.E. Piccone, R.W. De Doncker, J.A. Barrow, and W.H. Tobin, The MTO thyristor—a new high power
bipolar MOS thyristor, presented at IEEE Industry Applications Society 31st Annual Meeting, Oct.
6–10, 1996, 1472–1473.
5. E.R. Motto and M. Yamamoto, New High Power Semiconductors: High Voltage IGBTs and GCTs, in
PCIM’98 Power Electronics Conference Proceedings, 1998, 296–302.
6. Mitsubishi GCT FGC4000BX-90DS data sheet.
7. A.Q. Huang, Y. Li, K. Motto, and B. Zhang, MTO thyristor—an efficient replacement for standard
GTO, presented at IEEE Industry Applications Society 34th Annual Meeting, 1990, 364–372.
8. Y. Li, A.Q. Huang, and K. Motto, Experimental and numerical study of the emitter turn-off thyristor
(ETO), IEEE Trans. Power Electron., May 2000.
1.11 Comparison Testing of Switches
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Alex Q. Huang
Pulse Tester Used for Characterization
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In a typical power device dynamic test, the device under test (DUT) is initially off, and the high-voltage
capacitor bank is charged to set the voltage the DUT will experience during switching. A typical pulse
tester is shown in Fig. 1.100 and a typical waveform of the test is shown in Fig. 1.101. The so-called
double pulse testing will capture one device turn-on event and one device turn-off event. The doublepulse test consists of the following complete events:
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t0–t1: At time t0, the control system initiates a pulse to the gate driver for the DUT. The DUT turns
on and the high voltage capacitor bank charges the load inductor. After the current reaches
the desired value at t1, the DUT gate driver is commanded to turn off.
t1–t2: From time t1 to t2, no changes to the device are seen. During this time, referred to as the
storage time, internal processes in the device initiate the turn-off process.
FIGURE 1.100
Pulse tester schematic diagram.
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Double-pulse tester waveforms.
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FIGURE 1.101
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t2–t3: At time t2, the anode voltage begins to rise, as the turn-off process has begun. The freewheeling
diode is still reverse-biased so the current cannot yet begin to fall.
t3–t4: At time t3, the anode voltage reaches the bus voltage and the main device current begins to
fall. The current that had been flowing through the DUT is commutated into the freewheeling
diode. This is the highest stress interval of the turn-off transition, as the current and voltage
are simultaneously high during this interval
t4–t5: At time t4, the main current fall is completed and the current tail phase begins. The current
tail continues until t5. At this point the device can be said to have completed the turn-off process.
t5–t6: During this time, the dI/dt snubber resistor carries the current, inducing additional voltage
stress on the main DUT. The snubber inductor is charging during this time, and becomes
charged at t6. The snubber diode then goes through a reverse-recovery process.
t6–t7: During this time, the DUT is off and blocking a voltage equal to the input capacitor voltage.
The current is still freewheeling through the load inductor and the freewheeling diode. This
current will continue to circulate for a long time because the only energy dissipation is due
to the conduction voltage of the freewheeling diode.
t7–t2: At this time, the controller initiates the second pulse to test the turn-on of the device. Nothing
external occurs until t8, which is the end of the turn-on delay time.
t8–t9: During this time, the load current begins to commutate into the DUT from the freewheeling
diode. The dI/dt snubber inductor determines the rate of current transfer.
t9–t10: At time t9, the load inductor current is completely commutated into the DUT and out of the
freewheeling diode. The freewheeling diode undergoes reverse recovery during this period
and releases a significant amount of reverse current into the DUT. It is important that the
DUT have fully switched on by now or the diode recovery current will induce large power loss.
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Switching time definition waveform.
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FIGURE 1.102
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t10–t11: During this time, the device is on and the current is rising because of the input voltage divided
by the load inductance. This is equivalent to the interval t0–t1 from the first pulse. The same
sequence will continue for the turn-off of the second pulse as that for the first pulse.
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The current through the device under test is measured with a precision current shunt in series with
the cathode (or emitter for an IGBT). All delay times are defined with respect to the actual gate of the
device, so gate driver internal delays are not included. Conventionally, fall time is defined as when the
current decreases from 90% of its initial value to 10%, but a different definition is used here. For the highvoltage devices, the current tail value can be greater than 10% of the initial current value, so it is
unreasonable to include this time in the fall time. Therefore, the definition used here is that the fall time
ends and the tail time begins when the current slope visibly changes. This is physically justified because for
all three devices the current tail means that the main turn-off process is complete and the open-base pnp
transistor is removing the remaining carriers. A sample waveform is shown in Fig. 1.102. Current tail time
is defined from the end of the current fall time until the anode/collector current decreases to 1% of the
initial current.
Devices Used for Comparison
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To compare these various semiconductor technologies, two IGBTs, an IGCT, a GCT, and three ETOs
were used [1]. One IGBT and the GCT are made by Mitsubishi, and the ETOs have been developed by
researchers at Virginia Tech. The other IGBT is made by EUPEC, and the IGCT is from ABB. The IGBTs,
CM1200HA-66H and FZ1200R33KF2, are rated for 1200 A (DC) and 3300 V, and are packaged in plastic
modules 14 by 19 cm in size. The IGCT and the GCT are both 4500-V devices, which are rated for 4000 A
maximum controllable current. The first ETO used, ETO4060s, is rated for 6000 V and 4000 A controllable
current, and is based on a Toshiba GTO. The IGCT, the GCT, and the ETO4060s are packaged in 93-mm
press-packs and, with gate drivers, have a maximum width of around 20 cm. The second ETO used,
ETO1045s, is a small (53-mm) device rated for 4500 V and 1000 A. This ETO is based on a Westcode
GTO. The ETO1045s is obviously of a lower rating than the GCT and IGCT, but it uses a fast conventional
GTO, whereas the ETO4060s is based on a GTO designed for about 300 Hz. One final device used is a
newly designed ETO, the ETO4045A, which is based on an ABB GTO similar to the thyristor used in the
IGCT. The average current ratings for the IGCT, GCT, ETO4045A, and ETO4060s are 1200 A, whereas
the ETO1045 is suitable for about 450 A average. When the switching losses of the IGBT and a safe
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FIGURE 1.103 Clockwise from top left: ETO4060, EUPEC HVIGBT, Mitsubishi HVIGBT, ABB IGCT, Mitsubishi
GCT. (Photograph courtesy of Mitsubishi.)
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temperature margin are considered, the average operating current for this device should be between 600
and 800 A. Figure 1.103 shows most of the devices tested.
One significant difficulty in comparing this type of device is that the ratings, and even the ratings
system, are different for the different devices. For GTO-based devices, the current ratings are the peak
controllable current, whereas IGBTs use a DC current rating. The IGBTs tested have a controllable current
rating of twice the DC rating, which translates to a 2400 A rating in the GTO system. These IGBTs consist
of many small dies in parallel, giving a net current density much smaller than that of the GTO-based
devicess. The rms current for the IGCT, the GCT, and the ETO4045A is about 1800 A, and the RMS
current rating of the ETO 4060 is about 1600 A, although the devices have the same average rating (1200 A)
from the manufacturers.
Unity Gain Verification
Because of the strict requirements on the gate loop stray inductance for the IGCT and the ETO, it is very
difficult to insert a current probe directly to monitor the gate current. Fortunately, the unity gain of the
IGCT and the ETO can be verified by observing easily probed voltage signals. It is critical for the
performance of these devices that unity gain has been achieved, so some effort is made to verify unity
gain and predict the maximum current that can be turned off while maintaining the hard-driven
condition.
In the case of the IGCT, monitoring the gate-to-cathode voltage at the terminals of the IGCT thyristor
can show the unity gain. When the gate voltage becomes в€’20 V, which is equal to the power supply in
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GCT unity gain.
FIGURE 1.105
Mitsubishi GCT gate driver. (Photograph courtesy of Mitsubishi.)
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FIGURE 1.104
the gate driver, then clearly no voltage drop is occurring across the parasitic gate inductance. This in
turn implies that dIG/dt is zero, so the gate current has completed commutation. A typical GCT waveform
showing the gate voltage is shown in Fig. 1.104. The inside of the GCT driver box is shown in Fig. 1.105.
Unity gain of the ETO can be verified by observing the drain-to-source voltage of the series switch.
When the current is commutating, the voltage across this switch quickly rises to the breakdown voltage
of the MOSFETs (60 V). When the voltage across this switch begins to fall, then the net cathode current
of the GTO is negative, which discharges the output capacitors of the MOSFETs. Therefore, the ETO
unity gain corresponds to the falling edge of the emitter switch voltage. A turn-off waveform showing
the ETO emitter switch voltage is shown in Fig. 1.106.
Based on the unity gain observation, the rate of current commutation for the devices can be estimated
by dividing the anode current by the time required for unity gain. This method yields a lower result
than truly occurs because the total current commutated is slightly greater than the anode current due
to a reverse recovery effect of the gate to cathode pn-junction. Even with this conservative estimation
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ETO4060 unity gain.
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FIGURE 1.106
Gate Drive Circuits
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of the dI/dt of the gate current, the GCT and the ETO are both capable of approximately 6000 A/Вµs
commutation rate.
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The performance of all semiconductor switches depends on the gate driver circuit. This is especially true
for the GCT, where the device will be unable to operate in the snubberless mode if the gate driver is not
drawing the gate current out fast enough to achieve unity gain. The drivers for the ETO and IGBT are
less difficult to implement since the driver is not required to provide high current.
From a schematic point of view, the GCT driver is very simple, consisting primarily of a capacitor bank
and a switch made from many parallel MOSFETs. The PCB layout and component selection is critical
because of the very strict stray inductance requirement imposed on the switching loop. Additionally, there
is a portion of the driver devoted to turning on the GCT. This is done by injecting a high-current (200-A)
pulse into the gate for 5 Вµs and then injecting 10 A into the gate throughout the on time. This part of the
driver dissipates significant power because of the linear transistors controlling the exact current level, but
the implementation of this part of the gate driver is simple. The GCT driver contains minimum on-time
and off-time protection to allow the device to be always in a uniform state prior to switching. No
overcurrent protection is used for the GCT at the driver level. Although the total gating power is still very
small compared with the main power, all the gating power must be supplied by an external isolated supply
that must have an isolation capability and dV/dt rejection to match that of the GCT.
Because of the different thyristor design used by ABB in the IGCT, the driving power for this device
has been greatly reduced. This is accomplished by increasing the current gain of the thyristor so less gate
current is required to maintain the on state. This leads to a DC injection current of only 2 A. In addition,
the IGCT driver uses a switching rather than linear circuit for pulse injection, which reduces losses as well.
For the ETO driver, three gates have to be controlled—the GTO current injection, the emitter switch,
and the gate switch. Fortunately, the emitter switch and gate switch are easily controlled by using one
inverting driver and one non-inverting driver controlled by the same input. The only function of the
GTO gate is to inject the turn-on current just as in the case of the GCT. The ETO driver developed at
the Center for Power Electronic Systems (CPES) also contains minimum on-time and off-time protection.
In addition, the emitter switch MOSFET can be used as a linear resistor to approximate the anode current,
which can be used for on-driver overcurrent protection. Like the GCT driver, the ETO driver requires
an external isolated power supply, although the power consumption is much lower.
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Forward conduction voltage.
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FIGURE 1.107
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The IGBT driver is very easy to implement, since it has only a single MOS-gate to control. The peak
gate current for the tested IGBT is about 10 A, which flows for about 2 Вµs at every switching event. The
IGBT driver can be used to control actively the dI/dt and dV/dt of the collector, but this feature was not
implemented for this test. Information about active driver techniques can be found in many papers such
as Lee et al. [2]. The IGBT driver implements an overcurrent protection by means of desaturation
detection. IGBT drivers consume so little power that commercial DC-DC converter modules can be used
to provide the isolation internally for the high-side switch.
Forward Conduction Loss Characterization
The forward current vs. voltage characteristics for all of these devices can be found easily. As can be seen
from Fig. 1.107, the thyristors have a clear advantage in conduction loss over the IGBT, even though
their active die area is less than that of the IGBT. If the relationship between breakdown voltage and
conduction loss is found, the advantage of the latching devices becomes even greater. The 4.5-kV thyristors
have the lowest conduction loss, followed by the 6-kV thyristor, and then the IGBT are the worst even
if the loss is not normalized to die area. The ABB transparent anode and punch-through base design show
an advantage in the forward conduction test, as the higher gain allows the device to latch into an extremely
low loss conduction mode. This holds true for the ABB IGCT as well as for the ETO4045A, which is
based on an ABB GTO with the same transparent anode and punch-through base design.
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Switching Tests
Switching performance of high-power devices has been greatly enhanced by the hard-driven GTOs and
the HVIGBTs appearing to challenge the slow GTO technology. Typical operation frequencies of the
high-power GTOs range from line frequency (50/60 Hz) to a high of about 500 Hz. In contrast, the
HVIGBT can be operated at up to 1500 Hz, and the hard-driven GTOs can operate at 1 kHz or more.
This increase in frequency leads to dramatically reduced filters and lower distortion in the typical inverter
applications.
To evaluate the performance of these devices, they were operated with DC voltages of 1.5 and 2 kV
on the pulse tester without any turn-off snubbers. The limiting factor in the amount of current that
could be switched off safely was the clamping diode used to limit the voltage spike on the switch. During
reverse recovery, the voltage across this diode approaches its breakdown (4.5 kV) at the same time the
anode (or collector for IGBTs) voltage of the device under test approaches zero, as circled in Fig. 1.108.
For the GCT and the ETOs, no reverse voltage was acceptable because of the lack of either reverse
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Typical turn-off waveform.
FIGURE 1.109
A 2-kV snubberless switching loss.
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FIGURE 1.108
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conduction capability (such as an antiparallel diode) or reverse voltage blocking capability. GTO-based
devices can achieve reverse voltage blocking easily, but these tested GTOs are anode-shorted types, which
trade away the reverse blocking capability for better switching performance, especially in the current
tail phase. The ABB design uses a transparent anode rather than anode shorts, which also eliminates the
reverse blocking capability. The transparent anode technology makes the current gain of the device change
as a function of the current flowing so that it will have a high gain at low current and a lower gain at
high current. The switching losses for each device were calculated by first multiplying the voltage across
the device by the current being conducted, and then integrating this instantaneous power during the
switching time to find the switching loss. The results of the switching loss tests were compared for the
IGBT, the GCT, and the ETOs. These results are shown for a 2-kV bus in Fig. 1.109.
As expected, the IGBT holds the advantage in this test with the lowest turn-off loss overall. Surprisingly,
the loss of the GCT and the ETO1045 is only marginally higher than the IGBT loss. The primary advantage
of the IGBT in switching loss is in the initial voltage rising phase, which occurs much faster than in the
thyristors. This is because the MOSFET channel in the IGBT can turn off faster than the npn transistor
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Storage (or delay) time comparison.
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FIGURE 1.110
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in the GTOs, and the channel is better distributed through the IGBT than are the gates of the GTOs.
The amount of carrier stored in the GTOs is also higher than in the IGBT, resulting in slower dV/dt. It
is not surprising that the ultrahigh-voltage ETO4060 has significantly more switching loss than the lowervoltage devices. The probable reasons for the high switching loss of this device are a high carrier lifetime
in the GTO, a strong pnp transistor, which can maintain the current longer with the base open, and a
GTO design optimized for low-frequency, high-power operation. The theory of hard-driven GTOs
predicts no improvement in turn-off loss when compared with traditionally driven GTOs, only an
improved safe operating area and higher speed. This shows that the GCT is very well optimized for
performance as well as for a low internal inductance. The transparent anode of the ABB IGCT proved a
disadvantage in this test, as switching times and switching losses were noticeably worse than with the
anode-shorted devices.
As can be seen in Fig. 1.110, the switching times for all of these devices are short and very consistent.
The ETOs and the GCT have long storage times at very low current levels, but the storage time is very
consistent at 600 A and beyond. The current fall times for all devices characterized except the IGCT are
around 250 ns and are essentially independent of the current being switched, as shown in Fig. 1.111. The
IGCT has a very long current fall time at low current levels, although the speed improves at higher
currents. The IGCT tail has a very large magnitude, which again shows that the anode-shorted structure
of the GCT and the ETOs offers advantages in this area.
FIGURE 1.111
Fall time comparison.
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GCT current tail detail.
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FIGURE 1.112
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Because of the large (10 ВµH) turn-on inductor, the turn-on loss for all devices is negligible. All the
thyristors hold a very slight advantage over the IGBT in terms of voltage fall time at turn-on, but the current
is so low during this time that there is no significant difference in loss. It must be noted that the IGBT can
be operated without the turn-on snubber at the expense of significantly increased switching loss, but doing
so requires a more complex gate driver design. This is due to the ability of the IGBT to control the exact
collector current by operating in the linear region. The GCT completely lacks this operating mode. Theoretical analysis predicts the existence of this forward-biased Safe Operating Area for the ETO [3], but no
experimental verification has been performed except at low current [4]. For current tail comparison, the
tail current was examined on a very high resolution (10 A/div) to see all the effects. Immediately after the
main current fall, the tail current decreases rapidly for all the devices tested. However, the current tail can
take a long time to finish decreasing to zero after this initial fast fall. The detail of the GCT current tail is
shown in Fig. 1.112 after turning off 1200 A. The current tail can indicate the strength of the pnp transistor
within an IGBT or a GTO. The long tail observed for the ETO4060 indicates a stronger pnp, which helps
reduce the conduction losses. The GCT demonstrates the shortest current tail of all of the devices tested,
which is further evidence of the very good internal design. The drawback of this performance is that the
effective current gain of the GCT is reduced, thus requiring more DC gate current injection during conduction. The IGBT and the ETO1045 have only slightly worse current tails than the GCT.
Traditionally, the GTO switching frequencies were limited by the times required for the GTO to
complete the switching transitions. In particular, a very long minimum off-time had to be observed due
to some parts of the GTO remaining latched for more than 100 Вµs. The devices tested here all have very
fast switching times, but the switching loss is rather high because of the very large currents and voltages
considered. Therefore, the switching frequency is thermally limited by the switching loss. Soft switching
techniques may allow these devices to achieve much higher operating frequencies (в€ј10 kHz) if the
switching loss can be reduced.
Discussion
Packaging technology is very different for the IGBT modules compared with GTO packaging. The IGBT
modules use many parallel dies, which are wire-bonded and housed in a plastic module. Since a GTO
can be fabricated on a single wafer, press-pack (“hockey-puck”) housings are utilized. The reliability
record for the press-pack devices is much higher than wire-bond modules, largely due to a better tolerance
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for thermal cycling. Additionally, the press-pack allows double-sided cooling to lower the thermal impedance. However, the IGBT achieves similar thermal impedance overall because of the much larger die area
and the consequently large baseplate. The IGBT baseplate is electrically isolated from the heat sink, but
the press-pack heat sinks are directly connected to the anode and cathode terminals. As a result, liquidcooled systems with press-pack devices must rely on oil or deionized water to prevent the coolant from
conducting current. The main advantage of the IGBT module is its ease of use, with the isolated baseplate
leading to easy heat-sinking. The collector and emitter terminals are conveniently located for connection
to a laminated busbar to reduce the parasitic inductance and hence the voltage spike. Additionally, the
IGBT module does not require any external mechanical clamp for mounting, as the press-pack housing
requires. The reliability of the press-pack is a key issue, and this package is preferred for many applications
where long life is necessary.
Although failures are obviously unwanted, the characteristics of the device after a failure should be
considered. This can make a big difference in how much damage is done to the rest of a system and how
difficult repair will be. After a failure, any of these devices will become short-circuited. The current will
then increase until either all the energy available has been consumed or an external circuit acts to stop
the fault current. For the wire-bond IGBT, all the current will concentrate into the die that broke down.
This will usually destroy the wire bonds for that die as a result of the huge current flowing. After failure,
the IGBT can become an open circuit. This is a very dangerous condition for series-connected devices
or multilevel converters, as the voltage will no longer be shared, thus exposing the other devices in the
chain to the risk of overvoltage [5]. The press-pack devices will remain shorted since the die is directly
connected with the metal contacts. There is some concern about the wire-bond MOSFETs in the ETO
emitter and gate switches, although no failure of these MOSFETs has yet been seen even after destruction
of the GTO. Another issue related to the packaging is explosion damage. The press-pack is very strong,
and as a result explosions are very unlikely in this type of package. Plastic modules can easily shatter the
housing, which leads to damage to nearby components.
As previously mentioned, an IGBT can actively control the collector voltage and current during the
switching events. This feature of the device can lead to reduced EMI as well as elimination of the dI/dt
(turn-on) snubber. However, elimination of this snubber in high-power, quasi-zero impedance source
(voltage-fed) converters may not be desirable because of the other benefits the snubber offers. These
include elimination of damage due to cross-conduction of bridge switches (“shoot-through”), or load
short-circuiting, and improved fault management. If the rate of rise of current in a fault condition is
controlled, a fast device such as the (I)GCT, ETO, or IGBT can respond in time to turn off the fault
current with the semiconductor switches. For GTO systems, the GTO could not respond in time to
interrupt a fault current, so the protection commonly used was to turn all the bridge switches on and
wait for fuses to open. The ability of the ETO and IGBT to automatically detect and respond to overcurrents enhances the safe operation of high-power systems. In addition, the IGBT can self-limit the current
that will be conducted, so operation within the switching capability of the device’s can be ensured.
Thyristor devices will conduct an extremely high surge current that is much higher than their interrupting
capability, which requires control logic to prevent the devices from switching off during this time.
Comparison Conclusions
As can be seen from the switching times, all of the devices tested here offer very fast switching times
relative to their power ratings. In addition, even the worst conduction loss from the IGBT is still acceptable
when compared with the blocking voltage. For very high power systems, the IGCT, the GCT, the
ETO4045A, and the ETO4060s are capable of handling extremely high power levels. The GCT is very
fast for its high rating, and the only drawback is the difficult to construct gate driver and its power
consumption. The ABB IGCT and the ETO4045A trade away switching loss to reduce driver power and
conduction loss, so these devices are particularly suited to advanced topologies that reduce the necessary
switching frequency or to soft-switching applications that can reduce the switching loss. The ETO4060
offers very high ratings with minimal driving power, even though the switching is not quite as good as
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the GCT; however, it is better than the IGCT. The IGBT offers the best switching speed and loss of any
of the devices tested and the simplest drive. However, the GCT and small ETO are amazingly close to
the IGBT in switching loss considering their latching nature and nearly 50% higher voltage rating. The
performance of all devices tested here is very good, especially compared with the conventional GTO
applications.
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References
Pa
kw
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1. K. Motto, Y. Li, and A.Q. Huang, Comparison of the state-of-the-art in high power IGBTs, IGCTs,
and ETOs, in Conf. Rec. IEEE-APEC, 2000, 1129–1136.
2. H.-G. Lee, Y.-H. Lee, B.-S. Suh, and D.-S. Hyun, An improved gate control scheme for snubberless
operation of high power IGBTs, in Conf. Rec. IEEE-IAS, 1997, 975–982.
3. Y. Li, A.Q. Huang, and K. Motto, Experimental and numerical study of the Emitter Turn-Off thyristor
(ETO), IEEE Trans. Power Electron., 15(3), 2000, 561–574.
4. Z. Xu, Y. Bai, Y. Li, and A.Q. Huang, Experimental demonstration of the forward biased safe operation
area of the emitter turn-off thyristor, in Proc. CPES-VT Seminar, 2000, 448–455.
5. S. Bernet, R. Teichmann, A. Zuckerberger, and P. Steimer, Comparison of high power IGBTs and hard
driven GTOs for high power inverters, in Conf. Rec. IEEE-APEC, 1998, 711–718.
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II
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Power Electronic
Circuits and
Controls
2 DC-DC Converters Richard Wies, Bipin Satavalekar, Ashish Agrawal,
Javad Mahdavi, Ali Agah, Ali Emadi, Daniel Jeffrey Shortt
Overview • Choppers • Buck Converters • Boost Converters •
Cúk Converter • Buck–Boost Converters
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3 AC-AC Conversion SГЎndor HalГЎsz
Introduction • Cycloconverters • Matrix Converters
4 Rectifiers Sam Guccione, Mahesh M. Swamy, Ana Stankovic
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Uncontrolled Single-Phase Rectifiers • Uncontrolled and Controlled Rectifiers • ThreePhase Pulse-Width-Modulated Boost-Type Rectifiers
5 Inverters Michael Giesselmann, Attila Karpati, IstvГЎn Nagy, Dariusz Czarkowski,
Michael E. Ropp
Overview • DC-AC Conversion • Resonant Converters • Series-Resonant
Inverters • Resonant DC-Link Inverters • Auxiliary Resonant Commutated Pole Inverters
6 Multilevel Converters
Keith Corzine
Introduction • Multilevel Voltage Source Modulation • Fundamental Multilevel Converter
Topologies • Cascaded Multilevel Converter Topologies • Multilevel Converter Laboratory
Examples • Conclusions
7 Modulation Strategies
Tahmid Ur Rahman
Michael Giesselmann, Hossein Salehfar, Hamid A. Toliyat,
Pa
Introduction • Six-Step Modulation • Pulse Width Modulation • Third Harmonic Injection
for Voltage Boost of SPWM Signals • Generation of PWM Signals Using Microcontrollers
and DSPs • Voltage Source–Based Current Regulation • Hysteresis Feedback Control •
Space-Vector Pulse Width Modulation
8 Sliding-Mode Control of Switched-Model Power Supplies Giorgio Spiazzi,
Paolo Mattavelli
Introduction • Introduction to Sliding-Mode Control • Basics of Sliding-Mode Theory •
Application of Sliding-Mode Control to DC-DC Converters—Basic Principle • Sliding-Mode
Control of Buck DC-DC Converters • Extension to Boost and Buck–Boost DC-DC Converters •
Extension to Cúk and SEPIC DC-DC Converters • General-Purpose Sliding-Mode Control
Implementation • Conclusions
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2
2.1
One-Quadrant Choppers • Two-Quadrant Choppers •
Four-Quadrant Choppers
University of Alaska Fairbanks
2.3
Javad Mahdavi
2.4
Sharif University of Technology
2.5
Cedarville University
2.6
Buck–Boost Converters
Circuit-Analysis • Small Signal Transfer Functions •
Component Selection • Flyback Power Stage •
Summary • References
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Daniel Jeffrey Shortt
CГєk Converter
Nonisolated Operation • Practical Cúk Converter •
References
Ali Emadi
Illinois Institute of Technology
Boost Converters
Ideal Boost Circuit • Continuous-Conduction
Mode • Discontinuous-Conduction Mode • References
Sharif University of Technology
Ali Agah
Buck Converters
Ideal Buck Circuit • Continuous-Conduction
Mode • Discontinuous-Conduction Mode • References
University of Alaska Fairbanks
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Ashish Agrawal
Choppers
tc
2.2
Bipin Satavalekar
Overview
References
University of Alaska Fairbanks
fo
Richard Wies
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DC-DC Converters
2.1 Overview
Richard Wies, Bipin Satavalekar, and Ashish Agrawal
Pa
The purpose of a DC-DC converter is to supply a regulated DC output voltage to a variable-load resistance
from a fluctuating DC input voltage. In many cases the DC input voltage is obtained by rectifying a line
voltage that is changing in magnitude. DC-DC converters are commonly used in applications requiring
regulated DC power, such as computers, medical instrumentation, communication devices, television
receivers, and battery chargers [1, 2]. DC-DC converters are also used to provide a regulated variable
DC voltage for DC motor speed control applications.
The output voltage in DC-DC converters is generally controlled using a switching concept, as illustrated
by the basic DC-DC converter shown in Fig. 2.1. Early DC-DC converters were known as choppers with
silicon-controlled rectifiers (SCRs) used as the switching mechanisms. Modern DC-DC converters classified as switch mode power supplies (SMPS) employ insulated gate bipolar transistors (IGBTs) and metal
oxide silicon field effect transistors (MOSFETs).
The switch mode power supply has several functions [3]:
1. Step down an unregulated DC input voltage to produce a regulated DC output voltage using a
buck or step-down converter.
2. Step up an unregulated DC input voltage to produce a regulated DC output voltage using a boost
or step-up converter.
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S
+
+
Vi
Vo
Basic DC-DC converter.
vo,i
t on
t off
Vo
t
do
FIGURE 2.2 DC-DC converter voltage waveforms.
(From Mohan, N., Undeland, T. M., and Robbins, W. P.,
Power Electronics: Converters, Applications, and Design,
2nd ed., John Wiley & Sons, New York, 1995. With permission from John Wiley & Sons.)
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Vi
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FIGURE 2.1
R
Ts
ON
ON
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ON
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vrepetitive
OFF
OFF
vcontrol
Vrepetitive
t
Ts
FIGURE 2.3 Pulsewidth modulation concept. (From Mohan, N., Undeland, T. M., and Robbins, W. P., Power
Electronics: Converters, Applications, and Design, 2nd ed., John Wiley & Sons, New York, 1995. With permission from
John Wiley & Sons.)
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3. Step down and then step up an unregulated DC input voltage to produce a regulated DC output
voltage using a buck–boost converter.
4. Invert the DC input voltage using a CГєk converter.
5. Produce multiple DC outputs using a combination of SMPS topologies.
The regulation of the average output voltage in a DC-DC converter is a function of the on-time ton of the
switch, the pulse width, and the switching frequency fs as illustrated in Fig. 2.2. Pulse width modulation
(PWM) is the most widely used method of controlling the output voltage. The PWM concept is illustrated
in Fig. 2.3. The output voltage control depends on the duty ratio D. The duty ratio is defined as
t on
v control
- = ----------------D = ----Ts
V repetitive
(2.1)
based on the on-time ton of the switch and the switching period Ts . PWM switching involves comparing
the level of a control voltage vcontrol to the level of a repetitive waveform as illustrated in Fig. 2.3 [2]. The
on-time of the switch is defined as the portion of the switching period where the value of the repetitive
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waveform is less than the control voltage. The switching period (switching frequency) remains constant
while the control voltage level is adjusted to change the on-time and therefore the duty ratio of the switch.
The switching frequency is usually chosen above 20 kHz so the noise is outside the audio range [2, 3].
DC-DC converters operate in one of two modes depending on the characteristics of the output current
[1, 2]:
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1. Continuous conduction
2. Discontinuous conduction
tc
The continuous-conduction mode is defined by continuous output current (greater than zero) over the
entire switching period, whereas the discontinuous conduction mode is defined by discontinuous output
current (equal to zero) during any portion of the switching period. Each mode is discussed in relationship
to the buck and boost converters in subsequent sections.
References
2.2 Choppers
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1. Agrawal, J. P., Power Electronics Systems: Theory and Design, Prentice-Hall, Upper Saddle River, NJ,
2001, chap. 6.
2. Mohan, N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and
Design, 2nd ed., John Wiley & Sons, New York, 1995, chap. 7.
3. Venkat, R., Switch Mode Power Supply, University of Technology, Sydney, Australia, March 1, 2001,
available at http://www.ee.uts.edu.au/~venkat/pe_html/pe07_nc8.htm.
kw
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Javad Mahdavi, Ali Agah, and Ali Emadi
Choppers are DC-DC converters that are used for transferring electrical energy from a DC source into
another DC source, which may be a passive load. These converters are widely used in regulated switching
power supplies and DC motor drive applications.
DC-DC converters that are discussed in this section are one-quadrant, two-quadrant, and four-quadrant
choppers. Step-down (buck) converter and step-up (boost) converters are basic one-quadrant converter
topologies. The two-quadrant chopper, which, in fact, is a current reversible converter, is the combination
of the two basic topologies. The full-bridge converter is derived from the step-down converter.
One-Quadrant Choppers
Pa
In one-quadrant choppers, the average DC output voltage is usually kept at a desired level, as there are
fluctuations in input voltage and output load. These choppers operate only in first quadrant of v–i plane.
In fact, output and input voltages and currents are always positive. Therefore, these converters are called
one-quadrant choppers.
One method of controlling the output voltage employs switching at a constant frequency, i.e., a constant
switching time period (T = ton + toff ), and adjusting the on-duration of the switch to control the average
output voltage. In this method, which is called pulse-width modulation (PWM), the switch duty ratio d
is defined as the ratio of the on-duration to the switching time period.
t on
d = ----T
(2.2)
In the other control method, both the switching frequency and the on-duration of the switch are
varied. This method is mainly used in converters with force-commutated thyristors.
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iin
iL
S
L
+
+
Vin
vD
Vo
D
-
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FIGURE 2.4
-
Step-down buck converter.
tc
Choppers can have two distinct modes of operation, which have significantly different characteristics:
continuous-conduction and discontinuous-conduction modes. In practice, a converter may operate in
both modes. Therefore, converter control should be designed for both modes of operation.
Step-Down (Buck) Converter
T
1
v o ( t ) dt = --- пЈ«
пЈ­
T
0
∫
t on
∫
V in dt +
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in
1
v o, ave. = --T
fo
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A step-down converter produces an average output voltage, which is lower than the DC input voltage
Vin. The basic circuit of a step-down converter is shown in Fig. 2.4.
In continuous-conduction mode of operation, assuming an ideal switch, when the switch is on for
the time duration ton, the inductor current passes through the switch, and the diode becomes reversebiased. This results in a positive voltage (Vin в€’ Vo) across the inductor, which, in turn, causes a linear
increase in the inductor current iL. When the switch is turned off, because of the inductive energy storage,
iL continues to flow. This current flows through the diode and decreases. Average output voltage can be
calculated in terms of the switch duty ratio as:
0
T
∫
t on
t on
- V = dV in
0. 0пЈ¶ = ----пЈё
T in
(2.3)
v o, ave. can be controlled by varying the duty ratio (d = ton/T) of the switch. Another important observation is that the average output voltage varies linearly with the control voltage. However, in the
discontinuous-conduction mode of operation, the linear relation between input and output voltages
is not valid. Figure 2.5 shows (v o, ave. /v in, ave. ) – i o, ave. characteristic of a step-down converter in continuous and discontinuous conduction modes of operation.
Step-Up (Boost) Converter
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Schematic diagram of a step-up boost converter is shown in Fig. 2.6. In this converter, the output voltage
is always greater than the input voltage. When the switch is on, the diode is reversed-biased, thus isolating
the output stage. The input voltage source supplies energy to the inductor. When the switch is off, the
output stage receives energy from the inductor as well as the input source.
In the continuous-conduction mode of operation, considering d as the duty ratio, the input–output
relation is as follows:
1
v o, ave. = ----------- V in
1–d
(2.4)
If input voltage is not constant, Vin is the average of the input voltage. In this case, relation (2.3) is an
approximation. In the discontinuous-conduction mode of operation, relation (2.3) is not valid. Figure 2.7
shows (v in, ave. /v o, ave. ) – i L, ave. characteristic of a step-up converter in the continuous- and discontinuousconduction modes of operation.
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vo,ave.
vin,ave.
Continuous conduction
1
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d = 0.75
d = 0.5
Discontinuous
conduction
io,ave.
do
( v o,ave. /v in, ave. ) – i o,ave. characteristic of a step-down converter.
FIGURE 2.6
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in
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FIGURE 2.5
Vin
8Lf
tc
0
d = 0.25
Step-up boost converter.
vin,ave.
vo,ave.
Continuous conduction
Pa
1
Discontinuous
conduction
d = 0.25
d = 0.5
d = 0.75
0
FIGURE 2.7
vo,ave.
8Lf
iL,ave.
(v in, ave. /v o, ave. ) – i L, ave. characteristic of a step-down converter.
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D2
iin
io
S1
L
+
+
v
D1
-
-
A current reversible chopper.
tc
FIGURE 2.8
io
do
T
d1 T
d2 T
D1
S2
kw
eb
in
FIGURE 2.9
S1
t
fo
0
D2
Vo
S2
om
Vin
Output current of a two-quadrant chopper.
Two-Quadrant Choppers
Pa
A two-quadrant chopper has the ability to operate in two quadrants of the (v–i) plane. Therefore, input
and output voltages are positive; however, input and output currents can be positive or negative. Thus,
these converters are also named current reversible choppers. They are composed of two basic chopper
circuits. In fact, a two-quadrant DC-DC converter is achieved by a combination of two basic chopper
circuits, a step-down chopper and a step-up chopper, as is shown in Fig. 2.8.
The step-down chopper is composed of S1 and D1, and electric energy is supplied to the load. The
step-up chopper is composed of S2 and D2; electric energy is fed back to the source. Reversible current
choppers can transfer from operating in the power mode to operating in the regenerative mode very
smoothly and quickly by changing only the control signals for S1 and S2 , without using any mechanical
contacts.
Figure 2.9 depicts the output current of a two-quadrant chopper. d1 and d2 = 1 в€’ d1 are the duty ratios
of step-down and step-up converters, respectively. By changing d1 and d2 , not only the amplitude of the
average of the output current changes, but it can also be positive and negative, leading to two-quadrant
operation.
For each of step-down and step-up operating mode, relations (2.3) and (2.4) are applicable for
continuous currents. However, in discontinuous-conduction modes of operation, relations (2.3) and
(2.4) are not valid. Figure 2.10 shows the (v o, ave. /v in, ave. ) – i o, ave. characteristic of a two-quadrant converter in continuous- and discontinuous-conduction modes of operation. As is shown in Fig. 2.10, for
changing the operating mode both from step-down to step-up operation and in the opposite direction,
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vo ,ave.
vin ,ave.
d1 = 0.75
d 2 = 0.5
d1 = 0.5
om
d 2 = 0.25
d 2 = 0.75
Vin
8Lf
Vin
8 Lf
io ,ave.
tc
в€’
d1 = 0.25
D1
do
FIGURE 2.10 (v o, ave. /v in, ave. ) – i o, ave. characteristic of a two-quadrant converter.
S1
Vin
fo
io
+
FIGURE 2.11
S4
Vo
S3
D2
S2
-
kw
eb
in
D4
D3
A full-bridge four-quadrant chopper.
the operating mode must move from the discontinuous-current region. However, by applying d2 = 1 в€’
d1, the operating point will never move into the discontinuous-conduction region of the two basic
converters. In Fig. 2.10, the broken lines indicate passage from step-down operation to step-up operation,
and vice versa. In fact, because of this specific command—the relation between the two duty ratios—the
converter operating point always stays in the continuous-conduction mode.
Pa
Four-Quadrant Choppers
In four-quadrant choppers, not only can the output current be positive and negative, but the output
voltage also can be positive and negative. These choppers are full-bridge DC-DC converters, as is shown
in Fig. 2.11. The main advantage of these converters is that the average of the output voltage can be
controlled in magnitude as well as in polarity. A four-quadrant chopper is a combination of two twoquadrant choppers in order to achieve negative average output voltage and/or negative average output
current.
The four-quadrant operation of the full-bridge DC-DC converter, as shown in Fig. 2.12, for the first
two quadrants of the (v–i) plane is achieved by switching S1 and S2 and considering D1 and D2 like a
two-quadrant chopper. For the other two quadrants of the (v–i) plane, the operation is achieved by
switching S3 and S4 and considering D3 and D4 as another two-quadrant chopper, which is connected to
the load in the opposite direction of the first two-quadrant chopper.
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v o , ave.
пЈ±пЈґv o , ave. > 0
пЈІ
пЈґпЈіio , ave. < 0
om
пЈ±пЈґv o , ave. > 0
пЈІ
пЈґпЈіio , ave. > 0
io , ave.
Four-quadrant operation of a full-bridge chopper.
2.3 Buck Converters
do
FIGURE 2.12
пЈ±пЈґv o , ave. < 0
пЈІ
пЈґпЈіio , ave. > 0
tc
пЈ±пЈґv o , ave. < 0
пЈІ
пЈґпЈіio , ave. < 0
Richard Wies, Bipin Satavalekar, and Ashish Agrawal
kw
eb
in
fo
The buck or step-down converter regulates the average DC output voltage at a level lower than the input
or source voltage. This is accomplished through controlled switching where the DC input voltage is
turned on and off periodically, resulting in a lower average output voltage [1]. The buck converter is
commonly used in regulated DC power supplies like those in computers and instrumentation [1, 2].
The buck converter is also used to provide a variable DC voltage to the armature of a DC motor for
variable speed drive applications [2].
Ideal Buck Circuit
Pa
The circuit that models the basic operation of the buck converter with an ideal switch and a purely
resistive load is shown in Fig. 2.13. The output voltage equals the input voltage when the switch is in
position 1 and the output voltage is zero when the switch is in position 2. The resulting output voltage
is a rectangular voltage waveform with an average value as shown in Fig. 2.2 (in Section 2.1). The average
output voltage level is varied by adjusting the time the switch is in position 1 and 2 or the duty ratio.
The resulting average output voltage Vo is given in terms of the duty ratio and the input voltage Vi by
Eq. (2.5) [2].
Vo = DVi
(2.5)
The square wave output voltage for the ideal circuit of the buck converter contains an undesirable
amount of voltage ripple. The circuit is modified by adding an inductor L in series and a capacitor C in
parallel with the load resistor as shown in Fig. 2.14. The inductor reduces the ripple in the current through
S 1
+
Vi
FIGURE 2.13
+
2
R
Vo
Ideal buck converter.
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L
S 1
FIGURE 2.14 Modified buck converter with LC filter.
(From Mohan, N., Undeland, T. M., and Robbins, W. P.,
Power Electronics: Converters, Applications, and Design,
2nd ed., John Wiley & Sons, New York, 1995. With permission from John Wiley & Sons.)
+
2
+
+
C
tc
io
S
1
fo
do
rise
FIGURE 2.15 Rise and fall of load current in buck
converter.
2
+
Vi
Vo
fall
t
L
+
D
+
C
R
Vo
Buck converter with practical switch.
kw
eb
in
FIGURE 2.16
R
om
Vi
Pa
the load resistor, while the capacitor directly reduces the ripple in the output voltage. Since the current
through the load resistor is the same as that of the inductor, the voltage across the load resistor (output
voltage) contains less ripple.
The current through the inductor increases with the switch in position 1. As the current through the
inductor increases, the energy stored in the inductor increases. When the switch changes to position 2,
the current through the load resistor decreases as the energy stored in the inductor decreases. The rise
and fall of current through the load resistor is linear if the time constant due to the LR combination is
relatively large compared with the on- and off-time of the switch as shown in Fig. 2.15 [3]. A capacitor
is added in parallel with the load resistor to reduce further the ripple content in the output voltage. The
combination of the inductor and capacitor reduces the output voltage ripple to very low levels.
The circuit in Fig. 2.14 is designed assuming that the switch is ideal. A practical model of the switch is
designed using a diode and power semiconductor switch as shown in Fig. 2.16. A freewheeling diode is
used with the switch in position 2 since the inductor current freewheels through the switch. The switch
is controlled by a scheme such as pulse width or frequency modulation.
Continuous-Conduction Mode
The continuous-conduction mode of operation occurs when the current through the inductor in the
circuit of Fig. 2.14 is continuous. This means that the inductor current is always greater than zero. The
average output voltage in the continuous-conduction mode is the same as that derived in Eq. (2.5) for
the ideal circuit. As the conduction of current through the inductor occurs during the entire switching
period, the average output voltage is the product of the duty ratio and the DC input voltage. The operation
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L
L
+
+
Vi
D
C
R
+
+
+
Vo
+
C
D
Vi
(a)
Vo
R
om
(b)
tc
FIGURE 2.17 Buck converter switch states: (a) switch in position 1; (b) switch in position 2. (From Mohan, N.,
Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John Wiley &
Sons, New York, 1995. With permission from John Wiley & Sons.)
vL
Vo
do
Vi
t
kw
eb
in
fo
FIGURE 2.18 Inductor voltage and current for continuous mode of buck converter. (From Mohan, N., Undeland, T. M., and Robbins, W. P., Power Electronics:
Converters, Applications, and Design, 2nd ed., John Wiley
& Sons, New York, 1995. With permission from John
Wiley & Sons.)
IL
iL
t on
t off
Vo
of this circuit resembles a DC transformer according to Eq. (2.6) based on the time-integral of the
inductor voltage equal to zero over one switching period [2].
I
V
D = -----o = ---i
Vi
Io
(2.6)
Pa
The operation of the circuit in steady state consists of two states as illustrated in Fig. 2.17 [2, 4]. The
first state with the switch in position 1 has the diode reverse-biased and current flows through the inductor
from the voltage source to the load. The switch changes to position 2 at the end of the on-time and the
inductor current then freewheels through the diode. The process starts again at the end of the switching
period with the switch returning to position 1. A representative set of inductor voltage and current
waveforms for the continuous-conduction mode is shown in Fig. 2.18.
Discontinuous-Conduction Mode
The discontinuous mode of operation occurs when the value of the load current is less than or equal to
zero at the end of a given switching period. Assuming a linear rise and fall of current through the inductor,
the boundary point between continuous- and discontinuous-current conduction occurs when the average
inductor current over one switching period is half of the peak value, as illustrated in Fig. 2.19. The average
inductor current at the boundary point is calculated using Eq. (2.7) [2].
DT
1
I LB = -- i L ( peak ) = ---------s (V i – V o)
2
2L
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(2.7)
vL
Vo
Vi
iL
om
ILB
tc
FIGURE 2.19 Inductor current at boundary point for
discontinuous mode of buck converter. (From Mohan,
N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John
Wiley & Sons, New York, 1995. With permission from
John Wiley & Sons.)
t on
t off
t
Vo
do
The input voltage or output voltage is kept constant depending on the application. If the input voltage
remains constant, then the average inductor current at the boundary is calculated by replacing the output
voltage in Eq. (2.7) with Eq. (2.5), which yields the expression in Eq. (2.8) [2].
fo
DT
I LB = ---------s (V i ) (1 – D )
2L
(2.8)
The voltage ratio is now defined according to Eq. (2.9) [2]:
kw
eb
in
2
V
D
-----o = --------------------------------------Vi
Io пЈ¶
1
2
D + -- пЈ« --------------4 пЈ­ I LB ( max )пЈё
(2.9)
If the output voltage remains constant, then the average inductor current at the boundary is calculated
by replacing the input voltage in Eq. (2.7) with Eq. (2.5), which yields the expression in Eq. (2.10) [2]:
T
I LB = ------s ( V o ) (1 – D )
2L
(2.10)
Pa
The duty ratio is defined according to Eq. (2.11) by manipulating Eq. (2.9) [2]:
1
пЈ«
пЈ¶ --2
V пЈ¬ I o /I LB ( max )пЈ·
D = -----o пЈ¬ ---------------------пЈ·
Vi пЈ¬
oпЈ¶ пЈ·
пЈ«V
----–
1
пЈ­
пЈ­ ViпЈё пЈё
(2.11)
Output Voltage Ripple
In DC-DC converters the output voltage ripple is a measure of the deviation in the output voltage from
the average value. The peak-to-peak voltage ripple for the buck converter in Figure 2.16 for the continuous
conduction mode can be calculated for a specified value of output capacitance by calculating the additional charge ∆Q provided by the ripple current in the inductor. This analysis assumes that all of the
ripple current flows through the capacitor, while the average value of the inductor current flows through
the load resistor. The peak-to-peak voltage ripple is calculated by taking the area under the inductor
current iL (the additional charge ∆Q) and dividing by the capacitance resulting in Equation 2.12 [2]:
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В© 2002 by CRC Press LLC
T V
1 1 ∆I T
∆Q
∆V o = -------- = --- -- --------L -----s = ------s- -----o ( 1 – D )T s
C2 2 2
8C L
C
(2.12)
It is customary to refer to ripple in terms of the percentage ripple as illustrated in Equation 2.13 [2]:
2
(2.13)
om
2
f 2
∆V o
ПЂ
1 Ts ( 1 – D )
- = ----- ( 1 – D )  ---c
--------- = -- ----------------------пЈ­ f sпЈё
2
8 LC
Vo
where fs is the switching frequency and fc is the corner frequency of the low-pass LC filter on the output.
The voltage ripple is minimized by selecting a corner frequency for the lowpass filter which is much less
than the switching frequency.
tc
References
2.4 Boost Converters
fo
do
1. Agrawal, J. P., Power Electronics Systems: Theory and Design, Prentice-Hall, Upper Saddle River, NJ,
2001, chap. 6.
2. Mohan, N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and
Design, 2nd ed., John Wiley & Sons, New York, 1995, chap. 7.
3. Hoft, R. G., Semiconductor Power Electronics, Van Nostrand Reinhold, New York, 1986, chap. 5.
4. Venkat, R., Switch Mode Power Supply, University of Technology, Sydney, Australia, 01 March 2001,
available at http://www.ee.uts.edu.au/~venkat/pe_html/pe07_nc8.htm.
kw
eb
in
Richard Wies, Bipin Satavalekar, and Ashish Agrawal
A boost converter regulates the average output voltage at a level higher than the input or source voltage.
For this reason the boost converter is often referred to as a step-up converter or regulator. The DC input
voltage is in series with a large inductor acting as a current source. A switch in parallel with the current
source and the output is turned off periodically, providing energy from the inductor and the source to
increase the average output voltage. The boost converter is commonly used in regulated DC power supplies
and regenerative braking of DC motors [1, 2].
Ideal Boost Circuit
Pa
The circuit that models the basic operation of the boost converter is shown in Fig. 2.20 [2, 3]. The ideal
boost converter uses the same components as the buck converter with different placement. The input
voltage in series with the inductor acts as a current source. The energy stored in the inductor builds up
when the switch is closed. When the switch is opened, current continues to flow through the inductor
to the load. Since the source and the discharging inductor are both providing energy with the switch
open, the effect is to boost the voltage across the load. The load consists of a resistor in parallel with a
filter capacitor. The capacitor voltage is larger than the input voltage. The capacitor is large to keep a
constant output voltage and acts to reduce the ripple in the output voltage.
Continuous-Conduction Mode
The continuous-conduction mode of operation occurs when the current through the inductor in the
circuit of Fig. 2.20 is continuous with the inductor current always greater than zero. The operation of
the circuit in steady state consists of two states, as illustrated in Fig. 2.21 [2, 3]. The first state with the
switch closed has current charging the inductor from the voltage source. The switch opens at the end
of the on-time and the inductor discharges current to the load with the input voltage source still
connected. This results in an output voltage across the capacitor larger than the input voltage. The output
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В© 2002 by CRC Press LLC
D
L
FIGURE 2.20 Basic boost converter. (From Mohan, N.,
Undeland, T. M., and Robbins, W. P., Power Electronics:
Converters, Applications, and Design, 2nd ed., John Wiley
& Sons, New York, 1995. With permission from John
Wiley & Sons.)
+
S C
Vi
L
D
+
+
+
+
C
R
Vo
Vo
D
+
+
C
Vi
Vo
R
tc
Vi
R
om
L
+
+
(b)
do
(a)
fo
FIGURE 2.21 Basic boost converter switch states: (a) switch closed; (b) switch open. (From Mohan, N., Undeland,
T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John Wiley & Sons, New
York, 1995. With permission from John Wiley & Sons.)
kw
eb
in
Vi
FIGURE 2.22 Inductor voltage and current waveforms
for continuous mode of boost converter. (From Mohan,
N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John
Wiley & Sons, New York, 1995. With permission from
John Wiley & Sons.)
vL
IL
iL
t
t on
t off
Vi Vo
Pa
voltage remains constant if the RC time constant is significantly larger than the on-time of the switch.
A representative set of inductor voltage and current waveforms for the continuous conduction mode is
shown in Fig. 2.22 [2].
The voltage ratio for a boost converter is derived based on the time-integral of the inductor voltage
equal to zero over one switching period. The voltage ratio is equivalent to the ratio of the switching
period to the off-time of the switch as illustrated by Eq. (2.14) [2].
Vo
I
T
Ts
T
----- = ---i = ------s = ---------------- = ------------Vi
Io
t off
T s – t off
1–D
(2.14)
The current ratio is derived from the voltage ratio assuming that the input power is equal to the output
power, as with ideal transformer analysis.
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vL
om
iL
t on
t
t off
do
Discontinuous-Conduction Mode
tc
FIGURE 2.23 Inductor current at boundary point for
discontinuous mode of boost converter. (From Mohan,
N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John
Wiley & Sons, New York, 1995. With permission from
John Wiley & Sons.)
ILB
fo
The discontinuous mode of operation occurs when the value of the load current is less than or equal to
zero at the end of a given switching period. Assuming a linear rise and fall of current through the inductor,
the boundary point between continuous- and discontinuous-current conduction occurs when the average
inductor current over one switching period is half the peak value, as illustrated in Fig. 2.23 [2]. The average
inductor current at the boundary point is calculated using Eq. (2.15) [2].
kw
eb
in
Vo Ts
1
- D(1 – D)
I LB = -- i L ( peak ) = ---------2L
2
(2.15)
The output current at the boundary condition is derived by using the current ratio of Eq. (2.14) in Eq. (2.15)
with the inductor current equal to the input current. This results in Eq. (2.16) [2]:
Vo Ts
2
- D(1 – D)
I OB = ---------2L
(2.16)
Pa
For the boost converter in discontinuous mode, the output voltage Vo is generally kept constant while
the duty ratio D varies in response to changes in the input voltage Vi.
The duty ratio is defined as a function of the output current for various values of the voltage ratio
according to Eq. (2.17) [2]:
Io
4 V V
D = ----- -----o  -----o – 1 --------------
пЈё
I oB ( max )
27 V i V i
1
-2
(2.17)
Output Voltage Ripple
The peak-to-peak voltage ripple for the boost converter in Figure 2.20 for the continuous conduction
mode can be calculated for a specified value of output capacitance by calculating the additional charge
∆Q provided by the ripple current in the inductor. This analysis is similar to that discussed for the buck
converter. The peak-to-peak voltage ripple is calculated by taking the area under the inductor current iL
(the additional charge ∆Q) and dividing by the capacitance resulting in Equation 2.18 [2]:
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V DT
I o DT s
∆Q
= -----o ---------s
∆V o = -------- = ------------R C
C
C
(2.18)
The percentage output voltage ripple is calculated as in Equation 2.19 [2]:
T
∆V o
DT
--------- = ---------s = D -----s
П„
Vo
RC
om
(2.19)
where П„ is the RC time constant of the output filter. The voltage ripple is minimized by increasing the
time constant of the output filter.
tc
References
do
1. Agrawal, J. P., Power Electronics Systems: Theory and Design, Prentice-Hall, Upper Saddle River, NJ,
2001, chap. 6.
2. Mohan, N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and
Design, 2nd ed., John Wiley & Sons, New York, 1995, chap. 7.
3. Venkat, R., Switch Mode Power Supply, University of Technology, Sydney, Australia, 01 March 2001,
available at http://www.ee.uts.edu.au/~venkat/pe_html/pe07_nc8.htm.
fo
2.5 CГєk Converter
Richard Wies, Bipin Satavalekar, and Ashish Agrawal
kw
eb
in
The CГєk converter is a switched-mode power supply named after the inventor Dr. Slobodan CГєk. The
basic nonisolated CГєk converter shown in Fig. 2.24 is designed based on the principle of using two
buck–boost converters to provide an inverted DC output voltage [1]. The advantage of the basic nonisolated Cúk converter over the standard buck–boost converter is to provide regulated DC output voltage
at higher efficiency with identical components due to an integrated magnetic structure, reduced ripple
currents, and reduced switching losses [2, 3]. The integrated magnetic structure of the isolated CГєk
converter consists of the isolation transformer and the two inductors in a single core. As a result, the
ripple currents in the inductors are driven into the primary and secondary windings of the isolation
transformer. Also, the single core results in reduced flux paths, which improves the overall efficiency of
the converter.
Nonisolated Operation
Pa
The basic nonisolated CГєk converter is a switching power supply with two inductors, two capacitors, a
diode, and a transistor switch as illustrated in Fig. 2.24 [1, 2]. The transfer capacitor Ct stores and transfers
energy from the input to the output. The average value of the inductor voltages for steady-state operation
is zero. As a result, the voltage across the transfer capacitor is assumed to be the average value V Ct in
steady state and is the sum of the input and output voltages. The inductor currents are assumed to be
continuous for steady-state operation.
L1
FIGURE 2.24 Nonisolated CГєk converter. (From
Mohan, N., Undeland, T. M., and Robbins, W. P., Power
Electronics: Converters, Applications, and Design, 2nd ed.,
John Wiley & Sons, New York, 1995. With permission
from John Wiley & Sons.)
+
Ct
L2
+
+
+
D
Vi
C
R
Vo
Q
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+ VCt
L1
+
Ct
VCt +
L2
L1
Ct
L2
+
+
+
+
+
+
C
D
R
Vo
D
Vi
C
Vo
R
om
Vi
+
(a)
(b)
tc
FIGURE 2.25 CГєk converter switch states: (a) switch open; (b) switch closed. (From Mohan, N., Undeland, T. M.,
and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John Wiley & Sons, New York,
1995. With permission from John Wiley & Sons.)
do
VL1
Vi
fo
iL1
kw
eb
in
FIGURE 2.26 Inductor 1, voltage and current waveforms
for CГєk converter. (From Mohan, N., Undeland, T. M.,
and Robbins, W. P., Power Electronics: Converters, Applications, and Design, 2nd ed., John Wiley & Sons, New
York, 1995. With permission from John Wiley & Sons.)
t
Vo
t off
t on
Pa
The operation of the basic nonisolated CГєk converter in steady state consists of two transistor states,
as illustrated in Fig. 2.25 [1, 2]. In the first state when the transistor is off, the inductor currents flow
through the diode and energy is stored in the transfer capacitor from the input and the inductor L1. The
energy stored in the inductor L2 is transferred to the output. As a result, both of the inductor currents are
linearly decreasing in the off-state. In the second state when the transistor is on, the inductor currents
flow through the transistor and the transfer capacitor discharges while energy is stored in the inductor L1.
As the transfer capacitor discharges through the transistor, energy is stored in the inductor L2. Consequently, both of the inductor currents are linearly increasing in the on-state. A representative set of inductor
voltage and current waveforms for the nonisolated CГєk converter are shown in Figs. 2.26 and 2.27 [1].
The voltage and current ratio for the nonisolated CГєk converter can be derived by assuming the
inductor currents, which correspond to the input current and output current, are ripple-free [1]. This results
in an equal charging and discharging of the transfer capacitor during the off-state and the on-state. The
charging and discharging are defined in Eq. (2.20) in terms of the product of current and time [1].
I L1 t off = I L2 t on
(2.20)
The resulting current ratio is expressed in Eq. (2.21) by substituting I L1 = I i , I L2 = I o , toff = (1 в€’ D)Ts ,
and ton = DTs into Eq. (2.20) [1].
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VL2
VCt Vo
om
iL2
FIGURE 2.27 Inductor 2, voltage and current waveforms for CГєk converter. (From Mohan, N., Undeland,
T. M., and Robbins, W. P., Power Electronics: Converters,
Applications, and Design, 2nd ed., John Wiley & Sons,
New York, 1995. With permission from John Wiley &
Sons.)
t
t on
do
Io
1–D
--- = ------------Ii
D
tc
Vo
t off
(2.21)
fo
If the input power is equal to the output power for the ideal case, the voltage ratio in Eq. (2.22) is
determined as the inverse of the current ratio using the analysis of an ideal transformer [1].
(2.22)
kw
eb
in
V
D
-----o = ------------Vi
1–D
Practical CГєk Converter
The advantages of the practical isolated CГєk converter discussed earlier are the integrated magnetic
structure, reduced ripple currents, and reduced switching losses. With the use of a single transformer to
provide isolation and the two inductors required in the circuit, the ripple in the inductor currents is
essentially reduced to zero. This reduces the amount of external filtering required, but the transfer
capacitor carries the ripple from both inductors. This requires a transfer capacitor with a large ripple
current capacity. For futher information and a more-detailed analysis of the practical CГєk converter, see
Refs. 2 and 3.
References
Pa
1. Mohan, N., Undeland, T. M., and Robbins, W. P., Power Electronics: Converters, Applications, and
Design, 2nd ed., John Wiley & Sons, New York, 1995, chap. 7.
2. TESLAco, CUKonverter Technology, 1996, 23 February 2001, available at http://www.teslaco.com/
inverter.htm.
3. CГєk, S. and Middlebrook, R. D., Advances in Switched-Mode Power Conversion, Vol. 1 and 2, TESLAco,
Pasadena, CA, 1981.
2.6 Buck–Boost Converters
Daniel Jeffrey Shortt
A schematic of the buck–boost converter circuit (in one of its simplest forms) is shown below in Fig. 2.28.
The main power switch is shown to be a bipolar transistor, but it could be a power MOSFET, or any
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FIGURE 2.28
Control
Control
Circuit
Circuit
L
C
fo
+
--
RL
vO
+
kw
eb
in
vS
do
tc
om
other device that could be turned on (and off) in a controlled fashion. This converter processes the power
from a DC-biased source (high-voltage ripple) to a DC output (containing low-voltage ripple). The DC
output voltage value can be chosen to be higher or lower than the input DC voltage. Note: The output
load is represented by a resistor, RL, but in real life can be something much more complicated. In a general
sense, this circuit processes power from input to output with “square wave” technology, that is, the circuit
produces waveforms that have sharp edges (such as those shown in Fig. 2.29). (There are converters that
develop sine waves and semi-sine waves in the power process. They will not be discussed here.) The
waveforms in Fig. 2.29 have a square-wave (or semi-square-wave) appearance and are indicative of current
waveforms in a typical DC-DC converter. In fact, the iL waveform is in a similar shape as the inductor
(L) current in the buck–boost converter of Fig. 2.28, iD can represent the diode current, and iC , the
capacitor current.
The operation of this converter is nonlinear and discrete; however, it can be represented by a cyclic
change of power stage topologies. The three topologies for this converter, the equations for those topologies,
and the small-signal transfer functions are presented in this section. For specific details of the derivation
of each of these items, see the technical articles and papers listed in the References.
Buck–boost converter.
iD
0
Pa
iC
0
iL
0
FIGURE 2.29
Typical current waveforms in a buck–boost converter.
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Circuit Analysis
The buck–boost converter has cyclic changes in topology due to the switching action of the semiconductor
devices. During a cycle of operation, the main power switch is turned on and off; the diode responds to
this by switching off and on.
Continuous-Current Mode
di L
v
------- = ----S
dt
L
(2.23)
(2.24)
do
dv
vO
-------C- = --------dt
RL C
tc
om
Figure 2.30 illustrates the topology where the main power switch is on and the diode is reverse-biased;
thus, it is off. For the purpose of illustration the semiconductor devices are assumed to be ideal.
There are two independent state variables that contain the information describing the operation of this
circuit: the inductor current, iL, and the capacitor voltage, vC . Two differential equations in terms of these
variables, the output voltage, vO , and the source voltage, vS , for the designated Topology 1 are shown below.
fo
Please note that the inductor is receiving energy from the source and being charged up, while the capacitor
is being discharged into the output load, RL, and the output voltage is falling.
Figure 2.31 shows the change in topology when the main power switch turns off. The inductor
maintains current flow in the same direction so that the diode is forward-biased. The differential
kw
eb
in
iL
vS
L
C
+
vC RL
-
vO
+
Topology 1 for the buck–boost converter.
Pa
FIGURE 2.30
+
-
vS
+
-
iL
-
+
L
C
vC
-
RL
vO
+
FIGURE 2.31
Topology 2 for the buck–boost converter.
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equations for the designated Topology 2 are shown below. Please note that the inductor is transferring
the energy it has obtained from the source into the capacitor; the capacitor is being charged up as the
inductor is being discharged, and the output voltage is rising.
di L
v
------- = – ----Cdt
L
(2.25)
om
i
vO
dv
-------C- = ---L + --------dt
C RL C
(2.26)
Pa
kw
eb
in
fo
do
tc
Another topology change will occur if the inductor has transferred all of its energy out into the capacitor.
In that case the inductor current will fall to zero. This will be examined later in the section. The inductor
current is assumed to be nonzero.
These four linear time-invariant differential equations describe the state of the buck–boost converter.
The power stage analysis is linear for each interval; however, for the complete operational cycle, it becomes
a piecewise linear problem. The on-time or off-time of the main power switch may vary from cycle to cycle,
further complicating the analysis.
Various modeling schemes have been proposed using nonlinear techniques that would in essence
“combine” these equations. Basically there are two approaches: numerical (universal) and analytical (mathematical) techniques [1, 2]. In analytical techniques, a closed-form expression representing the operation
of the converter is obtained, enabling a qualitative analysis to be performed [1]. The numerical techniques
use various algorithms to produce an accurate quantitative result. However, simple relations among the
system parameters are not easily obtainable. Numerical techniques are not to be considered at this time,
because the desire at this point is to obtain a closed-form solution from which a considerable amount of
design insight can be obtained.
Analytical techniques can be divided into two different system descriptions, discrete and continuous. The
discrete system description makes no simplifying assumption on the basis of converter application. This
description could be used in any application where the linearization of a periodically changing structure is
sought. This method is accurate, but very complicated. The derived expressions are complex and cumbersome,
which impedes its practical usefulness, and physical insight into the system operation is not easily obtainable.
An important continuous analytical technique is the averaging technique by Wester and Middlebrook [3].
It is easy to implement and gives physical insight into the operation of a buck–boost converter. Through
circuit manipulation, analytical expressions were derived to determine the appropriate expressions.
Middlebrook and CГєk [4, 5] modified the technique to average the state space descriptions (variables)
over a complete cycle. Shortt and Lee [6–8] used a discrete sample of the average state space representation
to develop a modeling technique that would enable a judicious control selection to be made. VorpГ©rian
et al. [9] developed an equivalent circuit model for a pulse width modulation (PWM) switch that can
be used in the analysis of this converter.
For the averaging technique each interval in the cycle is described by its state space representation (differential equation). Figure 2.32 shows the waveform of the continuous, instantaneous inductor current (that is,
T ON
0
T OFF
TOFF
Instantaneous Inductor Current
TP
FIGURE 2.32
TON
TP
TON
T OFF
Average Inductor Current
TP
Continuous inductor current.
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iL does not equal zero at any point in time) and the average inductor current for the buck–boost converter
(Fig. 2.30). The instantaneous current is cyclic with a time period equal to TP s; the main power switch
is on for TON s and off for TOFF s. The equations are averaged to give a single period representation, as
shown below:
v
v
i˙L = – d ′ ----C- + d ----S
L
L
om
(2.27)
i
vO
v˙C = d′ ---L + --------C RL C
(2.28)
do
tc
where iL and vC are average state variables, d = TON/TP and d′ = TOFF/TP . Please note: d + d′ = 1.
To study the small signal behavior, the time-varying system described in Eqs. (2.27) and (2.28) can
be linearized using small signal perturbation techniques. By using these techniques, the inputs are
assumed to vary around a steady-state operating point. Taking a first-order Fourier series approximation,
the inputs are represented by the sum of a DC or steady-state term and an AC variation or sinusoidal
term. Introducing variations in the line voltage and duty cycle by the following substitutions
d = D + dˆ ,
v S = V S + vˆ s,
d′ = D′ – dˆ
vЛ™C = VЛ™ C + vЛ™Л† C ,
kw
eb
in
Л™
i˙L = I˙L + iˆL ,
fo
cause perturbations in the state and output, as shown below. In the above and following equations the
variables in capital letters represent the DC or steady-state term; and the variables with the symbol “ ˆ ”
above them represent the AC variation or sinusoidal term.
i L = I L + iˆL ,
v C = V C + vˆ c ,
v O = V O + vˆ O
Figure 2.33 shows the type of change that is being modeled for an inductor current perturbation of
Fig. 2.32. Note the TON and TOFF slowly change from cycle to cycle, which produces a slight change in
the inductor current from cycle to cycle.
The derivative of a DC term is zero, so the above equations can be simplified to the following:
Л™
i˙L = iˆL ,
vЛ™C = vЛ™Л†C ,
i L = I L + iˆL ,
v C = V C + vˆ c ,
v O = V O + vˆ O
Pa
Substituting these equations into (2.27) and (2.28), separating the DC (steady-state) terms and the AC
(sinusoidal) terms results in the following:
TON
TOFF
TON
TOFF
Average Inductor Current
TON
TOFF
Instantaneous Inductor Current
0
TP
FIGURE 2.33
TP
TP
Inductor current perturbation.
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DC terms:
(2.29)
D′I L V O
---------- + ---------- = 0
C
RL C
(2.30)
AC terms (neglecting the higher-order terms):
I
D′
vˆ O
- + ----L dˆ
vˆ˙C = ------ ˆi L + --------C
RL C C
(2.32)
do
The equation
(2.31)
tc
Л™
D′vˆ
Dvˆ V S – V C ˆ
-d
iˆL = ----------C- + ---------S + ----------------L
L
L
om
D′V C DV S
------------- + ---------- = 0
L
L
V
D
-----C- = – -----VS
D′
fo
is derived from Eq. (2.29). Note that from Fig. 2.28, vC = −vO , giving VC = −VO and vˆ C = –vˆ O ; substituting
this into the previous equation results in:
VO
D
------ = -----VS
D′
kw
eb
in
(2.33)
Equation (2.33) states that the ratio of the DC output voltage to the DC input voltage is equal to the
ratio of the power switch on-time to the power switch off-time. The expression for the DC inductor
current term is
VO
I L = – ----------D′R L
(2.34)
Pa
Equations (2.31) and (2.32) constitute the small signal model of a buck–boost converter.
Another method that is utilized to extract the small signal model is to realize an equivalent circuit
model from Eqs. (2.27) and (2.28). Figure 2.34 is the average circuit model of the buck–boost converter.
vS
L
iL
C
+
vC
+
-
-
RL
vO
+
1:d
FIGURE 2.34
d:1
Average circuit model of the buck–boost converter.
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^
(vS -vO)d
L
-+
^i
L
+
-
^
I Ld
^
IL d
1:D
D :1
The small signal circuit model.
TF2
T ON T OFF
Discontinuous inductor current.
FIGURE 2.37
+
-
L
kw
eb
in
vS
fo
FIGURE 2.36
-
RL v^O
+
do
iL
0
+
tc
FIGURE 2.35
C
v^C
om
^v
S
C
+
vC
-
RL
vO
+
Topology 3 for the buck–boost converter (discontinuous inductor current).
Pa
(For a quantitative, numerical analysis, this circuit can be simulated with SPICE or an equivalent simulation package, as demonstrated in Ref. 10.)
Introducing perturbations into the state and output, removing the DC conditions, neglecting the small
nonlinear terms, and simplifying the structure, results in Fig. 2.35.
Discontinuous-Current Mode
Figure 2.36 shows the waveform of the discontinuous inductor current for the buck–boost converter
(Fig. 2.30). Note that the inductor current is equal to zero for TF2s. This results in an additional (third)
topology change, shown in Fig. 2.37.
Since the inductor current is zero for this portion of the switching cycle, there is only one state equation
that can be determined.
dv
vO
-------C- = --------dt
RL C
(2.35)
This equation indicates that the capacitor is now discharging its energy into the load resistor, RL, and
the output voltage is falling.
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iL
TON
TOFF
TF2
iAV
FIGURE 2.38
om
IR
General form of discontinuous inductor current.
T ON
di L
1
------- = ----dt
TP
T ON
∫
0
0
vS
1
---- dt + ----L
TP
T ON +T OFF
∫
T ON
kw
eb
in
dv C
1
-------- = ----dt
TP
∫
fo
do
tc
The modeling of this particular mode is presented in Refs. 5,11, and 12. A general discussion is provided
here as it applies to the model of the buck–boost converter in the discontinuous inductor current mode.
For this case, the inductor current does not behave as a true state variable, since diL/dt = 0, thereby
reducing the system order by one. Figure 2.38 illustrates the general form of the inductor current. The
в€—
equations for the Ton time interval are the same as Eqs. (2.23) and (2.24), except iL = IR + i L , where IR
в€—
represents the DC level at which the inductor current begins and i L , the value of the time-varying
inductor current. The equations for the TOFF interval are the same as Eqs. (2.25) and (2.26) except iL =
в€—в€—
в€—в€—
IR + i L , where i L represents the value of the time-varying inductor current. By combining these sets of
equations with Eq. (2.31) by the averaging technique, the equations listed below are obtained.
vC пЈ¶
1
 – --------- dt + ---- R L C
TP
T ON +T OFF
∫
T ON
в€—
v CпЈ¶
 –------- dt = 0
пЈ­ L пЈё
vC пЈ¶
1
R + iL
 I------------- – --------- dt + ---- C
TP
R L CпЈё
T ON +T OFF +T F2
∫
T ON +T OFF
(2.36)
vC пЈ¶
 – --------- dt
пЈ­ R L CпЈё
(2.37)
For the buck–boost converter case IR = 0; also, from Fig. 2.38, note that
T ON +T OFF
∫
T ON
1v
в€—
i L dt = пЈ« -- ----S T ONпЈ¶ T OFF = i AV T OFF
пЈ­2 L
пЈё
(2.38)
Pa
The variable iAV is the average value of the inductor during the TON + TOFF time, not for the whole cycle.
Substituting into Eqs. (2.36) and (2.37) results in the following:
dv C
T OFF i AV T OFF v C
T F2 v C
T ON v C
- ------ – ---------- ---------- – -------------- = – ----------------- ---------- + --------dt
TP C
TP RL C TP RL C
TP RL C
Let
T ON
-,
d 1 = -------TP
T OFF
-,
d 2 = --------TP
(2.39)
T F2
d 3 = ------TP
and substitute into the above equation.
vC
i AV
- – --------v˙C = d 2 ----C
RL C
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(2.40)
v^O
Id^
C
R
Gd^
R L/2
^
I(D1/D2)d
Buck–boost converter small signal model for the discontinuous mode.
Note that, d1 + d2 + d3 = 1 and
1v
i AV = -- ----s T ON
2L
tc
FIGURE 2.39
+
-
om
^v
S
(2.41)
v S = V S + vˆ S ,
d 1 = D 1 + dˆ1 ,
do
At this point, the same perturbation techniques, as presented previously, are used to obtain the small
signal model. Introducing variations in the line voltage and duty cycle
d 2 = D 2 + dˆ2 ,
d 3 = D 3 + dˆ3
fo
produce perturbations in the state and output; separating the DC and AC terms and simplifying results in
(2.42)
V 2LT
T ON = ----- -----------PVS
RL
(2.43)
kw
eb
in
V S T OFF Л†
V S T ON Л†
T ON T OFF
vˆ C
- vˆ S
- + ---------------- d + ------------------vЛ™C = в€’ --------d + -------------RL C
2LC 1
2LC 2
2LCT P
where
T OFF =
2LT P
-----------RL
D
VC
------ = ------1
VS
D2
(2.44)
(2.45)
Pa
A circuit model (Fig. 2.39) can be realized from the above equations. The process is not shown here;
however, please see Ref. 5 for the details of the circuit derivation and presentation. This concludes the
circuit analysis portion of this section. In the next section the appropriate transfer functions to be used
in the design and implementation of the buck–boost converter are presented. The above small signal
model is used to derive them. For more detail, please see Refs. 3, 5, and 9.
Small Signal Transfer Functions
The analysis done in the previous section enables the development of transfer functions that describe
the buck–boost converter stability performance and input to output signal attenuation. The transfer
functions are illustrated in Fig. 2.40. This figure assumes there is only one feedback (the output voltage)
loop; for more complicated feedback schemes, please see Refs. 7 and 8.
The continuous-current mode transfer functions are derived by using the Laplace transform to solve
for the output voltage and duty cycle variations in Eqs. (2.31) and (2.32). Equation (2.42) is used to
derive the discontinuous-current mode transfer functions.
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v^S
D
D’
+
1
+
пЈ« D RL пЈ¶ пЈ« LC пЈ¶
 s +  ’ s 
1 + пЈ¬пЈ¬
пЈ­ L пЈё пЈ­D пЈё
^
d
VS пЈ«
DL
1 − ’2
’2 
D пЈ­ D RL
пЈ¶
s пЈ·пЈ·
пЈё
Feedback Control
Small Signal Model
^
d
+
1
RC
1+ L s
2
^v
O
fo
VS
D2
+
do
VO
VS
tc
(a)
v^S
^v
O
2
om
’3
kw
eb
in
Feedback Control
Small Signal Model
(b)
FIGURE 2.40 Block diagram of transfer functions for the buck–boost converter: (a) continuous-current mode;
(b) discontinuous-current mode.
Component Selection
Pa
The component values can be chosen based on several constraints. The constraints that are to be discussed
here are not exhaustive, but are only mentioned to provide an introduction into the selection process.
Some component values can be based on an arbitrary selection. For example, the frequency of the
converter is the designer’s choice. As the frequency rises, the volume of the inductor (which is usually
the biggest component in the converter) decreases and its temperature rises. The component value can
be selected based on a given frequency value, which is assumed to be optimized based on the previously
mentioned constraints. However, the frequency value can also be chosen based on experience. There is
to be no discussion on the optimization of the switching frequency in this section; please see Ref. 13 for
a detailed explanation of the process to optimize the converter switching frequency. Thus, an assumption
made at this point is that the switching frequency has been selected.
Inductor Value
The inductor has to be large enough to handle the output power, according to the energy transfer equation
shown below.
1 2
-- Li peak = PT P
2
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(2.46)
where, ipeak is the maximum value of the inductor current, P is the output power, and TP is the time
period of the switching cycle. If the desire for the current is to be continuous, then the inequality shown
below must be satisfied.
TP
2L
------ > ---------------------2
RL пЈ« V
----- + 1пЈ¶
пЈ­ VS
пЈё
om
(2.47)
tc
The inequality (2.47) is derived from Eqs. (2.43) and (2.44). The total of those equations has to be greater
than the cycle time, TP , for the converter to be in the continuous-current mode. If the designer desires the
converter to be in the discontinuous mode, then the inequality sign in (2.47) is reversed so that TON + TOFF
is less than TP .
Satisfying the above two constraints, (2.46) and (2.47), should provide an inductor that is minimal,
but probably not optimal. Using a circuit simulation package, such as PSpice, to simulate and check the
converter action can help determine an optimal value.
do
Capacitor Value
The capacitor value is chosen based on the specified ripple voltage, VPP , the switching frequency (actually
the TOFF for the buck–boost converter), and the allowable capacitor ripple current, iallowable. The following
inequality describes the relationship of the previously mentioned items:
fo
i allowable
C > --------------T OFF
V pp
Pa
kw
eb
in
As with the inductor value, this constraint provides a minimal
capacitor value, but probably not an optimal one. Any value
that is chosen should be used in a simulation to test the
value for feasibility. The latent assumption made here is that
the capacitor is an ideal one. In actuality, a practical capacitor can be modeled as a linear combination of resistors,
inductors, and capacitors. This complicates the previously
discussed models greatly. The equivalent series resistance
(ESR) and the equivalent series inductance (ESL) (Fig. 2.41),
probably have the biggest influence on the effective capacitance, because of their effect on the capacitor ripple voltage.
Both, in general, tend to raise ripple voltage. This may require
an iteration involving a simulation using the catalog or given
values for the ESR and ESL in a more realistic model of the
capacitor.
(2.48)
ESL
ESR
C
FIGURE 2.41
A practical capacitor model.
Main Power Switch and Output Power Diode
The main switching transistor and diode should be chosen based on the inductor current peak value. As
with all of these components, the final component value selection should have appropriate design
margins. These margins, however, do vary with the scope of the mission of the individual converter.
The main power switch function is to provide a path for the inductor to receive energy from the source;
that is, the switch connects the source to the inductor at the appropriate time in the switching cycle. The
switch can dissipate a significant amount of power if not chosen properly or not connected to an
appropriately designed heat sink. So, in addition to ensuring that the switch can handle the peak current
and voltage values, the power dissipation must be checked. For a bipolar transistor, assuming the efficiency
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of the converter is very high, the on-state power dissipation can be expressed as the following:
D
P DISS = V CEsat пЈ« ------ I OпЈ¶
 D′ 
(2.49)
2
D
P DISS = пЈ« ------ I OпЈ¶ R DS ( on )
 D′ 
om
where IO = P/VO . If a MOSFET device is chosen, the on-state dissipation is the following:
(2.50)
P DISS = V D I O
tc
The output power diode provides the path for the inductor to discharge its energy to the output; it
connects the inductor to the output when the main power switch is off. Its voltage drop is primarily
responsible for power dissipation. If Vd is the on-state voltage drop of the diode, then its power dissipation
is expressed as
(2.51)
do
A judicious selection for the diode can be made using the above calculated power value, the peak output
current, and output voltage.
Flyback Power Stage
kw
eb
in
fo
A popular version of the buck–boost converter, shown in Fig. 2.28, is the variation shown in Fig. 2.42,
the flyback converter. The flyback converter provides isolation from input to output: note the output
voltage is not inverted as in the simpler buck–boost converter version of Fig. 2.28. These things are
accomplished because of the two-winding or coupled inductor. The inductor now serves a dual purpose:
it transfers energy from the source to the output and provides input to output voltage. This is a popular
power stage used in off-line (110 VAC or 220 VAC) applications, particularly with multiple output voltages.
Power diodes, capacitors, and windings on the two-winding inductor (power transformer) are added in
the appropriate fashion to provide additional outputs.
The process discussed previously can be used to determine the small signal model and DC operating
point of this converter. The state variables for this converter are the capacitor voltage, vC , and the flux
density, П†, of the two-winding inductor:
dφ
di
N P ------ = L P ----dt
dt
and
dφ
di
N S ------ = L S ----dt
dt
Pa
NP :NS
vS
+
-
vO
+
C
-
RL
Feedback
Control
and Drive
Circuit
FIGURE 2.42
Flyback power converter.
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The input voltage can be expressed as (NS /NP)vS , instead of just vs . Making these substitutions allows
the development of the small signal model that is shown below.
The continuous-current mode small signal model is
with
do
N D
VO
------ = ------S -----VS
N P D′
tc
NS
------ О¦
Л†
v
N
LS Л†
D′
O
- + -----------d
vˆ˙C = ------ ------S φˆ + --------RL C
C
C LS
om
N
N
D ------S vˆ S ------S V S – V C
NP
NP
Л™
D′vˆ
- + ------------------------- dˆ
iˆL = ----------C- + ----------------L
L
L
and
(2.53)
(2.54)
(2.55)
fo
VO LS
Φ = – ----------- -----D′R L N S
(2.52)
kw
eb
in
The discontinuous-current mode small signal model is
N
N
------S V S T OFF
------S V S T ON
Л†vC
NP
NP
T ON T OFF N S
v˙ C = – ---------- + ------------------------ dˆ1 + ----------------------- dˆ2 + ------------------- ------ vˆ S
2L S C
2L S C
2L S CT P N P
RL C
with
Pa
2L S T
V
T ON = -------------- -------------PNS
RL
------ V S
NP
T OFF =
2L S T P
-------------RL
VC
N D
------ = ------S ------1
VS
NP D2
(2.56)
(2.57)
(2.58)
(2.59)
Summary
This section has presented and analyzed a buck–boost converter (Fig. 2.28). The topological changes have
been presented and a discussion of the state space has shown a modeling process (averaging), which can
be used to design the converter. This process models a linear time-varying structure in a relatively simple
way so that significant design insight can be obtained. The small signal model that was presented can be
used to analyze the stability and the input-to-output signal attenuation of the buck–boost converter.
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References
Pa
kw
eb
in
fo
do
tc
om
1. Middlebrook, R. D. and CГєk, S., Modeling and analysis methods for DC-to-DC switching converters,
presented at IEEE Int. Semiconductor Power Converter Conference, 1977.
2. Owen, H. A., Capel, A., and Ferrante, J. G., Simulation and analysis methods for sampled power
electronic systems, in IEEE Power Electronics Specialists Conference Record, 1976, 45–55.
3. Wester, G. W. and Middlebrook, R. D., Low-frequency characterization of switched DC-DC converters, IEEE Trans. Aerospace Electron. Syst., AES-9(3), 376–385, 1973.
4. Middlebrook R. D. and CГєk, S., A general unified approach to modeling switching-converter power
stages, in IEEE Power Electronics Specialists Conference Record, 1976, 18–34.
5. CГєk, S. and Middlebrook, R. D., A general unified approach to modeling switching DC-to-DC converters in discontinuous conduction mode, in IEEE Power Electronics Specialists Conference Record,
1977, 36–57.
6. Lee, F. C. and Shortt, D. J., Improved model for predicting the dynamic performance of high
bandwidth and multiloop power converters, in POWERCON 11 Record, 1984, E-3, 1–14.
7. Shortt, D. J. and Lee, F. C., Extensions of the discrete-average models for converter power stages, in
PESC Record, 1983, 23–37; IEEE Trans. Aerospace Electron. Syst., AES-20(3), 279–289, 1984.
8. Shortt, D. J. and Lee, F. C., An improved switching converter model using discrete and average
techniques, in PESC Record, 1982, 199–212; IEEE Trans. Aerospace Electron. Syst., AES-19(2), 1983.
9. VorpГ©rian, V., Tymerski, R., and Lee, F. C., Equivalent circuit models for resonant and PWM switches,
IEEE Trans. Power Electron., 4(2), 1989.
10. Bello, V., Computer-aided analysis of switching regulators using SPICE2, in IEEE Power Electronics
Specialists Conference Record, 1980, 3–11.
11. Chen, D. Y., Owen, H. A., and Wilson, T. G., Computer-aided design and graphics applied to the
study of inductor energy storage DC-to-DC electronic converters, IEEE Trans. Aerospace Electronic
Syst., AES-9(4), 585–597, 1973.
12. Lee, F. C., Yu, Y., and Triner, J. E., Modeling of switching regulator power stages with and without
zero-inductor current dwell time, in IEEE Power Electronics Specialists Conference Record, 1976,
62–72.
13. Rahman, S. and Lee, F. C., Nonlinear program based optimization of boost and buck–boost converter
designs, in IEEE Power Electronics Specialists Conference Record, 1981, 180–191.
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3
3.1
3.2
3.3
Introduction
Cycloconverters
Matrix Converters
tc
SГЎndor HalГЎsz
Budapest University of Technology
and Economics
om
AC-AC Conversion
3.1 Introduction
do
AC-AC converters as shown in Fig. 3.1 are frequency converters. They produce an AC voltage in which both
the frequency and voltage can be varied directly from the AC line voltage, e.g., from a 60- or 50-Hz source.
There are two major classes of AC-AC, or so-called direct static frequency converters, as shown in Fig. 3.1.
kw
eb
in
3.2 Cycloconverters
fo
1. Cycloconverters, which are constructed using naturally commutated thyristors. The commutation
voltage is ensured by the supply voltage. These are so-called line commutated converters.
2. Matrix converters, which are constructed using full-controlled static devices, such as transistors
or GTOs (gate turn-off thyristors).
In Figs. 3.2 and 3.3, the two typical types of cycloconverters are presented. In the first case there are
two three-phase midpoint controlled rectifiers connected back to back. The second case shows two
three-phase bridge rectifier converters connected back to back. Both are used for three-phase to threephase conversion. In Fig. 3.4 the single-phase output voltage and current waves are presented for the
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Frequency
converters
AC-AC Converters
(direct frequency
converters)
Cycloconverters
FIGURE 3.1
AC-DC-AC
Converters
Matrix
converters
Classification of frequency converters.
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~V ; 60 Hz
R
S
n converter
a
в€’
ia
b
LOAD
Cycloconverter scheme with three-phase midpoint controlled rectifier.
R
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FIGURE 3.2
c
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p
converter
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T
S
T
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Transformer
p
converter
n
converter
ia
a
b
c
FIGURE 3.3
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LOAD
Cycloconverter scheme with three-phase bridge controlled rectifier.
bridge rectifier circuits. The output voltage Va and current ia have Va1 and ia1 fundamental components
with П†1 phase displacement and numerous harmonics. Because of the load inductance, the current harmonics
will be significantly lower than the voltage harmonics. The firing angles are О±P and О±N for the p and n
converters, respectively. In general, the controls are designed so that only the thyristors of either the p
or n converter is firing, which produces a current in the desired direction. During this period the other
converter is blocked. When the current changes direction, both converters must be blocked for a short time.
It is possible to operate without blocking the converters. In this case, their average voltage must be
the same, and therefore the relation О±p = 180 в€’ О±n is valid. However, additional inductances are necessary
to limit the circulating currents between two converters since the instantaneous voltages of the two
converters differ from one another.
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О±p
Va
Va 1
V
П‰1t
П‰1t
ia1
n
inverter
p and n blocked
p rectifier
p
inverter
p and n blocked
p blocked
n rectifier
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n blocked
Voltage and current vs. time for cycloconverter with three-phase bridge converters.
Nine
independent
bidirectional
switches
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(60 HZ) S
a
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FIGURE 3.4
О±n
ia
П†1
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i
b
Load
(variable
voltage
and
variable
frequency)
T
FIGURE 3.5
c
Three-phase to three-phase matrix converter.
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The phase control of the p and n converters is modulated by a sine or trapezoidal wave. The content
of the harmonics for sine modulation is lower; however, the maximum value of the output voltage is
lower than that for trapezoidal modulation. During every cycle of the output voltage both of the converters
must work as rectifiers and inverters.
The shape of the output voltage goes from bad to worse with an increase in the output voltage and
the output frequency. If the frequency reaches the well-defined value the current harmonics become
unacceptable. This frequency is usually 33% of supply frequency for three-phase midpoint (Fig. 3.2) and
50% for three-phase bridge (Fig. 3.3) converters.
The cycloconverter is usually used for three-phase, high-power, low-speed synchronous motor drives
and rarely employed for induction motor drives.
3.3 Matrix Converters
The three-phase to three-phase matrix converter is presented in Fig. 3.5. Using the bidirectional switches,
any phase of the load can be connected to any phase of the input voltage, e.g., the zero value of the load
phase voltages is maintained by connecting all the load phases to the same input phase. Using pulse-width
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Bidirectional switch.
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FIGURE 3.6
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modulation techniques, the load voltage and the load frequency are controlled from zero to their maximum values. The maximum voltage is usually close to the input voltage, but the maximum frequency
can be several times that of the input frequency and is only limited by practical considerations. The
bidirectional switches must be capable of permitting current flow in either direction. In Fig. 3.6 one
possible configuration of the bidirectional switch is shown.
Matrix converters require the use of numerous switches and well-established control methods. Some
additional elements are necessary for the safe commutation of the bidirectional switches. These disadvantages of matrix converters prevent their use in industrial applications.
References
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Guggi, L. and Pelly, B. R. 1976. Static Power Frequency Changes, John Wiley & Sons, New York.
Pelly, B. R. 1976. Thyristor Phase-Controlled Converters, John Wiley & Sons, New York.
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4
4.1
Mahesh M. Swamy
Yaskawa Electric America
Uncontrolled Single-Phase Rectifiers
Single-Phase Half-Wave Rectifiers • Single-Phase Full-Wave
Rectifiers
Eastern Illinois University
4.2
tc
Sam Guccione
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Rectifiers
Uncontrolled and Controlled Rectifiers
Uncontrolled Rectifiers • Controlled Rectifiers • Conclusion
4.3
Ana Stankovic
Three-Phase Pulse-Width-Modulated Boost-Type
Rectifiers
Introduction • Indirect Current Control of a Unity Power
Factor Sinusoidal Current Boost-Type Rectifier • Appendix
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Cleveland State University
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4.1 Uncontrolled Single-Phase Rectifiers
Sam Guccione
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Single-Phase Half-Wave Rectifiers
Operation
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A single-phase half-wave rectifier consists of a single diode connected as shown in Fig. 4.1. This is the
simplest of the rectifier circuits. It produces an output waveform that is half of the incoming AC voltage
waveform. The positive pulse output waveform shown in Fig. 4.1 occurs because of the forward-bias
condition of the diode. A diode experiences a forward-bias condition when its anode is at a higher
potential than its cathode. Reverse bias occurs when its anode is lower than its cathode.
During the positive portion of the input waveform, the diode becomes forward biased, which allows
current to pass through the diode from anode to cathode, such that it flows through the load to produce
a positive output pulse waveform. Over the negative portion of the input waveform, the diode is reversebiased ideally so no current flows. Thus, the output waveform is zero or nearly zero during this portion
of the input waveform.
Because real diodes have real internal electrical characteristics, the peak output voltage in volts of a
real diode operating in a half-wave rectifier circuit is
V P(out) = V P(in) – V F
(4.1)
where VP(in) is the peak value of the input voltage waveform and VF is the forward-bias voltage drop across
the diode. This output voltage is used to determine one of the specification values in the selection of a
diode for use in a half-wave rectifier.
Other voltage and current values are important to the operation and selection of diodes in rectifier
circuits.
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Single-phase half-wave rectifier.
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FIGURE 4.1
Important Diode Current Characteristics
(4.2)
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I FM = V P(out) /R L
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Peak Forward Current
The peak forward or rectified forward current, IFM, in amperes is the current that flows through the diode
as a result of the current demand of the load resistor. It is determined from the peak output voltage Eq.
(4.1) as
where RL is the load resistance in ohms. IFM is also a specification value used to select a diode for use in
a rectifier. Choose a diode with an IFM that is equal to or greater than the IFM calculated in Eq. (4.2).
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rms Forward Current
Since rms values are useful, the rms value of forward current in amperes is determined from
I FRMS = I FM Г— 0.707
(4.3)
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This value is sometimes called the maximum rms forward current.
Mean Forward Current
To find the continuous forward current that the diode in a half-wave rectifier circuit is subjected to, the
mean or average rectified current, IFAV , can be found from
I FAV = I FM / ПЂ
(4.4)
Because this average current is a continuous value, it is sometimes suggested that a diode be selected that
has an IFAV value of 1.25 times that determined from Eq. (4.4).
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Single Cycle Surge Current
One additional current is important in rectifier circuits. That current is the single cycle surge current,
IFSM. This is the peak forward surge current that exists for one cycle or one half cycle for nonrepetitive
conditions. This could be due to a power-on transient or other situations.
Important Diode Voltage Characteristics
Average Output Voltage
The average output voltage of a half-wave rectifier is determined from
V AVG (out) = V P(in) / ПЂ
(4.5)
Repetitive Peak Reverse Voltage
Another characteristic that is important to the operation of rectifier circuits is the voltage that the diode
experiences during reverse bias. When the diode is reversed, it experiences a voltage that is equal to the
value of the negative peak input voltage. For example, if the negative peak input voltage is 300 V, then
the peak reverse voltage (prv) rating of the diode must be at least 300 V or higher. The prv rating is for
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a repetitive input waveform, thus producing a repetitive peak reverse voltage value. A nonrepetitive prv
is also an important specification value, as will be described below.
The repetitive peak reverse voltage is given different names. It is called variously the peak reverse
voltage, peak inverse voltage, maximum reverse voltage (VRM), and maximum working peak reverse
voltage (VRWM). The most common name is the repetitive peak reverse voltage, VRRM. The repetitive peak
reverse voltage is one of the critical specification values that are important when selecting a diode for
operation in half-wave rectifier circuits.
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Forward Voltage Drop
The value of the maximum forward voltage, VF , is the voltage value that occurs across a diode when it
becomes forward biased. It is a small value usually in the range of 0.5 V to several volts. VF is sometimes
identified as the maximum forward voltage drop, VFM. The threshold value of the forward voltage is
sometimes listed in specifications as VF(TO).
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Nonrepetitive Peak Reverse Voltage
Diodes used in rectifiers are also specified in terms of their characteristics to nonrepetitive conditions.
This is usually identified as the voltage rating for a single transient wave. The symbol, VRSM, is used. VRSM
is a specification value. This voltage is sometimes identified as the nonrepetitive transient peak reverse
voltage.
Single-Phase Full-Wave Rectifiers
Operation
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A single-phase full-wave rectifier consists of four diodes arranged as shown in Fig. 4.2 in what is called
a bridge. This rectifier circuit produces an output waveform that is the positive half of the incoming AC
voltage waveform and the inverted negative half. The bias path for the positive output pulse is through
diode D1, then the load, then D4, and back to the other side of the power supply. The current flow through
the load is in the down direction for the figure shown. Diodes D2 and D3 are reverse-biased during this part.
The bias path for the negative cycle of the input waveform is through diode D3, then the load, then
D2, and back to the opposite side of the power supply. The current flow through the load resistor is once
again down. That is, it is flowing through the load in the same direction as during the positive cycle of
the input waveform. Diodes D1 and D4 are reverse-biased during this part. The resulting output waveform
is a series of positive pulses without the “gaps” of the half-wave rectifier output.
As in the half-wave rectifier circuit description, real diodes have real characteristics, which affect the
circuit voltages and currents. The peak output voltage in volts of a full-wave bridge rectifier with real diodes is
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V P(out) = V P(in) – 2 × V F
FIGURE 4.2
Single-phase full-wave bridge rectifier.
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(4.6)
where VF is the forward-bias voltage drop across one diode. Because there are two forward-biased diodes
in the current path, the total drop would be twice the drop of one diode.
As in the half-wave rectifier, there are other voltages and currents that are important to the operation
and selection of diodes in a full-wave rectifier. Only those values that are different from the half-wave
circuit will be identified here. The other values are the same between a half-wave and a full-wave rectifier.
Important Diode Current Characteristics
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Peak Rectified Forward Current
The peak rectified forward current, IFM, in amperes has the same equation (4.1) as for the half-wave
rectifier. The difference is that the value VP(out) is as shown in Eq. (4.6).
rms Forward Current
The rms value is computed using the same Eq. (4.2).
Single-Cycle Surge Current
This current is the same for either type of rectifier.
Important Diode Voltage Characteristics
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I FAV = 2 Г— I FM / ПЂ
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Average Forward Current
The mean or average forward current for a full-wave rectifier is twice the value for a half-wave rectifier.
The equation is
(4.7)
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Average Output Voltage
The average output voltage of a full-wave rectifier is twice that of a half-wave rectifier. It is determined
from
V AVG (out) = 2 Г— V P (in) / ПЂ
(4.8)
Repetitive Peak Reverse Voltage
The repetitive peak reverse voltage, VRRM, is slightly different for a full-wave bridge rectifier. It is determined by
V RRM = V P (out) – V F
(4.9)
where VP(out) and VF have been defined before in Eq. (4.1).
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Forward Voltage Drop
This voltage is the same for either type of rectifier.
Nonrepetitive Peak Reverse Voltage
This voltage is the same for either type of rectifier.
4.2 Uncontrolled and Controlled Rectifiers
Mahesh M. Swamy
Rectifiers are electronic circuits that convert bidirectional voltage to unidirectional voltage. This process
can be accomplished either by mechanical means like in the case of DC machines employing commutators
or by static means employing semiconductor devices. Static rectification is more efficient and reliable
compared to rotating commutators. This section covers rectification of electric power for industrial and
commercial use. In other words, we will not be discussing small signal rectification that generally involves
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low power and low voltage signals. Static power rectifiers can be classified into two broad groups. They
are (1) uncontrolled rectifiers and (2) controlled rectifiers. Uncontrolled rectifiers make use of power
semiconductor diodes while controlled rectifiers make use of thyristors (SCRs), gate turn-off thyristors
(GTOs), and MOSFET-controlled thyristors (MCTs).
Rectifiers, in general, are widely used in power electronics to rectify single-phase as well as three-phase
voltages. DC power supplies used in computers, consumer electronics, and a host of other applications
typically make use of single-phase rectifiers. Industrial applications include, but are not limited to,
industrial drives, metal extraction processes, industrial heating, power generation and transmission, etc.
Most industrial applications of large power rating typically employ three-phase rectification processes.
Uncontrolled rectifiers in single-phase as well as in three-phase circuits will be discussed, as will
controlled rectifiers. Application issues regarding uncontrolled and controlled rectifiers will be briefly
discussed within each section.
Uncontrolled Rectifiers
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The simplest uncontrolled rectifier use can be found in single-phase circuits. There are two types of
uncontrolled rectification. They are (1) half-wave rectification and (2) full-wave rectification. Half-wave
and full-wave rectification techniques have been used in single-phase as well as in three-phase circuits.
As mentioned earlier, uncontrolled rectifiers make use of diodes. Diodes are two-terminal semiconductor
devices that allow flow of current in only one direction. The two terminals of a diode are known as the
anode and the cathode.
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Mechanics of Diode Conduction
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The anode is formed when a pure semiconductor material, typically silicon, is doped with impurities
that have fewer valence electrons than silicon. Silicon has an atomic number of 14, which according to
Bohr’s atomic model means that the K and L shells are completely filled by 10 electrons and the remaining
4 electrons occupy the M shell. The M shell can hold a maximum of 18 electrons. In a silicon crystal,
every atom is bound to four other atoms, which are placed at the corners of a regular tetrahedron. The
bonding, which involves sharing of a valence electron with a neighboring atom is known as covalent
bonding. When a Group 3 element (typically boron, aluminum, gallium, and indium) is doped into the
silicon lattice structure, three of the four covalent bonds are made. However, one bonding site is vacant
in the silicon lattice structure. This creates vacancies or holes in the semiconductor. In the presence of
either a thermal field or an electrical field, electrons from a neighboring lattice or from an external agency
tend to migrate to fill this vacancy. The vacancy or hole can also be said to move toward the approaching
electron, thereby creating a mobile hole and hence current flow. Such a semiconductor material is also
known as lightly doped semiconductor material or p-type. Similarly, the cathode is formed when silicon
is doped with impurities that have higher valence electrons than silicon. This would mean elements
belonging to Group 5. Typical doping impurities of this group are phosphorus, arsenic, and antimony.
When a Group 5 element is doped into the silicon lattice structure, it oversatisfies the covalent bonding
sites available in the silicon lattice structure, creating excess or loose electrons in the valence shell. In the
presence of either a thermal field or an electrical field, these loose electrons easily get detached from the
lattice structure and are free to conduct electricity. Such a semiconductor material is also known as heavily
doped semiconductor material or n-type.
The structure of the final doped crystal even after the addition of acceptor impurities (Group 3) or
donor impurities (Group 5), remains electrically neutral. The available electrons balance the net positive
charge and there is no charge imbalance.
When a p-type material is joined with an n-type material, a pn-junction is formed. Some loose electrons
from the n-type material migrate to fill the holes in the p-type material and some holes in the p-type
migrate to meet with the loose electrons in the n-type material. Such a movement causes the p-type structure to develop a slight negative charge and the n-type structure to develop some positive charge.
These slight positive and negative charges in the n-type and p-type areas, respectively, prevent further
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Single-Phase Half-Wave Rectifier Circuits
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migration of electrons from n-type to p-type and holes from p-type to n-type areas. In other words, an
energy barrier is automatically created due to the movement of charges within the crystalline lattice
structure. Keep in mind that the combined material is still electrically neutral and no charge imbalance
exists.
When a positive potential greater than the barrier potential is applied across the pn-junction, then
electrons from the n-type area migrate to combine with the holes in the p-type area, and vice versa. The
pn-junction is said to be forward-biased. Movement of charge particles constitutes current flow. Current
is said to flow from the anode to the cathode when the potential at the anode is higher than the potential
at the cathode by a minimum threshold voltage also known as the junction barrier voltage. The magnitude
of current flow is high when the externally applied positive potential across the pn-junction is high.
When the polarity of the applied voltage across the pn-junction is reversed compared to the case described
above, then the flow of current ceases. The holes in the p-type area move away from the n-type area and
the electrons in the n-type area move away from the p-type area. The pn-junction is said to be reversebiased. In fact, the holes in the p-type area get attracted to the negative external potential and similarly
the electrons in the n-type area get attracted to the positive external potential. This creates a depletion
region at the pn-junction and there are almost no charge carriers flowing in the depletion region. This
phenomenon brings us to the important observation that a pn-junction can be utilized to force current
to flow only in one direction, depending on the polarity of the applied voltage across it. Such a semiconductor device is known as a diode. Electrical circuits employing diodes for the purpose of making
the current flow in a unidirectional manner through a load are known as rectifiers. The voltage-current
characteristic of a typical power semiconductor diode along with its symbol is shown in Fig. 4.3.
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A single-phase half-wave rectifier circuit employs one diode. A typical circuit, which makes use of a halfwave rectifier, is shown in Fig. 4.4.
A single-phase AC source is applied across the primary windings of a transformer. The secondary of
the transformer consists of a diode and a resistive load. This is typical since many consumer electronic
items including computers utilize single-phase power.
FIGURE 4.3
Typical v–i characteristic of a semiconductor diode and its symbol.
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Typical waveforms at various points in the circuit of Fig. 4.4. For a purely resistive load, V o =
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FIGURE 4.5
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FIGURE 4.4 Electrical schematic of a single-phase half-wave rectifier circuit feeding a resistive load. Average output
voltage is Vo.
2 Г— V sec / ПЂ .
Typically, the primary side is connected to a single-phase AC source, which could be 120 V, 60 Hz,
100 V, 50 Hz, 220 V, 50 Hz, or any other utility source. The secondary side voltage is generally stepped
down and rectified to achieve low DC voltage for consumer applications. The secondary voltage, the
voltage across the load resistor, and the current through it is shown in Fig. 4.5.
As one can see, when the voltage across the anode-cathode of diode D1 in Fig. 4.4 goes negative, the
diode does not conduct and no voltage appears across the load resistor R. The current through R follows
the voltage across it. The value of the secondary voltage is chosen to be 12 VAC and the value of R is
chosen to be 120 Ω. Since, only one half of the input voltage waveform is allowed to pass onto the output,
such a rectifier is known as a half-wave rectifier. The voltage ripple across the load resistor is rather large
and, in typical power supplies, such ripples are unacceptable. The current through the load is discontinuous and the current through the secondary of the transformer is unidirectional. The AC component in
the secondary of the transformer is balanced by a corresponding AC component in the primary winding.
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Modified circuit of Fig. 4.4 employing smoothing filters.
FIGURE 4.7
Voltage across load resistor R and current through it for the circuit in Fig. 4.6.
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FIGURE 4.6
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However, the DC component in the secondary does not induce any voltage on the primary side and
hence is not compensated for. This DC current component through the transformer secondary can cause
the transformer to saturate and is not advisable for large power applications. In order to smooth the
output voltage across the load resistor R and to make the load current continuous, a smoothing filter
circuit comprised of either a large DC capacitor or a combination of a series inductor and shunt DC
capacitor is employed. Such a circuit is shown in Fig. 4.6.
The resulting waveforms are shown in Fig. 4.7. It is interesting to see that the voltage across the load
resistor has very little ripple and the current through it is smooth. However, the value of the filter components
employed is large and is generally not economically feasible. For example, in order to get a voltage waveform
across the load resistor R, which has less than 6% peak-peak voltage ripple, the value of inductance that
had to be used is 100 mH and the value of the capacitor is 1000 ВµF. In order to improve the performance
without adding bulky filter components, it is a good practice to employ full-wave rectifiers. The circuit
in Fig. 4.4 can be easily modified into a full-wave rectifier. The transformer is changed from a single
secondary winding to a center-tapped secondary winding. Two diodes are now employed instead of one.
The new circuit is shown in Fig. 4.8.
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Electrical schematic of a single-phase full-wave rectifier circuit. Average output voltage is Vo.
FIGURE 4.9
Typical waveforms at various points in the circuit of Fig. 4.8. For a purely resistive load, Vo =
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FIGURE 4.8
2 Г— 2 Г— V sec / ПЂ .
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Full-Wave Rectifiers
The waveforms for the circuit of Fig. 4.8 are shown in Fig. 4.9. The voltage across the load resistor is a
full-wave rectified voltage. The current has subtle discontinuities but can be improved by employing
smaller size filter components. A typical filter for the circuit of Fig. 4.8 may include only a capacitor. The
waveforms obtained are shown in Fig. 4.10.
Yet another way of reducing the size of the filter components is to increase the frequency of the supply.
In many power supply applications similar to the one used in computers, a high frequency AC supply is
achieved by means of switching. The high frequency AC is then level translated via a ferrite core
transformer with multiple secondary windings. The secondary voltages are then rectified employing a
simple circuit as shown in Fig. 4.4 or Fig. 4.6 with much smaller filters. The resulting voltage across the
load resistor is then maintained to have a peak-peak voltage ripple of less than 1%.
Full-wave rectification can be achieved without the use of center-tap transformers. Such circuits make
use of four diodes in single-phase circuits and six diodes in three-phase circuits. The circuit configuration
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FIGURE 4.11
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FIGURE 4.10 Voltage across the load resistor and current through it with the same filter components as in Fig. 4.6.
Notice the conspicuous reduction in ripple across R.
Schematic representation of a single-phase full-wave H-bridge rectifier.
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is typically referred to as the H-bridge circuit. A single-phase full-wave H-bridge topology is shown in
Fig. 4.11. The main difference between the circuit topology shown in Figs. 4.8 and 4.11 is that the Hbridge circuit employs four diodes while the topology of Fig. 4.8 utilizes only two diodes. However, a
center-tap transformer of a higher power rating is needed for the circuit of Fig. 4.8. The voltage and
current stresses in the diodes in Fig. 4.8 are also greater than that occurring in the diodes of Fig. 4.11.
In order to comprehend the basic difference in the two topologies, it is interesting to compare the
component ratings for the same power output. To make the comparison easy, let both topologies employ
very large filter inductors such that the current through R is constant and ripple-free. Let this current
through R be denoted by Idc. Let the power being supplied to the load be denoted by Pdc . The output
power and the load current are then related by the following expression:
2
P dc = I dc Г— R
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The rms current flowing through the first secondary winding in the topology in Fig. 4.8 will be I dc / 2.
This is because the current through a secondary winding flows only when the corresponding diode is
forward-biased. This means that the current through the secondary winding will flow only for one half
cycle. If the voltage at the secondary is assumed to be V, the VA rating of the secondary winding of the
transformer in Fig. 4.8 will be given by:
VA 2 = V Г— I dc / 2
VA = VA 1 + VA 2 =
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VA = V Г— I dc / 2
2 Г— V Г— I dc
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This is the secondary-side VA rating for the transformer shown in Fig. 4.8.
For the isolation transformer shown in Fig. 4.11, let the secondary voltage be V and the load current
be of a constant value Idc . Since, in the topology of Fig. 4.11, the secondary winding carries the current
Idc when diodes D1 and D2 conduct and as well as when diodes D3 and D4 conduct, the rms value of the
secondary winding current is Idc . Hence, the VA rating of the secondary winding of the transformer
shown in Fig. 4.11 is V Г— I dc , which is less than that needed in the topology of Fig. 4.8. Note that the
primary VA rating for both cases remains the same since in both cases the power being transferred from
the source to the load remains the same.
When diode D2 in the circuit of Fig. 4.8 conducts, the secondary voltage of the second winding Vsec2
(= V) appears at the cathode of diode D1. The voltage being blocked by diode D1 can thus reach two
times the peak secondary voltage (= 2 Г— V pk ) (Fig. 4.9). In the topology of Fig. 4.11, when diodes D1
and D2 conduct, the voltage Vsec (= V), which is same as Vsec2 appears across D3 as well as across D4. This
means that the diodes have to withstand only one times the peak of the secondary voltage, Vpk. The rms
value of the current flowing through the diodes in both topologies is the same. Hence, from the diode
voltage rating as well as from the secondary VA rating points of view, the topology of Fig. 4.11 is better
than that of Fig. 4.8. Further, the topology in Fig. 4.11 can be directly connected to a single-phase AC
source and does not need a center-topped transformer. The voltage waveform across the load resistor is
similar to that shown in Figs. 4.9 and 4.10.
In many industrial applications, the topology shown in Fig. 4.11 is used along with a DC filter capacitor
to smooth the ripples across the load resistor. The load resistor is simply a representative of a load. It
could be an inverter system or a high-frequency resonant link. In any case, the diode rectifier-bridge
would see a representative load resistor. The DC filter capacitor will be large in size compared to an Hbridge configuration based on three-phase supply system. When the rectified power is large, it is advisable
to add a DC-link inductor. This can reduce the size of the capacitor to some extent and reduce the current
ripple through the load. When the rectifier is turned on initially with the capacitor at zero voltage, a
large amplitude of charging current will flow into the filter capacitor through a pair of conducting diodes.
The diodes D1 в€ј D4 should be rated to handle this large surge current. In order to limit the high inrush
current, it is a normal practice to add a charging resistor in series with the filter capacitor. The charging
resistor limits the inrush current but creates a significant power loss if it is left in the circuit under normal
operation. Typically, a contactor is used to short-circuit the charging resistor after the capacitor is charged
to a desired level. The resistor is thus electrically nonfunctional during normal operating conditions. A
typical arrangement showing a single-phase full-wave H-bridge rectifier system for an inverter application
is shown in Fig. 4.12.
The charging current at time of turn-on is shown in a simulated waveform in Fig. 4.13. Note that the
contacts across the soft-charge resistor are closed under normal operation. The contacts across the softcharge resistor are initiated by various means. The coil for the contacts could be powered from the input
AC supply and a timer or it could be powered on by a logic controller that senses the level of voltage
across the DC bus capacitor or senses the rate of change in voltage across the DC bus capacitor. A
simulated waveform depicting the inrush with and without a soft-charge resistor is shown in Fig. 4.13a
and b, respectively.
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FIGURE 4.12
Single-phase H-bridge circuit for use with power electronic circuits.
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For larger power applications, typically above 1.5 kW, it is advisable to use a higher power supply. In
some applications, two of the three phases of a three-phase power system are used as the source powering
the rectifier of Fig. 4.11 The line-line voltage could be either 240 or 480 VAC. Under those circumstances,
one may go up to 10 kW of load power before adopting a full three-phase H-bridge configuration. Beyond
10 kW, the size of the capacitor becomes too large to achieve a peak-peak voltage ripple of less than 5%.
Hence, it is advisable then to employ three-phase rectifier configurations.
Three-Phase Rectifiers (Half-Wave and Full-Wave)
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Similar to the single-phase case, there exist half-wave and full-wave three-phase rectifier circuits. Again,
similar to the single-phase case, the half-wave rectifier in the three-phase case also yields DC components
in the source current. The source has to be large enough to handle this. Therefore, it is not advisable to
use three-phase half-wave rectifier topology for large power applications. The three-phase half-wave
rectifier employs three diodes while the full-wave H-bridge configuration employs six diodes. Typical
three-phase half-wave and full-wave topologies are shown in Fig. 4.14.
In the half-wave rectifier shown in Fig. 4.14a, the shape of the output voltage and current through the
resistive load is dictated by the instantaneous value of the source voltages, L1, L2, and L3. These source
voltages are phase shifted in time by 120 electrical degrees, which corresponds to approximately 5.55 ms
for a 60 Hz system. This means that if one considers the L1 phase to reach its peak value at time t1, the
L2 phase will achieve its peak 120 electrical degrees later (t1 + 5.55 ms), and L3 will achieve its peak 120
electrical degrees later than L2 (t1 + 5.55 ms + 5.55 ms). Since all three phases are connected to the same
output resistor R, the phase that provides the highest instantaneous voltage is the phase that appears
across R. In other words, the phase with the highest instantaneous voltage reverse biases the diodes of
the other two phases and prevents them from conducting, which consequently prevents those phase
voltages from appearing across R. Since a particular phase is connected to only one diode in Fig. 4.14a,
only three pulses, each of 120В° duration, appear across the load resistor, R. Typical output voltage across
R for the circuit of Fig. 4.14a is shown in Fig. 4.15a.
A similar explanation can be provided to explain the voltage waveform across a purely resistive load
in the case of the three-phase full-wave rectifier shown in Fig. 4.14b. The output voltage that appears
across R is the highest instantaneous line-line voltage and not simply the phase voltage. Since there are
six such intervals, each of 60 electrical degrees duration in a given cycle, the output voltage waveform
will have six pulses in one cycle (Fig. 4.15b). Since a phase is connected to two diodes (diode pair), each
phase conducts current out and into itself, thereby eliminating the DC component in one complete cycle.
The waveform for a three-phase full-wave rectifier with a purely resistive load is shown in Fig. 4.15b.
Note that the number of humps in Fig. 4.15a is only three in one AC cycle, while the number of humps
in Fig. 4.15b is six in one AC cycle.
In both the configurations shown in Fig. 4.14, the load current does not become discontinuous due
to three-phase operation. Comparing this to the single-phase half-wave and full-wave rectifier, one can
say that the output voltage ripple is much lower in three-phase rectifier systems compared to single-phase
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FIGURE 4.13 (a) Charging current and voltage across capacitor for a typical value of soft-charge resistor of 2 Ω.
The DC bus capacitor is about 1000 µF. The load is approximately 200 Ω. (b) Charging current and voltage across
capacitor for no soft charge resistor. The current is limited by the system impedance and by the diode forward
resistance. The DC bus capacitor is about 1000 µF. The load is approximately 200 Ω.
rectifier systems. Hence, with the use of moderately sized filters, three-phase full-wave rectifiers can be
operated at hundred to thousands of kilowatts. The only limitation would be the size of the diodes used
and power system harmonics, which will be discussed next. Since there are six humps in the output
voltage waveform per electrical cycle, the three-phase full-wave rectifier shown in Fig. 4.14b is also known
as a six-pulse rectifier system.
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FIGURE 4.14 Schematic representation of three-phase rectifier configurations: (a) half-wave rectifier needing a
neutral point, N; and (b) full-wave rectifier.
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Average Output Voltage
In order to evaluate the average value of the output voltage for the two rectifiers shown in Fig. 4.14, the
output voltages in Fig. 4.15a and b have to be integrated over a cycle. For the circuit shown in Fig. 4.14a,
the integration yields the following:
5 ПЂ /6
∫
fo
3
V o = -----2ПЂ
ПЂ6
2V Lв€’N sin ( wt )d ( wt )
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3 Г— 3 Г— 2 Г— V Lв€’N
V o = ---------------------------------------------2Г—ПЂ
Similar operations can be performed to obtain the average output voltage for the circuit shown in
Fig. 4.14b. This yields:
3
V o = --ПЂ
2 ПЂ /3
∫
ПЂ3
2V Lв€’L sin ( wt )d ( wt )
3 Г— 2 Г— V Lв€’L
3 Г— 2 Г— 3 Г— V Lв€’N
- = ---------------------------------------------V o = -------------------------------ПЂ
ПЂ
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In other words, the average output voltage for the circuit in Fig. 4.14b is twice that for the circuit in
Fig. 4.14a.
Influence of Three-Phase Rectification on the Power System
Events over the last several years have focused attention on certain types of loads on the electrical system
that result in power quality problems for the user and utility alike. When the input current into the
electrical equipment does not follow the impressed voltage across the equipment, then the equipment is
said to have a nonlinear relationship between the input voltage and input current. All equipment that
employs some sort of rectification (either single phase or three phase) are examples of nonlinear loads.
Nonlinear loads generate voltage and current harmonics that can have adverse effects on equipment
designed for operation as linear loads. Transformers that bring power into an industrial environment
are subject to higher heating losses due to harmonic generating sources (nonlinear loads) to which they
are connected. Harmonics can have a detrimental effect on emergency generators, telephones, and other
electrical equipment. When reactive power compensation (in the form of passive power factor improving
capacitors) is used with nonlinear loads, resonance conditions can occur that may result in even higher
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FIGURE 4.15 (a) Typical output voltage across a purely resistive network for the half-wave rectifier shown in
Fig. 4.6a. (b) Typical output voltage across a purely resistive network for the full-wave rectifier shown in Fig. 4.6b.
levels of harmonic voltage and current distortion, thereby causing equipment failure, disruption of power
service, and fire hazards in extreme conditions.
The electrical environment has absorbed most of these problems in the past. However, the problem
has now reached a magnitude where Europe, the United States, and other countries have proposed
standards to responsibly engineer systems considering the electrical environment. IEEE 519-1992 and
IEC 1000 have evolved to become a common requirement cited when specifying equipment on newly
engineered projects.
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FIGURE 4.16 Typical pulsed-current waveform as seen at input of a three-phase diode rectifier with DC capacitor
filter. The lower trace is input line-line voltage.
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Why Diode Rectifiers Generate Harmonics
The current waveform at the inputs of a three-phase full-wave rectifier is not continuous. It has multiple
zero crossings in one electrical cycle. The current harmonics generated by rectifiers having DC bus
capacitors are caused by the pulsed current pattern at the input. The DC bus capacitor draws charging
current only when it gets discharged due to the load. The charging current flows into the capacitor when
the input rectifier is forward-biased, which occurs when the instantaneous input voltage is higher than
the steady-state DC voltage across the DC bus capacitor. The pulsed current drawn by the DC bus capacitor
is rich in harmonics due to the fact that it is discontinuous as shown in Fig. 4.16. Sometimes there are
also voltage harmonics that are associated with three-phase rectifier systems. The voltage harmonics
generated by three-phase rectifiers are due to the flat-topping effect caused by a weak AC source charging
the DC bus capacitor without any intervening impedance. The distorted voltage waveform gives rise to
voltage harmonics that could lead to possible network resonance.
The order of current harmonics produced by a semiconductor converter during normal operation is
termed characteristic harmonics. In a three-phase, six-pulse rectifier with no DC bus capacitor, the
characteristic harmonics are nontriplen odd harmonics (e.g., 5th, 7th, 11th, etc.). In general, the characteristic harmonics generated by a semiconductor recitifier are given by:
h = kq В± 1
where h is the order of harmonics; k is any integer, and q is the pulse number of the semiconductor
rectifier (six for a six-pulse rectifier). When operating a six-pulse rectifier system with a DC bus capacitor
(as in voltage source inverters, or VSI), one may start observing harmonics of orders other than those
given by the above equation. Such harmonics are called noncharacteristic harmonics. Though of lower
magnitude, these also contribute to the overall harmonic distortion of the input current. The per-unit
value of the characteristic harmonics present in the theoretical current waveform at the input of the
semiconductor converter is given by 1/h, where h is the order of the harmonics. In practice, the observed
per-unit value of the harmonics is much greater than 1/h. This is because the theoretical current waveform
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is a rectangular pattern made up of equal positive and negative halves, each occupying 120 electrical
degrees. The pulsed discontinuous waveform observed commonly at the input of a three-phase full-wave
rectifier system depends greatly on the impedance of the power system, the size of the DC bus capacitors,
and the level of loading of the DC bus capacitors. Total harmonic current distortion is defined as:
n=в€ћ
2
n
n=2
THD I = --------------I1
om
∑I
where I1 is the rms value of the fundamental component of current; and In is the rms value of the nth
harmonic component of current.
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Harmonic Limits Based on IEEE Std. 519-1992
The IEEE Std. 519-1992 relies strongly on the definition of the point of common coupling or PCC. The
PCC from the utility viewpoint will usually be the point where power comes into the establishment (i.e.,
point of metering). However, IEEE Std. 519-1992 also suggests that “within an industrial plant, the
point of common coupling (PCC) is the point between the nonlinear load and other loads” (IEEE Std.
519-1992). This suggestion is crucial since many plant managers and building supervisors feel that it is
equally, if not more important to keep the harmonic levels at or below acceptable guidelines within their
facility. In view of the many recently reported problems associated with harmonics within industrial
plants, it is important to recognize the need for mitigating harmonics at the point where the offending
equipment is connected to the power system. This approach would minimize harmonic problems, thereby
reducing costly downtime and improving the life of electrical equipment. If one is successful in mitigating
individual load current harmonics, then the total harmonics at the point of the utility connection will
in most cases meet or exceed the IEEE recommended guidelines. In view of this, it is becoming increasingly
common for specifiers to require nonlinear equipment suppliers to adopt the procedure outlined in IEEE
Std. 519-1992 to mitigate the harmonics to acceptable levels at the point of the offending equipment.
For this to be interpreted equally by different suppliers, the intended PCC must be identified. If the PCC
is not defined clearly, many suppliers of offending equipment would likely adopt the PCC at the utility
metering point, which would not benefit the plant or the building, but rather the utility.
Having established that it is beneficial to adopt the PCC to be the point where the nonlinear equipment
connects to the power system, the next step is to establish the short circuit ratio. Short circuit ratio
calculations are key in establishing the allowable current harmonic distortion levels. For calculating the
short circuit ratio, one has to determine the available short circuit current at the input terminals of the
nonlinear equipment. The short-circuit current available at the input of nonlinear equipment can be
calculated by knowing the value of the short-circuit current available at the secondary of the utility
transformer supplying power to the establishment (building) and the series impedance in the electrical
circuit between the secondary of the transformer and the nonlinear equipment. In practice, it is common
to assume the same short circuit current level as at the secondary of the utility transformer feeding
the nonlinear equipment. The next step is to compute the fundamental value of the rated input current
into the nonlinear equipment (three-phase full-wave rectifier in this case). An example is presented here
to recap the above procedure. A widely used industrial equipment item that employs a three-phase fullwave rectifier is the voltage source inverter (VSI). These are used for controlling speed and torque of
induction motors. Such equipment is also known as an Adjustable Speed Drive (ASD) or Variable Frequency
Drive (VFD).
A 100-hp ASD/motor combination connected to a 480-V system being fed from a 1500-kVA, threephase transformer with impedance of 4% is required to meet IEEE Std. 519-1992 at its input terminals.
The rated current of the transformer is 1500 Г— 1000/(в€љ(3) Г— 480 ) , and is calculated to be 1804.2 A.
The short-circuit current available at the secondary of the transformer is equal to the rated current divided
by the per unit impedance of the transformer. This is calculated to be 45,105.5 A. The short-circuit ratio,
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TABLE 4.1
Current Distortion Limits for General Distribution Systems
(120 V through 69,000 V)
Maximum Harmonic Current Distortion in percent of IL
a
Individual Harmonic Order (Odd Harmonics)
<11
11 ≤ h ≤ 17
17 ≤ h ≤ 23
23 ≤ h ≤ 35
35 ≤ h
TDD
c
4.0
7.0
10.0
12.0
15.0
2.0
3.5
4.5
5.5
7.0
1.5
2.5
4.0
5.0
6.0
0.6
1.0
1.5
2.0
2.5
0.3
0.5
0.7
1.0
1.4
5.0
8.0
12.0
15.0
20.0
<20
20 < 50
50 < 100
100 < 1000
>1000
a
b
om
Isc /IL
Even harmonics are limited to 25% of the odd harmonic limits above.
TDD is Total Demand Distortion and is defined as the harmonic current distortion
in % of maximum demand load current. The maximum demand current could either
be a 15-min or a 30-min demand interval.
c
All power generation equipment is limited to these values of current distortion,
regardless of actual Isc/IL; where Isc is the maximum short circuit current at PCC and IL
is the maximum demand load current (fundamental frequency) at PCC.
Source: IEEE Std. 519-1992.
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which is defined as the ratio of the short-circuit current at the PCC to the fundamental value of the
nonlinear current is computed next. NEC amps for 100-hp, 460-V is 124 A. Assuming that the shortcircuit current at the ASD input is practically the same as that at the secondary of the utility transformer,
the short-circuit ratio is calculated to be: 45,105.5/124, which equals 363.75. On referring to IEEE Std.
519-1992, Table 10.3 (IEEE Std. 519-1992), the short-circuit ratio falls in the 100 to 1000 category. For
this ratio, the total demand distortion (TDD) at the point of ASD connection to the power system network
is recommended to be 15% or less. For reference, see Table 4.1.
Harmonic Mitigating Techniques
Various techniques of improving the input current waveform are discussed below. The intent of all
techniques is to make the input current more continuous so as to reduce the overall current harmonic
distortion. The different techniques can be classified into four broad categories:
1.
2.
3.
4.
Introduction of line reactors and/or DC link chokes
Passive filters (series, shunt, and low pass broadband filters)
Phase multiplication (12-pulse, 18-pulse rectifier systems)
Active harmonic compensation
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The following paragraphs will briefly discuss the available technologies and their relative advantages and
disadvantages. The term three-phase line reactor or just reactor is used in the following paragraphs to
denote three-phase line inductors.
Three-Phase Line Reactors
Line reactors offer a significant magnitude of inductance that can alter the way the current is drawn by
a nonlinear load such as a rectifier bridge. The reactor makes the current waveform less discontinuous,
resulting in lower current harmonics. Since the reactor impedance increases with frequency, it offers
larger impedance to the flow of higher order harmonic currents. Therefore, it is instrumental in impeding
higher frequency current components while allowing the fundamental frequency component to pass
through with relative ease.
On knowing the input reactance value, one can estimate the expected current harmonic distortion. A
table illustrating the typically expected input current harmonics for various amounts of input reactance
is shown in Table 4.2.
Input reactance is determined by the accumulated impedance of the AC reactor, DC link choke (if
used), input transformer, and cable impedance. To maximize the input reactance while minimizing AC
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TABLE 4.2
Percent Harmonics vs. Total Line Impedance
Total Input Impedance
3%
4%
5%
6%
7%
8%
9%
10%
5th
7th
11th
13th
17th
19th
%THID
True rms
40
16
7.3
4.9
3
2.2
44
1.09
34
13
6.3
4.2
2.4
2
37
1.07
32
12
5.8
3.9
2.2
0.8
35
1.06
30
11
5.2
3.6
2.1
0.7
33
1.05
28
10
5
3.3
0.9
0.4
30
1.05
26
9
4.3
3.15
0.7
0.3
28
1.04
24
8.3
4.2
3
0.5
0.25
26
1.03
23
7.5
4
2.8
0.4
0.2
25
1.03
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Harmonic
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voltage drop, one can combine the use of both AC-input reactors and DC link chokes. One can approximate the total effective reactance and view the expected harmonic current distortion from Table 4.2.
The effective impedance value in percent is based on the actual loading and is:
3 Г— 2 Г— ПЂ Г— f Г— L Г— I act (fnd.)
- Г— 100
Z eff = ---------------------------------------------------------------V Lв€’L
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where Iact(fnd.) is the fundamental value of the actual load current and VLв€’L is the line-line voltage. The
effective impedance of the transformer as seen from the nonlinear load is:
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Z eff, x-mer Г— I act (fnd.)
Z eff, x-mer = ---------------------------------------Ir
where Zeff,x-mer is the effective impedance of the transformer as viewed from the nonlinear load end; Zx-mer
is the nameplate impedance of the transformer; and Ir is the nameplate rated current of the transformer.
On observing one conducting period of a diode pair, it is interesting to see that the diodes conduct
only when the instantaneous value of the input AC waveform is higher than the DC bus voltage by at
least 3 V. Introducing a three-phase AC reactor in between the AC source and the DC bus makes the
current waveform less pulsating because the reactor impedes sudden change in current. The reactor also
electrically differentiates the DC bus voltage from the AC source so that the AC source is not clamped
to the DC bus voltage during diode conduction. This feature practically eliminates flat topping of the
AC voltage waveform caused by many ASDs when operated with weak AC systems.
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DC-Link Choke
Based on the above discussion, it can be noted that any inductor of adequate value placed between the AC
source and the DC bus capacitor of the ASD will help in improving the current waveform. These observations lead to the introduction of a DC-link choke, which is electrically present after the diode rectifier
and before the DC bus capacitor. The DC-link choke performs very similar to the three-phase line inductance. The ripple frequency that the DC-link choke has to handle is six times the input AC frequency for
a six-pulse ASD. However, the magnitude of the ripple current is small. One can show that the effective
impedance offered by a DC-link choke is approximately half of that offered by a three-phase AC inductor.
In other words, a 6% DC-link choke is equivalent to a 3% AC inductor from an impedance viewpoint.
This can be mathematically derived equating AC side power flow to DC side power flow as follows:
2
3 Г— V Lв€’N
-;
P ac = ------------------R ac
P ac = P dc
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VLв€’N is the line-neutral voltage at the input to the three-phase rectifier.
2
V
P dc = ------dc- ;
R dc
3 × 3 × 2 × VL – N
-;
V dc = ----------------------------------------------ПЂ
Hence, R dc = 2 пЈ« -----2пЈ¶ R ac
9
пЈ­ПЂ пЈё
2
om
Since 9/ПЂ is approximately equal to 1, the ratio of DC impedance to AC impedance can be said to be
approximately 1:2. The DC link choke is less expensive and smaller than a three-phase line reactor and
is often included inside an ASD. However, as the derivation shows, one has to keep in mind that the
effective impedance offered by a DC link choke is only half its numerical impedance value when referred
to the AC side. DC link chokes are electrically after the diode bridge and so they do not offer any significant
spike or overvoltage surge protection to the diode bridge rectifiers. It is a good engineering practice to
incorporate both a DC link choke and a three-phase line reactor in an ASD for better overall performance.
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Passive Filters
Passive filters consist of passive components like inductors, capacitors, and resistors arranged in a predetermined fashion either to attenuate the flow of harmonic components through them or to shunt the
harmonic component into them. Passive filters can be of many types. Some popular ones are series
passive filters, shunt passive filters, and low-pass broadband passive filters. Series and shunt passive filters
are effective only in the narrow proximity of the frequency at which they are tuned. Low-pass broadband
passive filters have a broader bandwidth and attenuate almost all harmonics above their cutoff frequency.
However, applying passive filters requires good knowledge of the power system because passive filter
components can interact with existing transformers and power factor correcting capacitors and could
create electrical instability by introducing resonance into the system. Some forms of low-pass broadband
passive filters do not contribute to resonance but they are bulky, expensive, and occupy space. A typical
low-pass broadband filter structure popularly employed by users of ASDs is shown in Fig. 4.17.
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Phase Multiplication
As discussed previously, the characteristic harmonics generated by a full-wave rectifier bridge converter
is a function of the pulse number for that converter. A 12-pulse converter will have the lowest harmonic
order of 11. In other words, the 5th, and the 7th harmonic orders are theoretically nonexistent in a 12-pulse
converter. Similarly, an 18-pulse converter will have harmonic spectrum starting from the 17th harmonic
and upwards. The lowest harmonic order in a 24-pulse converter will be the 23rd. The size of the passive
harmonic filter needed to filter out the harmonics reduces as the order of the lowest harmonic in the current
spectrum increases. Hence, the size of the filter needed to filter the harmonics out of a 12-pulse converter
is much smaller than that needed to filter out the harmonics of a 6-pulse converter. However, a 12-pulse
FIGURE 4.17 Schematic representation of a low-pass broadband harmonic filter connected to an ASD with diode
rectifier front end. (U.S. Patent 5,444,609.)
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FIGURE 4.18 Schematic of a 12-pulse converter employing a three-winding transformer. Note that the input
transformer has to be sized for rated power operation.
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converter needs two 6-pulse bridges and two sets of 30В° phase shifted AC inputs. The phase shift is achieved
either by using an isolation transformer with one primary and two phase-shifted secondary windings or
by using an autotransformer that provides phase-shifted outputs. Many different autotransformer topologies exist and the choice of a topology over the other involves a compromise between ease of construction,
performance, and cost. An 18-pulse converter would need three 6-pulse diode bridges and three sets of
20В° phase-shifted inputs; similarly, a 24-pulse converter would need four 6-pulse diode bridges and four
sets of 15В° phase-shifted inputs. The transformers providing the phase-shifted outputs for multipulse
converters have to be properly designed to handle circulating harmonic flux.
A typical 12-pulse structure is shown in Fig. 4.18. In one electrical cycle, the DC voltage will have
12 humps and hence the name 12-pulse rectifier.
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Active Harmonic Compensation
Most passive techniques discussed above aim to cure the harmonic problems once nonlinear loads have
created them. However, motor-drive manufacturers are developing rectification techniques that do not
generate low-order harmonics. These drives use active front ends. Instead of using diodes as rectifiers,
the active front-end ASDs make use of active switches like IGBTs along with parallel diodes. Power flow
through a switch becomes bidirectional and can be manipulated to recreate a current waveform that
linearly follows the applied voltage waveform.
Apart from the active front ends, there also exist shunt active filters used for actively introducing a
current waveform into the AC network, which, when combined with the harmonic current, results in an
almost perfect sinusoidal waveform.
One of the most interesting active filter topologies for use in retrofit applications is the combination of a
series active filter along with shunt tuned passive filters. This combination is also known as the hybrid structure.
Most active filter topologies are complicated and require active switches and control algorithms that
are implemented using digital signal processing (DSP) chips. The active filter topology also needs current
and voltage sensors and corresponding analog-to-digital (A/D) converters. This extra hardware increases
the cost and component count, reducing the overall reliability and robustness of the design. Manufacturers of smaller power equipment like computer power supplies, lighting ballast, etc. have successfully
employed active circuits, employing boost converter topologies.
Controlled Rectifiers
Controlled rectifier circuits make use of devices known as “thyristors.” A thyristor is a four-layer (pnpn),
three-junction device that conducts current only in one direction similar to a diode. The last (third)
junction is utilized as the control junction and consequently the rectification process can be initiated at
will provided the device is favorably biased and the load is of favorable magnitude. The operation of a
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Virtual representation of a thyristor to explain its operation.
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FIGURE 4.19
I
О± 1 = ----c1-;
I e1
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thyristor can be explained by assuming it to be made up of two transistors connected back-to-back as
shown in Fig. 4.19.
Let О±1 and О±2 be the ratio of collector to emitter currents of transistors Q1 and Q2, respectively. In
other words:
I
О± 2 = ----c2- ;
I e2
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Also, from Fig. 4.19: Ie1 = Ie2 = IA where IA is the anode current flowing through the thyristor. From
transistor theory, the value of Ie2 is equal to Ic2 + Ib2 + Ilkg ; where Ilkg is the leakage current crossing the
n1 = p2-junction. From Fig. 4.19, Ib2 = Ic1 . Hence, the anode current can be rewritten as:
I A = I c1 + I c2 + I lkg
Substituting the collector currents by the product of ratio О± and emitter current, the anode current
becomes:
I A = ( О± 1 Г— I e1 ) + ( О± 2 Г— I e2 ) + I lkg
I A = ( О± 1 + О± 2 )I A + I lkg
I lkg
I A = -----------------------------1 – ( α1 + α2 )
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If the ratios of the collector current to base current (gain) of the transistors are assumed to be ОІ1 and
ОІ2, respectively, then the relationship between to ОІ1 , ОІ2 and О±1 , О±2 can be written as:
ОІ1
-;
О± 1 = ------------1 + ОІ1
ОІ2
О± 2 = ------------1 + ОІ2
Substituting for О±1 and О±2 in the expression for IA yields the following expression:
( 1 + ОІ 1 ) ( 1 + ОІ 2 )I lkg
I A = ---------------------------------------------.
1 – β1 β2
If the values of О±1 and О±2 are low (low gains), then the anode current is low and comparable to the
leakage current. Under this condition, the thyristor is said to be in its OFF state. However, if the effective
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gain of the transistor is such that the product of the gains are close to 1 (i.e., sum of the ratios of О±1 and
О±2 are close to 1), then there is a large increase in anode current and the thyristor is said to be in
conduction. External circuit conditions can be changed to influence the product of the gains (ОІ1ОІ2). Some
techniques of achieving this are briefly discussed next.
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Increasing Applied Voltage
On applying a voltage across the anode to cathode terminals of the thyristor (anode being more positive
than the cathode), one can see that junctions J1 and J3 in Fig. 4.19 are forwardв€’biased while junction J2
is reverse-biased. The thyristor does not conduct any current and is said to be in a blocking state. On
increasing the applied voltage, minority carriers in junction J2 (i.e., holes in n1, n2 and electrons in p1, p2)
start acquiring more energy and hence start to migrate. In the process, these holes could dislodge more
holes. Recombination of the electrons and holes also occur, which creates more motion. If the voltage is
increased beyond a particular level, the movement of holes and electrons becomes great and junction J2
ceases to exist. The product of the gains of the two transistors in the two-transistor model is said to
achieve values close to unity. This method of forcing current to flow through the thyristor is not
recommended since junction J2 gets permanently damaged and the thyristor ceases to block forward
voltage. Hence, this method is a destructive method.
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High dv/dt
As explained earlier, junction J2 is the forward blocking junction when a forward voltage is applied across
anode to cathode of a thyristor. Any pn-junction behaves like a depletion region when it is reverse-biased.
Since J2 is reverse-biased, this junction behaves like a depletion region. Another way of looking at a
depletion region is that the boundary of the depletion region has abundant holes and electrons while
the region itself is depleted of charged carriers. This characteristic is similar to that of a capacitor. If the
voltage across the junction (J2) changes very abruptly, then there will be rapid movement of charged
carriers through the depleted region. If the rate of change of voltage across this junction (J2) exceeds a
predetermined value, then the movement of charged carriers through the depleted region is so high that
junction J2 is again annihilated. After this event, the thyristor is said to have lost its capability to block
forward voltage and even a small amount of forward voltage will result in significant current flow, limited
only by the load impedance. This method is destructive too, and is hence not recommended.
Temperature
Temperature affects the movement of holes and electrons in any semiconductor device. Increasing the
temperature of junction J2 will have a very similar effect. More holes and electrons will begin to move,
causing more dislodging of electrons and holes from neighboring lattice. If a high temperature is maintained, this could lead to an avalanche breakdown of junction J2 and again render the thyristor useless
since it would no longer be able to block forward voltage. Increasing temperature is yet another destructive
method of forcing the thyristor to conduct.
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Gate Current Injection
If a positive voltage is applied across the gate to cathode of a thyristor, then one would be forward-biasing
junction J3. Charged carriers will start moving. The movement of charged carriers in junction J3 will
attract electrons from n2 region of the thyristor (Fig. 4.19). Some of these electrons will flow out of the
gate terminal but there would be ample of electrons that could start crossing junction J2. Since electrons
in p2 region of junction J2 are minority carriers, these can cause rapid recombination and help increase
movement of minority carriers in junction J2. By steadily increasing the forward-biasing potential of
junction J3, one could potentially control the depletion width of junction J2. If a forward-biasing voltage
is applied across anode to cathode of the thyristor with its gate to cathode favorably biased at the same
time, then the thyristor can be made to conduct current. This method achieves conduction by increasing
the leakage current in a controlled manner. The gain product in the two-transistor equivalent is made
to achieve a value of unity in a controlled manner and the thyristor is said to turn ON. This is the only
recommended way of turning ON a thyristor. When the gate–cathode junction is sufficiently forwardbiased, the current through the thyristor depends on the applied voltage across the anode–cathode and
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FIGURE 4.20
v–i characteristic of a thyristor along with its symbol.
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the load impedance. The load impedance and the externally applied anode–cathode voltage should be
such that the current through the thyristor is greater than a minimum current known as latching current, Il.
Under such a condition, the thyristor is said to have latched ON. Once it has latched ON, the thyristor
remains ON. In other words, even if the forward-biasing voltage across the gate–cathode terminals is
removed, the thyristor continues to conduct. Junction J2 does not exist during the ON condition. The
thyristor reverts to its blocking state only when the current through it falls below a minimum threshold
value known as holding current, Ih. Typically, holding current is lower than latching current (Ih < Il).
There are two ways of achieving this. They are either (1) increase the load impedance to such a value
that the thyristor current falls below Ih or (2) apply reverse-biasing voltage across the anode-cathode
of the thyristor.
An approximate v–i characteristic of a typical thyristor and its symbol are shown in Fig. 4.20.
Since the thyristor allows flow of current only in one direction like a diode and the instant at which
it is turned ON can be controlled, the device is a key component in building a controlled rectifier unit.
One can replace the diode in all the circuits discussed so far with the thyristor. Because of its controllability,
the instant at which the thyristor conducts can be delayed to alter the average and rms output voltages.
By doing so, one can choose to control the output voltage and power of a rectifier circuit. Hence, rectifiers
that employ thyristors are also known as silicon controlled rectifiers or SCR.
A typical single-phase, R-L rectifier circuit with one thyristor as the rectifier is shown in Fig. 4.21. The
figure also shows the relevant circuit waveforms. The greatest difference between this circuit and its diode
counterpart is also shown for comparison. Both circuits conduct beyond ПЂ radians due to the presence of
the inductor L since the average voltage across an inductor is zero. If the value of the circuit components
and the input supply voltage are the same in both cases, the duration for which the current flows into
the output R-L load depends on the values of R and L. In the case of the diode circuit, it does not depend
on anything else; while in the case of the thyristor circuit, it also depends on the instant the thyristor is
given a gate trigger.
From Fig. 4.21, it is interesting to note that the energy stored in the inductor during the conduction
interval can be controlled in the case of a thyristor is such a manner so as to reduce the conduction
interval and thereby alter (reduce) the output power. Both the diode and the thyristor show reverse
recovery phenomenon. The thyristor, like the diode, can block reverse voltage applied across it repeatedly,
provided the voltage is less than its breakdown voltage.
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Q
L
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Gate Circuit Requirements
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FIGURE 4.21 Comparing a single diode rectifier circuit with a single thyristor rectifier circuit. Note that the thyristor
conduction is delayed deliberately to bring out the differences.
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The trigger signal should have voltage amplitude greater than the minimum gate trigger voltage of the
thyristor being turned ON. It should not be greater than the maximum gate trigger voltage, either. The
gate current should likewise be in between the minimum and maximum values specified by the thyristor
manufacturer. Low gate current driver circuits can fail to turn ON the thyristor. The thyristor is a current
controlled switch and so the gate circuit should be able to provide the needed turn ON gate current into
the thyristor. Unlike the bipolar transistor, the thyristor is not an amplifier and so the gate current requirement does not absolutely depend on the voltage and current rating of the thyristor. Sufficient gate trigger
current will turn ON the thyristor and current will flow from the anode to the cathode provided that
the thyristor is favorably biased and the load is such that the current flowing is higher than the latching
current of the thyristor. In other words, in single phase AC to DC rectifier circuits, the gate trigger will
turn ON the thyristor only if it occurs during the positive part of the AC cycle (Fig. 4.21). Any trigger
signal during the negative part of the AC cycle will not turn ON the thyristor and the thyristor will
remain in blocking state. Keeping the gate signal ON during the negative part of the AC cycle does not
typically damage a thyristor.
Single-Phase H-Bridge Rectifier Circuits with Thyristors
Similar to the diode H-bridge rectifier topology, there exist SCR-based rectifier topologies. Because of
their unique ability to be controlled, the output voltage and hence the power can be controlled to desired
levels. Since the triggering of the thyristor has to be synchronized with the input sinusoidal voltage in
an AC to DC rectifier circuit, one can achieve a soft-charge characteristic of the filter capacitor. In other
words, there is no need for employing soft-charge resistor and contactor combination as is required in
single-phase and three-phase AC to DC rectifier circuits with DC bus capacitors.
In controlled AC-to-DC rectifier circuits, it is important to discuss control of resistive, inductive, and
resistive-inductive load circuits. DC motor control falls into the resistive-inductive load circuit. DC motors
are still an important part of the industry. However, the use of DC motors in industrial applications is
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FIGURE 4.23
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FIGURE 4.22 Single-phase DC motor control circuit for controlling a separately excited DC motor. Ra indicates
equivalent armature resistance and E is the back emf.
Armature current and output voltage of AC-to-DC rectifier employed to control a DC motor.
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declining rapidly. Control of DC motors are typically achieved by controlled rectifier circuits employing
thyristors. Small motors of less than 3 kW (approximately 5 hp) rating can be controlled by single-phase
SCR circuits while larger ratings require three-phase versions. A typical single-phase H-bridge SCR-based
circuit for the control of a DC motor is shown in Fig. 4.22. Typical output waveforms are shown in
Fig. 4.23. The current in the load side can be assumed continuous due to the large inductance of the
armature of the DC motor.
In Fig. 4.22, Vf is the field voltage, which is applied externally and generally is independent of the
applied armature voltage. Such a DC motor is known as a separately excited motor. Ia is the armature current
while If is the field current. The output of the controlled rectifier is applied across the armature. Since
the output voltage can be controlled, one can effectively control the armature current. Since the torque
produced by a DC motor is directly proportional to the armature current, the torque developed can thus
be controlled.
T = K П† Ia
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E = K П†П‰ = K ( K 1 I f ) П‰
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where K is the motor constant and depends on the number of armature conductors, number of poles,
and type of winding employed in the DC machine. П† is flux produced by the field and is proportional
to the field current, If . Hence, the torque produced by a DC machine can be rewritten as T = K(K1If )Ia.
By keeping the field current constant, the torque then becomes directly proportional to the armature
current, which is controlled by controlling the output voltage of the AC-to-DC controlled rectifier. In
the circuit shown in Fig. 4.22, it is interesting to note that the current Ia, cannot flow in the opposite
direction. Hence, the motor cannot generate negative torque. In order to make the motor run in the
opposite direction, the direction of the field has to be changed. Speed control within the base speed can
also be accomplished by controlling the armature voltage as is shown below.
Va – ( Ia Ra )
П‰ = -------------------------KK 1 I f
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E = V a – ( I a R a );
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П‰ is the speed of the armature in rad/s. The back emf, E, is the difference between the output DC voltage
of the AC-to-DC controlled rectifier and the drop across the equivalent armature resistance. Hence, E
can be rewritten as:
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For control of speed beyond base speed, the field current has to be altered. Hence, it can be shown that
controlling the armature current can control the speed and torque produced by a DC machine. Controlling the output DC voltage can control the armature current. Because of the large inductance of the
armature circuit, the current through it can be assumed to be continuous for a practical operating region.
The average output voltage of a single-phase AC-to-DC rectifier circuit for continuous current operation
is given by (referring to Fig. 4.23):
1
V o = --ПЂ
ПЂ+О±
∫
О±
2 Г— 2 Г— V rms Г— cos ( О± )
-.
( 2 Г— V rms ) d( wt ) = ------------------------------------------------------ПЂ
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for continuous current condition. By controlling the triggering angle, О±, one can control the average
value of the output voltage, VO. If armature current control is the main objective (to control output
torque), then one can configure the controller of Fig. 4.22 with a feedback loop. The measured current
can be compared with a set reference and the error can be used to control the triggering angle, О±. Since
the output voltage and hence the armature current are not directly proportional to О± but to cos(О±), the
above method will yield a nonlinear (co-sinusoidal) relationship between the output voltage and control
angle, О±. However, one could choose to use the error signal to control cos(О±) instead of О±. This would
then yield a linear relationship between the output voltage and cos of control angle, О±.
It is interesting to note from the equation for the output average voltage that the output average voltage
can become negative if the triggering angle is greater than 90 electrical degrees. This leads us to the topic
of regeneration. AC-to-DC controlled rectifiers employing thyristors and having large inductance on the
DC side can be made to operate in the regeneration mode by simply delaying the trigger angle. This is
quite beneficial in overhauling loads like cranes. When the load on a hook of the crane has to be lifted
up, electrical energy is supplied to the motor. The voltage across the motor is positive and the current
through the armature is positive. Positive torque is generated and hence the load moves up. When the
load is to be brought down, the load starts to rotate the motor in the opposite direction due to gravity.
The voltage at the terminals becomes negative since speed is negative. The thyristors are gated at an angle
greater than 90 electrical degrees to match the generated (negative) voltage of the DC motor. Since current
through the thyristors cannot go negative, current is forced to flow into the DC motor in the positive
direction. The large inductance of the motor helps to maintain the positive direction of current through
the armature. Positive torque is still produced since the direction of current is still positive and the field
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Four-quadrant operation of a crane or hoist.
FIGURE 4.25
Two rectifier-bridge arrangements for four-quadrant operation of DC motor.
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FIGURE 4.24
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remains unchanged. In other words, the motor develops positive torque and tries to move the load up
against gravity but the gravity is pushing the motor down. The product of current through the motor
and the voltage across it is negative, meaning that the motor is not consuming energy, and on the contrary,
is producing electrical energy—the kinetic energy due to the motor’s downward motion is partly converted to electrical energy by the field and armature. This energy produced by the motor is routed out
to the supply via the appropriately gated thyristors. Conversion of kinetic energy to electrical energy acts
like a dynamic-brake and slows the rapid downward descent of the load.
A typical crane is required to operate in all four quadrants (Fig. 4.24). In the first quadrant, the motor
develops positive torque and the motor runs in the positive direction, meaning its speed is positive—
the product of torque and speed is power, and so positive electric power is supplied to the motor from
the AC-to-DC rectifier. When the crane with a load is racing upward, close to the end of its travel, the
AC-to-DC controlled rectifier is made to stop powering the motor. The rectifier generates practically no
voltage. The inertia of the load moving upward generates a voltage in the form of a back emf. This voltage
is fed into a second rectifier bridge arranged in the opposite direction as shown in Fig. 4.25. The second
bridge is turned ON to let the generated voltage across the still upwardly mobile motor flow into the
utility, thereby converting the inertial motion to electric power. In the second quadrant, speed remains
positive but torque becomes negative, since the current through the motor flows in the opposite direction
into the second rectifier bridge arrangement (Fig. 4.25). The product of speed and torque is negative,
meaning that the motor behaves like a generator during this part of the travel.
The third quadrant, as explained earlier, occurs at the beginning of the lowering action. Both torque
and speed are negative and so the product of torque and speed is positive. Power is applied to the motor
to overcome static friction and accelerate the rotating parts of the mechanism to move the load downward.
In this case, the direction of armature current through the motor is opposite to that in quadrant 1, and
the electrical power needed by the motor is supplied by the second rectifier bridge arrangement (Fig. 4.25).
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Three-Phase Controlled AC-to-DC Rectifier Systems
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The mechanical load and motor arrangement goes into the fourth quadrant of operation for the larger
part of the downward motion. This is the period during which the motor resists the tendency of the load
to accelerate downward by developing positive torque. Since motion is downward, speed is negative
and the product of torque and speed is negative. This means the motor behaves like a generator as explained
earlier.
Since the thyristors cannot conduct in the opposite direction, a new inverter section had to be provided
to enable the four-quadrant operation needed in cranes and hoists. The method by which unidirectional
electrical power was routed to the bidirectional AC utility lines is known as inversion (opposite of
rectification). Since no external means of switching OFF the thyristors was employed, the process of
inversion was achieved by natural commutation. Such an inverter is also known as a line commutated
inverter.
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The observations made so far for the single-phase controlled AC-to-DC rectifiers can be easily extended
to three-phase versions. An important controlled rectification scheme that was not mentioned in the
single-phase case is the semiconverter circuit. In Fig. 4.22, if the thyristors Q2 and Q4 are replaced by
diodes (D2 and D4), then the circuit of Fig. 4.22 is converted into a semiconverter circuit. Such a circuit does
not have the potential to provide regeneration capability and hence is of limited use. However, in dual
converter applications, especially in three-phase versions, there are a few instances where a semiconverter
can be employed to reduce cost. A typical three-phase semiconverter circuit will consist of three thyristors
and three diodes arranged in an H-bridge configuration as shown in Fig. 4.26.
Three-phase dual converter schemes similar to the one shown in Fig. 4.25 are still employed to operate
large steel mills, hoists, and cranes. However, the advent of vector-controlled AC drives has drastically
changed the electrical landscape of the modern industry. Most DC motor applications are being rapidly
replaced by AC motors with field-oriented control schemes. DC motor application in railway traction
has also seen significant reduction due to the less expensive and more robust AC motors.
However, there are still a few important applications where three-phase controlled rectification (inversion) is the most cost-effective solution. One such application is the regenerative converter module that
many inverter-drive manufacturers provide as optional equipment to customers with overhauling loads.
Under normal circumstances, during the motoring mode of operation of an AC drive, the regenerative
unit does not come into the circuit. However, when the DC bus voltage tends to go higher than a
predetermined level due to overhauling of the load, the kinetic energy of the load is converted into
electrical energy and is fed back into the AC system via a six-pulse thyristor-based inverter-bridge. One
such scheme is shown in Fig. 4.27.
FIGURE 4.26
A typical three-phase semiconverter. Rarely employed in modern industry.
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FIGURE 4.27 Use of six-pulse thyristor bridge in the inverter mode to provide regeneration capability to an existing
AC drive system.
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Average Output Voltage
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In order to evaluate the average value of the output voltage for a three-phase full-bridge converter, the
process of integrating the output voltage similar to the one in Fig. 4.15b has to be undertaken. For the
circuit shown in Fig. 4.14b, where the diodes are replaced by thyristors, the integration yields the following:
3
V o = --ПЂ
О± + ( 2 ПЂ /3 )
∫
О±+(ПЂ 3)
2V Lв€’L sin ( wt ) d( wt )
3 Г— 2 Г— V Lв€’L Г— cos ( О± )
3 Г— 2 Г— 3 Г— V Lв€’N Г— cos ( О± )
V o = -------------------------------------------------------= ---------------------------------------------------------------------ПЂ
ПЂ
The average output voltage for the circuit in Fig. 4.14b with the diodes being replaced by thyristors is
only different in the cosine of the triggering angle, О±. If the triggering angle is zero, the circuit performs
similar to a three-phase diode rectifier and the average output voltages become the same.
HVDC Transmission Systems
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One area where it is difficult to replace the use of high voltage, high current carrying thyristors is high
voltage DC (HVDC) transmission systems. When a large amount of power is to be transported over long
distances, or under water, it has been found that high voltage DC transmission is more economical.
HVDC systems are in reality back-to-back rectifier systems. The sending end rectifier system consists
typically of 12- or 24-pulse thyristor bridges while the receiving end consists of a similar configuration
but in the opposite direction. The receiving end 12- or 24-pulse bridge operates in the inverter mode
while the sending end operates in the rectifier mode. 12-pulse configuration is achieved by cascading two
six-pulse bridges in series while 24-pulse configuration needs four six-pulse bridges cascaded in series.
Typical advantages of high voltage DC transmission over high voltage AC transmission is briefly listed below:
1. No stability problems due to transmission line length since no reactive power needs to be transmitted.
2. No limitation of cable lengths for underground cable or submarine cable transmission due to the
fact that no charging power compensation need be done.
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3. AC power systems can be interconnected employing a DC tie without reference to system frequencies, short circuit power, etc.
4. High-speed control of DC power transmission is possible due to the fact that the control angle,
О±, has a relatively short time constant.
5. Fault isolation between receiving end and sending end can be dynamically achieved due to fast
efficient control of the high voltage DC link.
6. Employing simple control logic can change energy flow direction very fast. This can help in meeting
peak demands at either the sending or the receiving station.
7. High reliability of thyristor converter and inverter stations makes this mode of transmission a
viable solution for transmission lengths typically over 500 km.
8. The right-of-way needed for high voltage DC transmission is much lower than that of AC transmission of the same power capacity.
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The advantages of DC transmission over AC should not be misunderstood and DC should not be
considered as a general substitute for AC power transmission. In a power system, it is generally believed
that both AC and DC should be considered as complementary to each other, so as to bring about the
integration of their salient features to the best advantage in realizing a power network that ensures high
quality and reliability of power supply. A typical rectifier-inverter system employing a 12-pulse scheme
is shown in Fig. 4.28.
Typical DC link voltage can be as high as 400 to 600 kV. Higher voltage systems are also in use. Typical
operating power levels are over 1000 MW. There are a few systems transmitting close to 3500 MW of
power through two bipolar systems. Most thyristors employed in large HVDC transmission systems are
liquid cooled to improve their performance.
Power System Interaction with Three-Phase Thyristor AC-to-DC Rectifier Systems
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Similar to the diode rectifiers, the thyristor based AC-to-DC rectifiers also suffer from low order current
harmonics. In addition to current harmonics, there is a voltage notching phenomenon occurring at the
input terminals of an AC-to-DC thyristor based rectifier system. The voltage notching is a very serious
problem. Since thyristors are generally slower to turn ON and turn OFF compared to power semiconductor diodes, there are nontrivial durations during which an outgoing thyristor and an incoming
FIGURE 4.28
scheme.
Schematic representation of a bipolar HVDC system employing 12-pulse rectification/inversion
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thyristor remain in conduction, thereby creating a short-circuit across the power supply phases feeding the
corresponding thyristors. Thyristors used in rectifiers are generally known as phase control type thyristors
and have typical turn OFF times of 50 to 100 Вµs. Thyristors employed in inverter circuits typically are
faster and have turn OFF times in the 10 to 50 Вµs range.
Notching can create major disturbances in sensitive electronic equipment that rely on the zero-crossing
of the voltage for satisfactory operation. Multiple pseudo-zero-crossings of the voltage waveform can occur
due to the notching effect of thyristor-based rectifier systems. Notching can create large magnitudes of
currents to flow into power-factor correcting capacitors, thereby potentially causing permanent damage
to them. IEEE Std. 519-1992 in the United States has strict regulations regarding the depth of the notch
as well as the duration of the notch. AC line inductors in series with the supply feeding power to the
three-phase bridge help to minimize the notching effect on the power system. The theory behind this
phenomenon is discussed next.
When an external inductance is added in front of a three-phase AC-to-DC rectifier employing thyristors, the duration of commutation increases. In other words, the time period for which the outgoing
thyristor remains in conduction along with the incoming thyristor increases. This overlap period causes
the average output voltage to reduce because during this period, the output voltage is composed of two
shorted phases and a healthy phase. The extent of reduction in the output voltage depends on the duration
of overlap in electrical degrees. The duration of overlap in electrical degrees is commonly represented
by Вµ. The overlap duration is directly proportional to the value of the external inductance used. If no
external line inductor is used, then this duration will depend on the existing inductance of the system
including the wiring inductance. In order to compute the factors influencing the overlap duration, a
simple model can be assumed. Assume that the line is comprised of inductance L in each phase. Let the
DC load current be Idc and let it be assumed that this current does not change during the overlap interval.
The current in the incoming thyristor is zero at start and by the end of the overlap interval, it rises to
Idc. Based on this assumption, the relationship between current and voltage can be expressed as:
v ab =
2 Г— V Lв€’L Г— sin ( wt ) = 2 Г— L Г— ( di/dt )
2 Г— V Lв€’L Г—
∫
О± + ( ПЂ /3 ) + Вµ
О± + ( ПЂ /3 )
sin ( wt ) d( t ) = 2 Г— L Г—
∫
I dc
di
0
2 × V L−L × ( cos ( α + π /3 ) – cos ( α + π /3 + µ ) )
I dc = -----------------------------------------------------------------------------------------------------------------2wL
2 Г— V Lв€’L Г— sin ( О± + ПЂ 3 + Вµ /2 ) Г— sin ( Вµ /2 )
= ------------------------------------------------------------------------------------------------------wL
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For small values of overlap angle Вµ, sin(Вµ/2) = Вµ/2 and sin(О± + ПЂ/3 + (Вµ/2)) = sin(О± + ПЂ/3). Rearranging
the above equation yields:
2wL Г— I dc
Вµ = -------------------------------------------------------------2 Г— V Lв€’L Г— sin ( О± + ПЂ вЃ„ 3 )
From the above expression, it is interesting to note the following:
1. If the inductance L in the form of either external inductance or leakage inductance of transformer
or lead length is large, the overlap duration will be large.
2. If the load current Idc is large, the overlap duration will be large.
3. If the delay angle is small, then the inductance will store more energy and so the duration of
overlap will be large. The minimum value of delay angle О± is 0В° and the maximum value typically
is 60В°.
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The average output voltage will reduce due to the overlap angle as mentioned before. In order to
compute the average output voltage with a certain overlap angle, the limits of integration have to be
changed. This exercise yields the following:
3
V o = --ПЂ
∫
О± + Вµ + ( 2 ПЂ /3 )
О±+Вµ+(ПЂ 3)
2V L – L sin ( wt ) d( wt )
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3 Г— 2 Г— V Lв€’L Г— cos ( О± + Вµ )
3 Г— 2 Г— 3 Г— V Lв€’N Г— cos ( О± + Вµ )
- = -------------------------------------------------------------------------------V o = ----------------------------------------------------------------ПЂ
ПЂ
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Thus, it can be seen that the overlap angle has an equivalent effect of advancing the delay angle, thereby
reducing the average output voltage. From the discussions in the previous paragraphs on notching, it is
interesting to note that adding external inductance increases the duration of the overlap and reduces the
average value of the output DC voltage. However, when viewed from the AC source side, the notching
effect is conspicuously reduced and in some cases not observable. Since all other electrical equipment in
the system will be connected to the line side of the AC inductor (in front of a thyristor-based AC-to-DC
rectifier), these equipment will not be affected by the notching phenomenon of thyristors. The external
inductance also helps limit the circulating current between the two thyristors during the overlap duration.
Conclusion
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Uncontrolled and controlled rectifier circuits have been discussed in this section. An introduction to the
theory of diode and thyristor conduction has been presented to explain the important operating characteristics of these devices. Rectifier topologies employing both diodes and thyristors and their relative
advantages and disadvantages have been discussed. Use of a dual thyristor bridge converter to achieve
four-quadrant operation of a DC motor has been discussed. The topic of high-voltage DC (HVDC)
transmission has been briefly introduced. Power quality issues relating to diode and thyristor-based
rectifier topologies has also been addressed. To probe further into the various topics briefly discussed in
this section, the reader is encouraged to refer to the references listed below.
References
Pa
Dewan, S. B. and Straughen, A., Power Semiconductor Circuits, John Wiley & Sons, New York, 1975.
Hoft, R. G., Semiconductor Power Electronics, Van Nostrand Reinhold, New York, 1986.
IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE
Std. 519–1992.
Laughton M. A. and Say, M. G., Eds., Electrical Engineer’s Reference Book, 14th ed., Butterworths, Boston,
1985.
Passive Harmonic Filter Systems for Variable Frequency Drives, U.S. Patent 5,444,609, 1995.
Sen, P. C., Principles of Electric Machines and Power Electronics, John Wiley & Sons, New York, 1997.
4.3 Three-Phase Pulse-Width-Modulated Boost-Type Rectifiers
Ana Stankovic
Introduction
The boost-type rectifier has been extensively developed and analyzed in recent years [1, 3, 6]. It offers
advantages over traditionally used phase-controlled thyristor rectifiers in AC-DC-AC converters for
variable-control drives because of its capability for nearly instantaneous reversal of power flow, power
factor management, and reduction of input harmonic distortion. Figure 4.29 shows the structure of the
pulse-width-modulated (PWM) boost-type rectifier.
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I0
SW1
SW3
SW2
VDC
i1
U2
+
L1
i2
U3
+
L1
i3
SW4
SW5
PWM boost-type rectifier.
I
1
U1
jXI
1
+
Vs1
kw
eb
in
U1
FIGURE 4.30
I
1
fo
jX
+
SW6
do
FIGURE 4.29
L
O
A
D
om
L1
tc
+
U1
Vs1
The per-phase equivalent circuit and phasor diagram.
Pa
Power flow in the PWM converter is controlled by adjusting the phase shift angle Оґ between the source
voltage U1 and the respective converter reflected input voltage Vs1 [2].
When U1 leads Vs1 the real power flows from the AC source into the converter. Conversely, if U1 lags
Vs1, power flows from the DC side of the converter into the AC source. The real power transferred is
given by the Eq. (4.10).
U 1 V s1
- sin (Оґ )
P = ------------X1
(4.10)
The AC power factor is adjusted by controlling the amplitude of Vs1. The phasor diagram in Fig. 4.30
shows that, to achieve a unity power factor, Vs1 must be
V s1 =
2
U1 + ( X1 I1 )
2
(4.11)
Indirect Current Control of a Unity Power Factor Sinusoidal Current
Boost-Type Rectifier
To control the DC output voltage of the PWM boost-type rectifier, the input line currents must be
regulated [4, 5]. In typical rectifier controllers presented to date, the DC bus voltage error is used to
synthesize a line current reference. Specifically, the line current reference is derived through the multiplication of a term proportional to the bus voltage error by a template sinusoidal waveform. The sinusoidal
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FIGURE 4.31
Indirect current control of the unity power factor boost-type rectifier.
Pa
template is directly proportional to the input voltage, resulting in a unity power factor. The line current
is then controlled to track this reference. Current regulation is accomplished through the use of hysteresis
controllers [5]. A proposed control method [4] is shown in Fig. 4.31.
To explain the closed-loop operation of the PWM boost-type rectifier, the switch matrix theory is
used. The output current I0 of the matrix converters is a function of the converter transfer function vector
T and the input current vector i and is given by,
I 0 = Ti
(4.12)
The converter transfer function vector T is composed of three independent line-to-neutral switching
functions: SW1, SW2, SW3.
T = [ SW 1 SW 2 SW 3 ]
(4.13)
The input current vector is given by
i1
i = i2
i3
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(4.14)
The line-to-neutral switching functions are balanced and are represented by their fundamental components only.
SW 1 ( t ) = S 1 sin ( wt – � )
SW 2 ( t ) = S 1 sin ( wt – � – 120° )
(4.15)
om
SW 3 ( t ) = S 1 sin ( wt + 120° – � )
Therefore, converter synthesized line-to-neutral voltages can be expressed as
tc
1
V s1 = --V dc S 1 sin ( wt – � )
2
1
V s2 = --V dc S 1 sin ( wt – 120° – � )
2
1
V s3 = --V dc S 1 sin ( wt + 120° – � )
2
(4.16)
do
Eq. (4.15) shows the rectifier synthesized voltages. Vdc represents the output DC voltage.
In the time domain, the fundamental components of the three-phase input currents are given by
i 1 ( t ) = I 1 sin ( wt – ϕ 1 )
i 2 ( t ) = I 1 sin ( wt – 120° – ϕ 1 )
(4.17)
fo
i 3 ( t ) = I 1 sin ( wt + 120° – ϕ 1 )
kw
eb
in
By combining Eqs. (4.12), (4.15), and (4.17), the output current I0(t) is obtained, given by
I 0 ( t ) = I 1 sin ( wt – ϕ 1 ) S 1 sin ( wt – � ) + I 1 sin ( wt – 120° – ϕ 1 )S 1 sin ( wt – 120° – � )
+ I 1 sin ( wt + 120° – ϕ 1 ) S 1 sin ( wt + 120° – � )
(4.18)
By using a trigonometric identity, I0(t) becomes
3
I 0 ( t ) = -- I 1 S 1 cos ( � – ϕ 1 )
2
(4.19)
Pa
Because the angle (О� в€’ П•1) is constant for any set value of the input power factor, the output DC
current, I0(t), is proportional to the magnitude of the input current, I1(t), and so is the output voltage,
Vdc. For unity power factor control, angle П•1 is equal to zero.
The output voltage, Vdc is
V dc = RI 0
(4.20)
( V dcref – V dc ) = KI 1
(4.21)
Figure 4.30 shows that the DC bus error, (Vdcref в€’ Vdc), is used to set the reference for the input current
magnitude. The input sinusoidal voltage, Ua, is multiplied by the DC bus error and it becomes a reference
for the input current in phase 1. The reference value for current in phase 2 is phase-shifted by 120В° with
respect to the current in phase 1. Since the sum of three input currents is always zero, the reference for
current in phase 3 is obtained from the following equation:
i 3ref ( t ) = −i 1ref ( t ) – i 2ref ( t )
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(4.22)
om
FIGURE 4.33
Input voltage and input current.
tc
Hysteresis controller for one phase.
kw
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in
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do
FIGURE 4.32
Pa
The input currents, i1(t), i2(t), i3(t) are measured and compared with the reference currents, i1ref(t),
i2ref(t), i3ref(t). The error is fed to a comparator with a prescribed hysteresis band 2∆I. Switching of the
leg of the rectifier (SW1 off and SW4 on) occurs when the current attempts to exceed a set value
corresponding to the desired current iref + ∆I. The reverse switching (SW1 on and SW4 off) occurs when
the current attempts to become less than iref − ∆I. The hysteresis controller produces a very good quality
waveform and is simple to implement. Unfortunately, with this type of control (hysteresis controller) the
switching frequency does not remain constant but varies along different portions of the desired current.
A hysteresis current controller for one phase is shown in Fig. 4.32.
Example
Simulate the three-phase PWM rectifier of Fig. 4.31 using SABER with the following parameters: U1 =
U2 = U3 = 120 V at 60 Hz, L1 = 1 mH, C = 100 µF, and Rload = 100 Ω. Set the reference value for the
output DC voltage, Vdcref, to 300 V and input a power factor to unity. Assume the switches to be ideal
and choose the time step ∆t = 20 µs.
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tc
Output voltage Vdc.
do
FIGURE 4.34
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Solution
The SABER program listing is included in the Appendix to this section, and the results are shown in
Figs. 4.33 and 4.34. Figure 4.33 shows input currents as well as input voltages of the PWM boost-type
rectifier shown in the Fig. 4.31. Figure 4.33 shows controlled output voltage, Vdc.
High-quality input currents and output DC voltage are obtained at a unity power factor.
References
Pa
1. L. Moran, P. D. Ziogas, and G. Joos, Design aspects of synchronous PWM rectifier-inverter system
under unbalanced input voltages conditions, IEEE Trans. Ind. Appl., 28(6), 1286–1293, Nov./Dec.
1992.
2. J. W. Wilson, The forced-commutated inverter as a regenerative rectifier, IEEE Trans. Ind. Appl., IA14(4), 335–340, July/Aug. 1978.
3. A. V. Stankovic and T. A. Lipo, A novel control method for input-output harmonic elimination of
the PWM boost type rectifier under unbalanced operating conditions, IEEE Trans. Power Electron.,
16(5), Sept. 2001.
4. J. W. Dixon, and B. T. Ooi, Indirect current control of a unity power factor sinusoidal current boost
type three-phase rectifier, IEEE Trans. Ind. Electron., 35(4), 508–515, Nov./Dec. 1988.
5. D. M. Brod and D. W. Novotny, Current control of VSI-PWM inverters, IEEE Trans. Ind. Appl., IA21(4), 769–775, Nov./Dec. 1984.
6. T. A. Lipo, Recent progress and development of solid state AC motor drives, IEEE Trans. Power
Electron., 3(2), 105–117, April 1988.
Appendix
#
#
#
#
#
Hysteresis controller-Balanced three-phase system
Reference voltage = 300 V
Small resistor is added in series with the inductor for current measurements
igbt pwm rectifier
main voltage sources
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# Program for balanced voltages and balanced impedances-Closed-loop solution
# Hysteresis Controller + P-I controller
r.111 n11 n111 = 0.001
r.222 n12 n122 = 0.001
r.333 n13 n133 = 0.001
tc
v.vpl vp 0 = dc = 10
v.vmi vn 0 = dc = -10
v.vco vnn 0 = dc = 5
v.vcont vc1 0 = 7
# reference voltage source
vac.va1 na 0 = va = 6.6655, theta = -1.0803, f
vac.vb1 nb 0 = va = 6.6655, theta = -121.0803,
vac.vc1 nc 0 = va = 6.6655, theta = -118.9197,
# sow generator
v.vsow vs 0 = tran = (pulse = (v1 = -10,v2 =
250u,pw = 0,per = 500u))
#r.r30 n30 n18 = 1
r.rou1 vp ou1 = 2.2k
r.rou2 vp ou2 = 2.2k
r.rou3 vp ou3 = 2.2k
r.rout11 vp out1 = 2.2k
r.rout22 vp out2 = 2.2k
r.rout33 vp out3 = 2.2k
om
vac.va n11 n17 = va = 120, theta = 0,
f = 60
vac.vb n12 n17 = va = 120, theta = -120, f = 60
vac.vc n13 n17 = va = 120, theta = 120, f = 60
do
= 50
f = 50
f = 50
#cmp1_oc.u1
#cmp1_l4.u2
#cmp1_oc.u3
#cmp1_l4.u4
#cmp1_oc.u5
#cmp1_l4.u6
kw
eb
in
fo
10,td = 0,tr = 250u,tf =
na vs ou1 vp vn 0 = model = (vos=0, tdr=2n, tdf=2n, av=10K)
ts1 vnn t4 vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
nb vs ou2 vp vn 0 = model = (vos = 0, tdr = 2n, tdf = 2n, av = 10K)
ts3 vnn t6 vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
nc vs ou3 vp vn 0 = model = (vos = 0, tdr = 2n, tdf = 2n,av = 10K)
ts5 vnn t2 vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
Pa
#Comparison between two
cmp1_oc.u20 n2000 n2001
= 2n, av = 100K)
cmp1_l4.u21 out1 vnn t4
cmp1_oc.u22 n2002 n2003
= 2n, av = 100K)
cmp1_l4.u23 out2 vnn t6
cmp1_oc.u24 n2004 n2005
= 2n, av = 100K)
cmp1_l4.u25 out3 vnn t2
inv_l4.in1 t4 t1
inv_l4.in2 t6 t3
inv_l4.in3 t2 t5
current signals
out1 vp vn 0 = model = (vos = 0, tdr = 2n, tdf
vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
out2 vp vn 0 = model = (vos = 0, tdr = 2n, tdf
vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
out3 vp vn 0 = model = (vos = 0, tdr = 2n, tdf
vp 0 0 = model = (vos = 0, tdr = 2n, tdf = 2n)
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ide_d2an.vd1
ide_d2an.vd2
ide_d2an.vd3
ide_d2an.vd4
ide_d2an.vd5
ide_d2an.vd6
t1
t2
t3
t4
t5
t6
tt1
tt2
tt3
tt4
tt5
tt6
0
0
0
0
0
0
=
=
=
=
=
=
model
model
model
model
model
model
=
=
=
=
=
=
(voh
(voh
(voh
(voh
(voh
(voh
=
=
=
=
=
=
5,vol
5,vol
5,vol
5,vol
5,vol
5,vol
=
=
=
=
=
=
0,tr
0,tr
0,tr
0,tr
0,tr
0,tr
=
=
=
=
=
=
500n,tf
500n,tf
500n,tf
500n,tf
500n,tf
500n,tf
=
=
=
=
=
=
250n)
250n)
250n)
250n)
250n)
250n)
kw
eb
in
fo
do
tc
om
# optical isolation : voltage-controlled voltage-source
#clipv.vgg1 tt1 0 vg1 0 = a = 100, vomax = 10, vomin = 0
#clipv.vgg2 tt2 0 vg2 0 = a = 100, vomax = 10, vomin = 0
#clipv.vgg3 tt3 0 vg3 0 = a = 100, vomax = 10, vomin = 0
#clipv.vgg4 tt4 0 vg4 0 = a = 100, vomax = 10, vomin = 0
#clipv.vgg5 tt5 0 vg5 0 = a = 100, vomax = 10, vomin = 0
#clipv.vgg6 tt6 0 vg6 0 = a = 100, vomax = 10, vomin = 0
# three-phase bridge
1.1a n111 n14 = 1m
1.1b n122 n15 = 1m
1.1c n133 n16 = 1m
sdr_thr2.s1 tt1 0 ps1 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sdr_thr2.s2 tt2 0 ps2 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sdr_thr2.s3 tt3 0 ps3 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sdr_thr2.s4 tt4 0 ps4 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sdr_thr2.s5 tt5 0 ps5 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sdr_thr2.s6 tt6 0 ps6 = vpull = 4.9,vdrop = 0.1,tdelay = 0m
sw_1pno.1 ps1 n10 n14 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
sw_1pno.2 ps2 n16 0 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
sw_1pno.3 ps3 n10 n15 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
sw_1pno.4 ps4 n14 0 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
sw_1pno.5 ps5 n10 n16 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
sw_1pno.6 ps6 n15 0 = ron = 0,roff = inf,tdbrk = 0,tdmk = 0,rfunc
=
=
=
=
=
=
Pa
d.d1 n14 n10
d.d2 0 n16
d.d3 n15 n10
d.d4 0 n14
d.d5 n16 n10
d.d6 0 n15
r.r10 n10 0 = 100
c.cr n10 0 = 100u
# Actual output voltage converted to nonelectrical value output n50
elec2var.1 n10 0 n50 = 1
constant.1 n51 = 300
#Difference between the set value and the actual value
sum.1 n51 n50 n52 = 1,-1
#Proportional-integral controller
gain.96 n52 n98 = 1
#prop_int.1 n52 n98 = 1,0,0
#Limit the input for comparators
limit.1 n98 n99 = 15,1
# Multiply the gain with three voltages Ea,Eb, and Ec
# Set values for currents
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cont
cont
cont
cont
cont
cont
Pa
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elec2var.2 n11 n17 n100 = 1/100
elec2var.3 n12 n17 n101 = 1/100
elec2var.4 n13 n17 n102 = 1/100
mult.2 n99 n100 n200 = 1
mult.3 n99 n101 n201 = 1
mult.4 n99 n102 n202 = 1
#Nonelectrical current values
#Current sensor
elec2var.12 n11 n111 n82 = 1000
elec2var.13 n12 n122 n83 = 1000
elec2var.14 n13 n133 n84 = 1000
#Current in phase 1. and current in phase 1 .Transformation
#to electrical quantities
var2elec.1 n200 n2000 0 = 1
var2elec.2 n82 n2001 0 = 1
# Conversion of peak values to nonelectrical quantities
var2elec.3 n201 n2002 0 = 1
var2elec.4 n83 n2003 0 = 1
#Addition current error + actual current
#Current and voltage in phase 3 Transformation to electrical
#quantities
var2elec.5 n202 n2004 0 = 1
var2elec.6 n84 n2005 0 = 1
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5
Michael Giesselmann
5.1
5.3
South Dakota State University
5.4
Series-Resonant Inverters
Voltage-Source Series-Resonant Inverters • Voltage-Source
Parallel-Resonant Inverters • Voltage-Source Series–ParallelResonant Inverters • Summary
Eric Walters
5.5
Resonant DC-Link Inverters
kw
eb
in
P. C. Krause and Associates
Resonant Converters
Survey of Second-Order Resonant Circuits • Load Resonant
Converters • Resonant Switch Converters • Resonant
DC-Link Converters with ZVS
Polytechnic University, Brooklyn
Michael E. Ropp
do
Dariusz Czarkowski
DC-AC Conversion
Basic DC-AC Converter Connections (Square-Wave
Operation) • Control of the Output Voltage • Harmonics in
the Output Voltage • Filtering of Output Voltage • Practical
Realization of Basic Connections • Special Realizations
(Application of Resonant Converter Techniques)
IstvГЎn Nagy
Budapest University of Technology
and Economics
tc
5.2
Budapest University of Technology
and Economics
fo
Attila Karpati
Purdue University
Overview
Fundamental Issues • Single-Phase Inverters • Three-Phase
Inverters • Multilevel Inverters • Line Commutated Inverters
Texas Tech University
Oleg Wasynczuk
om
Inverters
The Resonant DC-Link Inverter • The Parallel-Resonant
DC-Link Inverter • Current Research Trends
5.6
Auxiliary Resonant Commutated Pole Inverters
Losses in Hard-Switched Inverters • Analysis of ARCP Phase
Leg • Analysis of ARCP H-Bridge • Analysis of ARCP ThreePhase Inverter • Summary
5.1 Overview
Michael Giesselmann
Pa
Inverters are used to create single or polyphase AC voltages from a DC supply. In the class of polyphase
inverters, three-phase inverters are by far the largest group. A very large number of inverters are used for
adjustable speed motor drives. The typical inverter for this application is a “hard-switched” voltage source
inverter producing pulse-width modulated (PWM) signals with a sinusoidal fundamental [Holtz, 1992].
Recently research has shown detrimental effects on the windings and the bearings resulting from unfiltered
PWM waveforms and recommend the use of filters [Cash and Habetler, 1998; Von Jouanne et al., 1996].
A very common application for single-phase inverters are so-called “uninterruptable power supplies” (UPS)
for computers and other critical loads. Here, the output waveforms range from square wave to almost ideal
sinusoids. UPS designs are classified as either “off-line” or “online”. An off-line UPS will connect the load
to the utility for most of the time and quickly switch over to the inverter if the utility fails. An online UPS
will always feed the load from the inverter and switch the supply of the DC bus instead. Since the DC bus
is heavily buffered with capacitors, the load sees virtually no disturbance if the power fails.
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In addition to the very common hard-switched inverters, active research is being conducted on softswitching techniques. Hard-switched inverters use controllable power semiconductors to connect an output
terminal to a stable DC bus. On the other hand, soft switching inverters have an oscillating intermediate
circuit and attempt to open and close the power switches under zero-voltage and or zero-current
conditions.
A separate class of inverters are the line commutated inverters for multimegawatt power ratings, that use
thyristors (also called silicon controlled rectifiers, SCRs). SCRs can only be turned “on” on command. After
being turned on, the current in the device must approach zero in order to turn the device off. All other
inverters are self-commutated, meaning that the power control devices can be turned on and off. Line
commutated inverters need the presence of a stable utility voltage to function. They are used for DC-links
between utilities, ultralong distance energy transport, and very large motor drives [Ahmed, 1999; Barton,
1994; Mohan et al., 1995; Rashid, 1993; Tarter, 1993]. However, the latter application is more and more
taken over by modern hard-switched inverters including multilevel inverters [Brumsickle et al., 1998; Tolbert
et al., 1999].
Modern inverters use insulated gate bipolar transistors (IGBTs) as the main power control devices
[Mohan et al., 1995]. Besides IGBTs, power MOSFETs are also used especially for lower voltages, power
ratings, and applications that require high efficiency and high switching frequency. In recent years, IGBTs,
MOSFETs, and their control and protection circuitry have made remarkable progress. IGBTs are now
available with voltage ratings of up to 3300 V and current ratings up to 1200 A. MOSFETs have achieved
on-state resistances approaching a few milliohms. In addition to the devices, manufacturers today offer
customized control circuitry that provides for electrical isolation, proper operation of the devices under
normal operating conditions and protection from a variety of fault conditions [Mohan et al., 1995]. In
addition, the industry provides good support for specialized passive devices such as capacitors and
mechanical components such as low inductance bus-bar assemblies to facilitate the design of reliable
inverters. In addition to the aforementioned inverters, a large number of special topologies are used. A
good overview is given by Gottlieb [1984].
Fundamental Issues
Pa
Inverters fall in the class of power electronics circuits. The most widely accepted definition of a power
electronics circuit is that the circuit is actually processing electric energy rather than information. The
actual power level is not very important for the classification of a circuit as a power electronics circuit.
One of the most important performance considerations of power electronics circuits, like inverters, is
their energy conversion efficiency. The most important reason for demanding high efficiency is the
problem of removing large amounts of heat from the power devices. Of course, the judicious use of
energy is also paramount, especially if the inverter is fed from batteries such as in electric cars. For these
reasons, inverters operate the power devices, which control the flow of energy, as switches. In the ideal
case of a switching event, there would be no power loss in the switch since either the current in the switch
is zero (switch open) or the voltage across the switch is zero (switch closed) and the power loss is computed
as the product of both. In reality, there are two mechanisms that do create some losses, however; these
are on-state losses and switching losses [Bird et al., 1993; Kassakian et al., 1991; Mohan et al., 1995;
Rashid, 1993]. On-state losses are due to the fact that the voltage across the switch in the on state is not
zero, but typically in the range of 1 to 2 V for IGBTs. For power MOSFETs, the on-state voltage is often
in the same range, but it can be substantially below 0.5 V due to the fact that these devices have a purely
resistive conduction channel and no fixed minimum saturation voltage like bipolar junction devices
(IGBTs). The switching losses are the second major loss mechanism and are due to the fact that, during
the turn-on and turn-off transition, current is flowing while voltage is present across the device. In order
to minimize the switching losses, the individual transitions have to be rapid (tens to hundreds of
nanoseconds) and the maximum switching frequency needs to be carefully considered.
In order to avoid audible noise being radiated from motor windings or transformers, most modern
inverters operate at switching frequencies substantially above 10 kHz [Bose, 1992; 1996].
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om
Single-Phase Inverters
tc
Topology of a single-phase, full-bridge inverter.
do
FIGURE 5.1
Pa
kw
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in
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Figure 5.1 shows the basic topology of a full-bridge inverter with single-phase output. This configuration is
often called an H-bridge, due to the arrangement of the power switches and the load. The inverter can deliver
and accept both real and reactive power. The inverter has two legs, left and right. Each leg consists of two
power control devices (here IGBTs) connected in series. The load is connected between the midpoints of the
two phase legs. Each power control device has a diode connected in antiparallel to it. The diodes provide an
alternate path for the load current if the power switches are turned off. For example, if the lower IGBT in
the left leg is conducting and carrying current towards the negative DC bus, this current would “commutate”
into the diode across the upper IGBT of the left leg, if the lower IGBT is turned off. Control of the circuit is
accomplished by varying the turn on time of the upper and lower IGBT of each inverter leg, with the provision
of never turning on both at the same time, to avoid a short circuit of the DC bus. In fact, modern drivers
will not allow this to happen, even if the controller would erroneously command both devices to be turned
on. The controller will therefore alternate the turn on commands for the upper and lower switch, i.e., turn
the upper switch on and the lower switch off, and vice versa. The driver circuit will typically add some
additional blanking time (typically 500 to 1000 ns) during the switch transitions to avoid any overlap in the
conduction intervals.
The controller will hereby control the duty cycle of the conduction phase of the switches. The average
potential of the center-point of each leg will be given by the DC bus voltage multiplied by the duty cycle
of the upper switch, if the negative side of the DC bus is used as a reference. If this duty cycle is modulated
with a sinusoidal signal with a frequency that is much smaller than the switching frequency, the shortterm average of the center-point potential will follow the modulation signal. “Short-term” in this context
means a small fraction of the period of the fundamental output frequency to be produced by the inverter.
For the single phase inverter, the modulation of the two legs are inverse of each other such that if the
left leg has a large duty cycle for the upper switch, the right leg has a small one, etc. The output voltage
is then given by Eq. (5.1) in which ma is the modulation factor. The boundaries for ma are for linear
modulation. Values greater than 1 cause overmodulation and a noticeable increase in output voltage
distortion.
V ac1 ( t ) = m a в‹… V dc в‹… sin ( w 1 в‹… t )
0 ≤ ma ≤ 1
(5.1)
This voltage can be filtered using a LC low-pass filter. The voltage on the output of the filter will closely
resemble the shape and frequency of the modulation signal. This means that the frequency, wave-shape,
and amplitude of the inverter output voltage can all be controlled as long as the switching frequency is
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om
Topology of a three-phase inverter.
tc
FIGURE 5.2
Three-Phase Inverters
do
at least 25 to 100 times higher than the fundamental output frequency of the inverter [Holtz, 1992]. The
actual generation of the PWM signals is mostly done using microcontrollers and digital signal processors
(DSPs) [Bose, 1987].
kw
eb
in
fo
Figure 5.2 shows a three-phase inverter, which is the most commonly used topology in today’s motor
drives. The circuit is basically an extension of the H-bridge-style single-phase inverter, by an additional
leg. The control strategy is similar to the control of the single-phase inverter, except that the reference
signals for the different legs have a phase shift of 120В° instead of 180В° for the single-phase inverter. Due
to this phase shift, the odd triplen harmonics (3rd, 9th, 15th, etc.) of the reference waveform for each
leg are eliminated from the line-to-line output voltage [Shepherd and Zand, 1979; Rashid, 1993; Mohan
et al., 1995; Novotny and Lipo, 1996]. The even-numbered harmonics are canceled as well if the waveforms
are pure AC, which is usually the case. For linear modulation, the amplitude of the output voltage is
reduced with respect to the input voltage of a three-phase rectifier feeding the DC bus by a factor given
by Eq. (5.2).
3
--------------- в‹… 3 = 82.7%
(2 в‹… p)
(5.2)
Pa
To compensate for this voltage reduction, the fact of the harmonics cancellation is sometimes used to
boost the amplitudes of the output voltages by intentionally injecting a third harmonic component into
the reference waveform of each phase leg [Mohan et al., 1995].
Figure 5.3 shows the typical output of a three-phase inverter during a startup transient into a typical
motor load. This figure was created using circuit simulation. The upper graph shows the pulse-width
modulated waveform between phases A and B, whereas the lower graph shows the currents in all three
phases. It is obvious that the motor acts a low-pass filter for the applied PWM voltage and the current
assumes the waveshape of the fundamental modulation signal with very small amounts of switching
ripple.
Like the single-phase inverter based on the H-bridge topology, the inverter can deliver and accept both
real and reactive power. In many cases, the DC bus is fed by a diode rectifier from the utility, which
cannot pass power back to the AC input. The topology of a three-phase rectifier would be the same as
shown in Fig. 5.2 with all IGBTs deleted.
A reversal of power flow in an inverter with a rectifier front end would lead to a steady rise of the DC
bus voltage beyond permissible levels. If the power flow to the load is only reversing for brief periods of
time, such as to brake a motor occasionally, the DC bus voltage could be limited by dissipating the power
in a so-called brake resistor. To accommodate a brake resistor, inverter modules with an additional seventh
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om
tc
do
fo
Typical waveforms of inverter voltages and currents.
FIGURE 5.4
Topology of a three-phase inverter with brake-chopper IGBT.
Pa
kw
eb
in
FIGURE 5.3
IGBT (called “brake-chopper”) are offered. This is shown in Fig. 5.4. For long-term regeneration, the
rectifier can be replaced by an additional three-phase converter [Mohan et al., 1995]. This additional
converter is often called a controlled synchronous rectifier. The additional converter including its controller is of course much more expensive than a simple rectifier, but with this arrangement bidirectional
power flow can be achieved. In addition, the interface toward the utility system can be managed such
that the real and reactive power that is drawn from or delivered to the utility can be independently
controlled. Also, the harmonics content of the current in the utility link can be reduced to almost zero.
The topology for an arrangement like this is shown in Fig. 5.5.
The inverter shown in Fig. 5.2 provides a three-phase voltage without a neutral point. A fourth leg
can be added to provide a four-wire system with a neutral point. Likewise four-, five-, or n-phase inverters
can be realized by simply adding the appropriate number of phase legs.
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om
Topology of a three-phase inverter system for bidirectional power flow.
FIGURE 5.6
Topology of a three-level inverter.
kw
eb
in
fo
do
tc
FIGURE 5.5
Pa
As in single-phase inverters, the generation of the PWM control signals is done using modern microcontrollers and DSPs. These digital controllers are typically not only controlling just the inverter, but
through the controlled synthesis of the appropriate voltages, motors and attached loads are controlled
for high-performance dynamic response. The most commonly used control principle for superior
dynamic response is called field-oriented or vector control [Bose, 1987; 1996; DeDonker and Novotny,
1988; Lorenz and Divan, 1990; Trzynadlowski, 1994].
Multilevel Inverters
Multilevel inverters are a class of inverters where a DC source with several tabs between the positive and
negative terminal is present. The two main advantages of multilevel inverters are the higher voltage
capability and the reduced harmonics content of the output waveform due to the multiple DC levels.
The higher voltage capability is due to the fact that clamping diodes are used to limit the voltage stress
on the IGBTs to the voltage differential between two tabs on the DC bus. Figure 5.6 shows the topology
of a three-level inverter. Here, each phase leg consists of four IGBTs in series with additional antiparallel
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om
Line commutated converter in inverter mode.
tc
FIGURE 5.7
Line-Commutated Inverters
fo
do
and clamping diodes. The output is again at the center-point of the phase leg. The output of each phase
can be connected to the top DC bus, the center connection of the DC supply, or the negative DC bus.
This amounts to three distinct voltage levels for the voltage of each phase, which explains the name of
the circuit. It turns out that the resulting line-to-line voltage has five distinct levels in a three-phase
inverter.
Pa
kw
eb
in
Figure 5.7 shows the topology of a line commutated inverter. In Fig. 5.7 the SCRs are numbered according
to their firing sequence. The circuit can operate both as a rectifier and an inverter. The mode of operation
is controlled by the firing angle of the SCRs in the circuit [Ahmed, 1999; Barton, 1994; Mohan et al., 1995].
The reference value for the firing angle О± is the instant when the voltage across each SCR becomes positive;
i.e., when an uncontrolled diode would turn on. This time corresponds to 30В° past the positive going zero
crossing of each phase. By delaying the turn-on angle О± more than 90В° past this instant, the polarity of
the average DC bus voltage reverses and the circuit enters the inverter mode. The DC source in Fig. 5.7
shows the polarity of the DC voltage for inverter operation. The firing delay angle corresponds to the phase
of the utility voltage. The maximum delay angle must be limited to less than 180В°, to provide enough time
for the next SCR in the sequence to acquire the load current. Equation (5.3) gives the value of the DC
output voltage of the converter as a function of the delay angle О± and the DC current Idc , which is considered
constant.
3
V dc = --- ( 2 ⋅ V LL ⋅ cos ( a ) – w ⋅ L S ⋅ I dc )
p
(5.3)
VLL is the rms value of the AC line-to-line voltage, П‰ is the radian frequency of the AC voltage, and Ls is
the value of the inductors La, Lb, and Lc in Fig. 5.7. Line commutated inverters have a negative impact
on the utility voltage and a relatively low total power factor. Equation (5.4) gives an estimate of the total
power factor of the circuit shown in Fig. 5.7 for constant DC current and negligible AC line reactors.
3
PF = --- в‹… cos ( a )
p
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(5.4)
References
Pa
kw
eb
in
fo
do
tc
om
Ahmed, A., Power Electronics for Technology, Prentice-Hall, Upper Saddle River, NJ, 1999.
Barton, T. H., Rectifiers, Cycloconverters, and AC Controllers, Oxford University Press, New York, 1994.
Bird, B. M., King, K. G., and Pedder, D. A. G., An Introduction to Power Electronics, 2nd ed., John Wiley
& Sons, New York, 1993.
Bose, B. K., Modern Power Electronics, Evolution, Technology, and Applications, IEEE Press, Piscataway,
NJ, 1992.
Bose, B. K., Microcomputer Control of Power Electronics and Drives, IEEE Press, Piscataway, NJ, 1987.
Bose, B. K., Power Electronics and Variable Frequency Drives, IEEE Press, Piscataway, NJ, 1996.
Brumsickle, W. E., Divan, D. M., and Lipo, T. A., Reduced switching stress in high-voltage IGBT inverters
via a three-level structure, IEEE-APEC 2. 544–550, Feb. 1998.
Cash, M. A. and Habetler, T. G., Insulation failure prediction in induction machines using line-neutral
voltages, IEEE Trans. Ind. Appl., 34(6), 1234–1239, Nov./Dec. 1998.
De Donker, R. and Novotny, D. W., The universal field-oriented controller, Conf. Rec. IEEE-IAS 1988,
450–456.
Gottlieb, I. M., Power Supplies, Switching Regulators, Inverters and Converters, TAB Books, Blue Ridge
Summit, PA, 1984.
Holtz, J., Pulsewidth modulation—a survey, IEEE Trans. Ind. Electr., 39(5), 410–420, 1992.
Kassakian, J. G., Schlecht, M. F., and Verghese, G. C., Principles of Power Electronics, Addison-Wesley,
Reading, MA, 1991.
Lorenz, R. D. and Divan, D. M., Dynamic analysis and experimental evaluation of delta modulators for
field oriented induction machines, IEEE Trans. Ind. Appl., 26(2), 296–301, 1990.
Mohan, N., Undeland, T., and Robbins, W., Power Electronics: Converters, Applications, and Design, 2nd ed.,
John Wiley & Sons, New York, 1995.
Novotny, D. W. and Lipo, T. A., Vector Control and Dynamics of AC Drives, Oxford Science Publications,
New York, 1996.
Rashid, M. H., Power Electronics, Circuits, Devices, and Applications, Prentice-Hall, Englewood Cliffs, NJ,
1993.
Shepherd, W. and Zand, P., Energy Flow and Power Factor in Nonsinusoidal Circuits, Cambridge University
Press, London, 1979.
Tarter, R. E., Solid State Power Conversion Handbook, John Wiley & Sons, New York, 1993.
Tolbert, L. M., Peng, F. Z., and Habetler, T. G., Multilevel converters for large electric drives, IEEE Trans.
Ind. Appl., 35(1), 36–44, Jan./Feb. 1999.
Trzynadlowski, A. M., The Field Orientation Principle in Control of Induction Motors, Kluwer Academic
Publishers, Dordrecht, the Netherlands, 1994.
Von Jouanne, A., Rendusara, D., Enjeti, P., and Gray, W., Filtering techniques to minimize the effect of
long motor leads on PWM inverter fed AC motor drive systems, IEEE Trans. Ind. Appl., 32(4),
919–926, July/Aug. 1996.
5.2 DC-AC Conversion
Attila Karpati
The DC-AC converters, also known as inverters and shown in Fig. 5.8, produce an AC voltage from a
DC input voltage. The frequency and amplitude produced are generally variable. In practice, inverters
with both single-phase and three-phase outputs are used, but other phase numbers are also possible.
Electric power usually flows from the DC to the AC terminal, but in some cases reverse power flow is
possible. These types of inverters, where the input is a DC voltage source, are also known as voltagesource inverters (VSI). The other type of inverter is the current-source inverters (CSI), where the DC
input is a DC current source. These converters are used primarily in high-power AC motor drives.
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=
~
Vin
Vout
DC-AC converter
DC-AC converter.
id
S2
A
B
D3
S3
S4
il
Ll
Vl
Vd
a.
T
fo
il
T/2
D4
Rl
vl
v
i
D2
do
Vd
S1
tc
D1
om
FIGURE 5.8
t
b.
-Vd
-Vd
S2, 3
kw
eb
in
S1, 4
i
S1, 4
T/2
D1, 4
S2, 3
T
t
c.
D2, 3
id
+
FIGURE 5.9
T
t
d.
Pa
T/2
+
Voltage-source, single-phase, full-bridge inverter connection.
Basic DC-AC Converter Connections (Square-Wave Operation)
This section presents a short summary of the main types of voltage-source DC-AC converter connections
and a brief description of their functions. At the end of this subsection is also given a current-source
converter configuration with its short description. It is assumed that the circuits incorporate ideal
semiconductor switches.
The most frequently used types of single-phase inverters are full-bridge inverters, as shown in Fig. 5.9a,
the half-bridge inverters, as shown in Fig. 5.10a, and push-pull inverters, as shown in Fig. 5.11a.
The switching sequences for the switches and the most important time functions for the full-bridge,
half-bridge, and push-pull inverters during square-wave operation can be seen in Figs. 5.9 through 5.11.
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id1
D1
Vd/2
LI
S1
RI
Vd/2
S2
vl, il
D2
om
a.
id2
Vd/2
vl
v
i
il
T/2
t
-Vd/2
S2
S1
i
do
S1
b.
tc
T
S2
T
t
id1
id2
fo
D2
kw
eb
in
i
D1 T/2
c.
T
t
d.
T/2
FIGURE 5.10
Voltage-source, single-phase, half-bridge inverter connection.
It is assumed that the load on the output consists of a series resistance and inductance. The threephase basic inverter configuration is the full-bridge connection shown in Fig. 5.12a. The loads are assumed
to be symmetrical inductances in the three phases. The switching sequences of the switches and the most
important time functions at square-wave operation are demonstrated in Fig. 5.12b through g.
One can draw the following conclusions from these figures:
Pa
• The output voltage is nonsinusoidal.
• Due to the presence of the freewheeling diodes, the output voltage is independent of the direction
of the load current, and is only dependent on the on and off state of the switches.
• The semiconductor switches and freewheeling diodes form two rectifiers. They are connected in
inverse parallel. The semiconductor switches make the energy flow from the DC side to the AC
side possible. The freewheeling diodes allow the reverse situation.
• Accordingly, the freewheeling diodes are necessary if the converter outputs are connected to loads,
which require either reactive power or effective power feedback. In the case of reactive power, the
direction of the power flow in the converter changes periodically (see the iB currents in Figs. 5.9
through 5.12).
A three-phase current-source inverter configuration is shown in Fig. 5.13a. The switching sequences
of the switches and the most important time functions are demonstrated in Fig. 5.13b.
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RI
LI
iI
vI
N
N
N
D1
v
i
S1
S2
D2
vl
Vd
il
tc
Vd
om
id
T
T/2
S2
S1
S2
fo
S1
i
do
в€’Vd
t
T
t
T/2
D2
kw
eb
in
D1
id
id
T/2
FIGURE 5.11
T
t
Voltage-source, single-phase, push-pull inverter connection.
Pa
Because of jumps in the output current, capacitors must be used, which are connected in parallel to
the load. In most cases, the current-source inverters use thyristors as switching devices, and the aforementioned capacitances are the energy storage elements of the quenching circuits.
Control of the Output Voltage
In voltage-source inverters, the output voltage is controlled by following methods:
•
•
•
•
•
•
In inverters with square-wave operation, voltage changes on the DC side
Voltage cancellation, which is feasible in single-phase full-bridge inverters
Sinusoidal pulse-width modulation (sinusoidal PWM), with bipolar and unipolar voltage switching
Programmed harmonic elimination switching
Tolerance band control
Fixed-frequency control
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id
1
2
3
S
+
S1
S4
b.
a.
Vd
A
B
T
T/ 2
C
S5
S2
S3
vCN
S6
в€’
N
4
5
6
5
vB
iB
vA
iA
vAN
vC
iC
vAB
v
Ll
Ll
O
vA
fo
do
v
i
FIGURE 5.12
kw
eb
in
i
T/ 2
vBC
T
t
c.
vCA
tc
Ll
vBN
om
v
t
d.
t
T
T/ 2
iA
iB
e.
t
T
T/ 2
S3
iC
S1
S2
f.
t
T/ 2
D1
T
D2
D3
id
t
T/ 2
g.
T
Voltage-source, three-phase, bridge inverter connection.
Pa
In current-source inverters, the output is controlled by changing the input DC voltage. In most cases
the DC voltage is changed by a controlled rectifier or DC chopper. In voltage cancellation the Ton and
Toff times of the switches in the two legs of the full-bridge connection are shifted to one another as shown
in Fig. 5.14. The rms value of the AC voltage can be changed between 0 and a maximum value, as defined
by the square-wave operation. This is a very simple method, in which the switching frequency of the
semiconductor elements is equal to the output frequency, but the harmonic content of the AC side voltage
is rather high. Therefore, it is the preferred method used in converters with high-frequency output. At
lower output frequency, i.e., at 60 Hz, other methods are used, and the switching frequency of the
semiconductors is much higher than the output frequency. This method allows for extensive reduction
of the harmonic content in the output voltage or current. In inverter circuits the sinusoidal PWM is used
to minimize the output harmonic content. The basic principle employed in a one-phase half-bridge
converter with bipolar voltage switching is demonstrated in Fig. 5.15. The switches S+ and Sв€’ work with
an internal frequency, which is much higher than the output frequency. The on and off state of the
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id
ia
a
b
tc
(a)
do
va
ia
o
wt
fo
(b)
FIGURE 5.13
induction
motor
c
om
Ld
Three-phase current-source inverter circuit.
kw
eb
in
vAN
vd
S1,4
0
a
w1t
180o
vBN
vd
S2,3
w1t
0
Pa
180o
(180 в€’ a)o
Vc
vd
a
0
w1t
ОІ
в€’vd
(180 в€’ a)
o
ОІ =
FIGURE 5.14
(180 в€’ a)o
= (90 в€’ a )o
2
2
Voltage cancellation by full-bridge connection.
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Vcont
Vtri
0
t
( 1f )
(a)
Vl1 fundamental
V1
om
s
Vd
0
tc
t
Vd
t=0
S2,3 on
Vcont > Vtri
S1,4 on
FIGURE 5.15
do
Vcont< Vtri
Pulse-width modulation with bipolar voltage switching.
kw
eb
in
fo
switches is determined by the crossover points of the triangular comparison signal Vtri and the sinusoidal
control signal Vcont. The sinusoidal control signal causes constant changes in the duty ratio of the switches
S+ and Sв€’ during the half-period of the output so that the harmonic content of the output is minimized.
The output voltage or current can be changed by varying Vcont.
The most important definitions are as follows:
The amplitude-modulation ratio: m a = V contM /V tri
The frequency-modulation ratio: m f = f s /f 1
Pa
where fs is the internal switching frequency and f1 is the frequency of the fundamental of the output.
At small mf (mf ≤ 21), synchronous PWM should be used, namely, mf should be an integer and Vcont
and Vtri are synchronized to one another. (Asynchronous PWM in the mf в‰ integer output produces
subharmonics of the fundamental frequency, which are generally undesirable.)
At large values of mf (mf > 21), the amplitudes of subharmonics caused by asynchronous PWM are
small. Therefore asynchronous PWM may be used, except in AC motor drives, if the frequency approaches
zero. In this case, small subharmonic voltages can also occur as well as high and undesirable currents.
In the case of ma < 1.0, the sinusoidal PWM operates in the linear range. The amplitude of the
fundamental frequency component varies linearly with ma. In this range, the maximum value of the
fundamental is less than the allowable maximum, which is achieved by overmodulation, with ma > 1. In
this range, the relation is not linear between ma and the fundamental. The allowable maximum value is
given by square-wave operation. The relation between the fundamental and ma is illustrated in Fig. 5.16.
The operating principles for sinusoidal PWM with unipolar voltage switching for a full-bridge inverter
can be seen in Fig. 5.17. The two legs of the inverter are not switched simultaneously, and are controlled
separately. For this reason, two control signals, Vcont and в€’Vcont are used. The advantage of this method
is that of “effectively” doubling the switching frequency, which results from the cancellation of certain
harmonic components.
The operating principles for sinusoidal PWM with three-phase inverters are shown in Fig. 5.18. To
control the three legs of the bridge connection, three control signals are used, Vcont,A Vcont,B and Vcont,C.
The fundamental of the output as a function of ma is given in Fig. 5.19.
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Vl1m
Vd
4
ПЂ
(=1.278)
om
1.0
Linear
overmodulation
FIGURE 5.16
1.0
Voltage control by varying ma.
3.24
(for mf =15)
ma
Vcont
fo
Vtri
tc
0
do
0
Square-wave
kw
eb
in
0
S2
on
S1
on
Vcont > Vtri
-Vcont > Vtri
t
-Vcont
(a)
vAN
Vd
0
Pa
vBN
Vd
0
Vd
Vl
(=vAN -vBN)
t
(b)
t
(c)
vol
0
t
-Vd
(d)
FIGURE 5.17
Pulse-width modulation with unipolar switching.
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vtri
Vcont,B
Vcont,A
Vcont,C
t
om
0
vAN
tc
Vd
0
do
vBN
0
vAB=vAN-vBN
fo
Fundamental VAB1
FIGURE 5.18
Vd
t
Vd
t
kw
eb
in
0
t
Three-phase PWM waveforms.
VLI1/Vd
Square-wave
_ 0.78
6 в€ј
ПЂ
Pa
_ 0.612
3 в€ј
2
Linear
Overmodulation
Square-wave
ma
0
FIGURE 5.19
1
3.24
(for mf =15)
Three-phase inverter VLi1 = VLi1 (ma).
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Fundamental
component
Reference
wave
Third harmonic
Reference wave with third harmonic.
VAN
(a)
i*
A
-
iA
Switch
mode
inverter
io
A
B
C
kw
eb
in
+
io
ОЈ
t
fo
Comparator
tolerance
band
do
tc
Reference current i*
A
Actual current iA
t
om
FIGURE 5.20
W1t
(b)
FIGURE 5.21
Tolerance-band current control.
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In the case of overmodulation, low-order harmonics appear, and therefore the above-mentioned
natural sampling method is used only until the output fundamental voltage becomes equal to 78.5% of
its maximum possible value. For a three-phase system, this situation can be improved by using a reference
wave with the addition of the third harmonic, as shown in Fig. 5.20. In the output phase voltages, the
third harmonics have in all of the phases the same time functions (zero sequence components), and,
therefore, cannot produce current.
For programmed harmonic elimination switching, the moments of the semiconductor switching are
calculated so that the lower harmonics will be eliminated. This method permits the elimination of
undesirable lower harmonics, without a very high resulting switching frequency. Therefore, the power
losses in the converter can be reduced.
The principles of tolerance band control (following control) can be seen in Fig. 5.21. The difference
between the reference value and the actual value will be directed to one comparator with a tolerance band.
The output of the comparator controls the switches in the inverter so that the above-mentioned difference
will not be greater than that required. At the sinusoidal output, the reference value has the required
sinusoidal form, and the actual value fluctuates along the curve. The switching frequency varies in a large
interval and depends on the AC side load and the input DC voltage. The controlled variable can be the
output voltage or current.
The principles of fixed-frequency control are shown in Fig. 5.22. The difference between the reference
value and actual value will be directed to a regulator. The regulator output is the control signal, Vcontr ,
which is compared to a triangular waveform, Vtri, with the switching frequency fs. The switching moments
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Comparator
i*
A
+
ОЈ
-
iA
Amplifier
Vcontrol
io
ОЈ
A
+
- v
Switch
mode
inverter
tri
FIGURE 5.22
t
om
0
A
B
C
Fixed-frequency current control.
tc
are specified by the crossover points of the two signals. This type of control circuit is also used in following
control. At the sinusoidal output, the reference value has the required sinusoidal form.
Harmonics in the Output Voltage
do
The harmonics in the output voltage depend primarily on the control method for the output voltage.
For inverters with square-wave operation, the harmonic content is constant. For single-phase inverters
the harmonic numbers are
n = 1, 3, 5, 7, …
The amplitude of the nth harmonic can be calculated for the full-bridge inverter by the following formula:
fo
V onrmso = 0.9V d /n
kw
eb
in
For three-phase inverters the harmonic numbers are
n = 6c В± 1,
where c = 1, 2, 3, …
The rms value of the line voltage can be calculated as follows:
V onrms = 0.78V d /n
For voltage cancellation in a single-phase full-bridge inverter, the harmonic numbers are the same as
those for square wave operation, but the amplitude of the output voltage harmonics varies with the control
angle in the following form:
Pa
V onrms = V onrmso sin ( n в‹… ОІ )
For sinusoidal PWM with bipolar voltage switching and ma ≤ 1.0, the harmonic numbers are
n = jm f В± k
where the fundamental frequency is denoted by n = 1. For odd values of j, only even values of k are
possible, and vice versa.
The harmonic spectrum is presented in Fig. 5.23. In case of overmodulation, the harmonic content
will be higher, as shown in Fig. 5.24.
For sinusoidal PWM with unipolar voltage switching, the harmonic content is less than that for bipolar
voltage switching, due to the cancellation of some harmonics, as shown in Fig. 5.25.
For sinusoidal PWM with three-phase inverters, the harmonic spectrum of the output voltage is given
in Fig. 5.26.
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В© 2002 by CRC Press LLC
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