Texas Instruments | AN-2116 SolarMagic ICs in Micro-inverter Applications (Rev. B) | Application notes | Texas Instruments AN-2116 SolarMagic ICs in Micro-inverter Applications (Rev. B) Application notes

Texas Instruments AN-2116 SolarMagic ICs in Micro-inverter Applications (Rev. B) Application notes
Application Report
SNVA471B – February 2011 – Revised May 2013
AN-2116 SolarMagic™ ICs in Microinverter Applications
.....................................................................................................................................................
ABSTRACT
This application report explores some of the prevalent topologies used in microinverters today, and the
use of SolarMagic™ ICs in these demanding applications. In particular, the use of the SM72295
Photovoltaic Full-Bridge Driver is highlighted.
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Contents
Introduction ..................................................................................................................
SolarMagic Renewable Energy Grade Components ...................................................................
Single-Stage Microinverters ...............................................................................................
Two-Stage Microinverters ..................................................................................................
Housekeeping Power and Other Applications ...........................................................................
Conclusion ...................................................................................................................
References ...................................................................................................................
SolarMagic ICs in a Microinverter Application ..........................................................................
List of Devices ...............................................................................................................
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List of Figures
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Block Diagram of Microinverter Using a Single-Stage Topology ..................................................... 2
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Simplified Schematic of a Quasi-Resonant Interleaved Flyback Using the SM72295 ............................. 3
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Block Diagram of Two-Stage Inverter With A DC Bus ................................................................. 4
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Simplified Schematic of Two Stage Microinverter Using the SM72295.............................................. 5
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Block Diagram of SolarMagic ICs in a Microinverter Application ..................................................... 6
SolarMagic is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
SNVA471B – February 2011 – Revised May 2013
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1
Introduction
1
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Introduction
Microinverters are a growing and rapidly evolving part of the photovoltaic (PV) system. Modern
microinverters are designed to convert the DC power from one PV module (solar panel) to the AC grid,
and are designed for a max output power in the range of 180W to 300W. Compared to conventional string
or central inverters, microinverters have advantages in ease of installation, localized max power point
tracking, and redundancy that provides robustness to failure.
Since this area of power electronics is seeing such rapid innovation, there are many different topologies
and variations being developed.
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SolarMagic Renewable Energy Grade Components
The environment for electronics in PV systems is a very demanding one due to the extremes in
temperatures and requirements for long-lifetime. The ambient temperature behind a photovoltaic module
can range from below freezing in the winter to over 90°C on a summer day. With this in mind Texas
Instruments created the Renewable Energy Grade line of SolarMagic ICs that are all rated for operation
from -40°C to +125°C and have all been screened and tested to standards appropriate for products that
are designed for a 25 year lifetime. This line of products includes MOSFET gate drives, PWM controllers
with integrated switches, LDO regulators, amplifiers, and many other ICs necessary for photovoltaic
electronics. All of the ICs recommended in this article are being made available as Renewable Energy
Grade components.
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Single-Stage Microinverters
There have been a multitude of microinverter topologies developed (see [1]), and these topologies can be
broken up into two broad categories. The first category depicted in the block diagram of Figure 1 employs
a DC/DC converter and controls the converter output voltage to have the shape of a rectified sinusoid.
This rectified sinusoid waveform is then inverted into a full sinusoidal waveform using an “unfolding bridge”
that interfaces to the grid voltage. Though perhaps not the most accurate name, this category of
microinverter topologies is often referred to as a “single-stage microinverter” because the boosting of the
panel voltage and shaping of the AC waveform is accomplished in a single stage.
A more formal categorization of microinverter topologies (see [2]) refers to this as a PV-side decoupled
topology because the input capacitors decouple the AC power variation. The most widespread topology of
this category is a quasi-resonant interleaved flyback, however, there are other variants such as interleaved
flyback (not quasi-resonant) and interleaved forward converter. The unfolding inverter is generally
implemented with 4 SCR’s (silicon controlled rectifiers) that switch at the grid frequency.
173V
173V
-173V
PV Module
25 Vdc - 55 Vdc
DC/DC with
rectified sinusoidal
output
Unfolding Bridge
Grid
Microinverter
The DC/DC stage can be implemented as a quasi-resonant interleaved flyback or another topology.
Figure 1. Block Diagram of Microinverter Using a Single-Stage Topology
2
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Single-Stage Microinverters
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Figure 2 shows a simplified schematic of a single-stage microinverter using a quasi-resonant interleaved
flyback for the DC-DC stage. In the quasi-resonant interleaved flyback, the SM72295 provides the
microinverter designer with a high level of integration and enables maximized power density, reduced
component count, and reduced PCB space. The SM72295 combines four independent 3A MOSFET gate
drives with signal conditioning, power good, and overvoltage sensing functionality. Gate drive signal inputs
are compatible with both 3.3V and 5V logic.
The integrated signal conditioning provides two channels optimized for using high-side current sense
resistors with common-mode voltages up to 100V. A transconductance amplifier provides gain and is
followed by a low-impedance buffer suitable for interfacing into an analog to digital converter (ADC). The
use of current sense resistors is a lower cost alternative to commonly used current-sense transformers. In
addition, the ability to put the current sense resistors on the high-side (positive voltage) current path as
opposed to the low-side (ground) current return path can ease layout because it does not require
segmenting the ground plane and also eliminates the need for a negative rail voltage in cases where the
sense resistor voltages goes below ground.
PGND
LOA
OVS
LOB
HOA
HOB
SOB
SIB
SM72295
HIB
LIB
PGOOD
OVP
BIN
BOUT
SOA
SIA
LIA
HIA
The advantages of the single-stage topology microinverters are their lower component count, low
switching frequencies of the unfolding bridge, and ease of implementing isolation. Disadvantages include
high voltage ratings on both the primary side switches and the secondary side diode, and high amplitude
120Hz ripple current at the input. This input ripple current must be controlled to maintain an acceptable
efficiency level due to the nature of the photovoltaic module.
Current sensing is implemented with high-side current sense resistors, and output overvoltage shutdown is
implemented using voltage sense windings and the OVS pin.
Figure 2. Simplified Schematic of a Quasi-Resonant Interleaved Flyback Using the SM72295
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3
Two-Stage Microinverters
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A photovoltaic module has a load curve with a specific maximum power point Pmp that occurs when its
output voltage equals Vmp and output current equals Imp. To maximize energy harvest, the microinverter
maintains the module output voltage and current as closely as possible to Vmp and Imp using a max power
point tracking algorithm. Deviations from Vmp or Imp, such as those caused by input ripple current, would
cause power loss. Therefore the input ripple current of the single-stage inverters must be reduced to
minimize the power loss, and this necessitates large capacitors at the input of the microinverter. For
practical cost and size purposes these capacitors can only be implemented with electrolytic capacitors.
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Two-Stage Microinverters
The second category of microinverter topologies depicted in the block diagram of Figure 3 employ an
intermediate high voltage DC-bus. These topologies use a DC/DC converter with a high boost ratio to
boost from the PV module voltage to the intermediate DC-bus voltage, and then use a conventional PWM
controlled MOSFET or IGBT full-bridge to invert the waveform to the grid. This type of microinverter is also
referred to as a DC-link topology (see [2]).
There are many different options being implemented for the DC/DC stage in the designs being developed
today. Possibilities include:
1. Interleaved flyback
2. Push-pull converter (current-fed or voltage-fed, with passive or active clamp)
3. Full-bridge converter (voltage-fed, current-fed, or resonant)
From a high-level perspective, all of these topologies are more complex and costly than the single-stage
microinverter due to the additional high-frequency switching components. At first it may not be obvious
why these topologies are being developed. However, for applications in microinverters, there’s an
overriding focus on maximizing reliability, which puts emphasis on choosing a topology that enables the
selection of the highest reliability components. All of the these topologies have much lower input ripple
current at the PV input side, and therefore use lower capacitance values that make it practical to use
higher reliability film capacitors in the place of electrolytic capacitors.
Another benefit of the two-stage topologies is that it makes it possible to provide reactive power to the
grid, whereas it is not possible with single-stage inverters with an SCR unfolding bridge. The ability to
provide reactive power is a highly desirable feature for some commercial installations, and it is already a
requirement for larger photovoltaic installations in some countries.
200V
173V
- 173V
PV Module
25 Vdc - 55 Vdc
DC/DC with high voltage output
DC/AC Inverter
Grid
Microinverter
Figure 3. Block Diagram of Two-Stage Inverter With A DC Bus
4
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Housekeeping Power and Other Applications
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Figure 4 shows is a simplified schematic of a two-stage microinverter implemented with a voltage-fed fullbridge for the DC/DC stage and a MOSFET full-bridge for the DC/AC stage. In this application, the
SM72295 provides gate drives for the four primary side MOSFETs. The SM72295 is ideally suited as a
gate driver in many of these two-stage topologies, several of which use a MOSFET full-bridge on their
primary side.
As shown in Figure 4, the SM72295 is capable of driving all 4 MOSFETs in the primary side full-bridge. It
provides 2 high-side and 2 low-side gate drives, integrated bootstrap diodes, and is suitable for input
voltages up to 100V. The additional integration of signal conditioning, undervoltage lockout, and
overvoltage shutdown further reduce part count and conserve valuable PCB real-estate.
DC/AC
LOB
HOB
LOA
HOA
PGND
L IB
HIB
LIA
SM72295
HIA
SIA
PGOOD
OVP
BIN
SOA
DC/DC
PWM inputs
The DC/DC stage is implemented as a voltage-fed full-bridge converter, and the DC/AC stage is implemented as a
MOSFET Full-bridge.
Figure 4. Simplified Schematic of Two Stage Microinverter Using the SM72295
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Housekeeping Power and Other Applications
In addition to the SM72295, there is a broad range of SolarMagic ICs suitable for application in other
areas of the microinverter. As shown in Figure 5 and Table 1, this includes temperature sensors, voltage
references, precision amplifiers for current and voltage sensing, and switchers and LDOs for
housekeeping power. These ICs have application in both single-stage and two-stage microinverters of all
topologies.
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Conclusion
Microinverters are an exciting and growing application area for power electronics. This article gave a brief
overview of some of the topologies being used in microinverters today, and described the SM72295
Photovoltaic Full-bridge Driver which integrates the key functions of MOSFET gate drives, signal
conditioning, under-voltage lockout, and overvoltage shutdown. The SM72295 and other SolarMagic ICs
support the most prevalent topologies used in microinverters today, and help microinverter designers
maximize reliability, minimize complexity, minimize size, and minimize cost.
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5
References
7
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References
1. B. Burger, B. Goeldi, S. Rogalla, H. Schmidt. “Module integrated electronics – an overview” 25th
European Photovoltaic Solar Energy Conference and Exhibition. 6-10 Sept. 2010. pp. 3700–3707.
2. Haibing Hu; Harb, S.; Kutkut, N.; Batarseh, I.; Shen, Z.J. “Power decoupling techniques for microinverters in PV systems-a review” Energy Conversion Congress and Exposition (ECCE), 2010.
pp. 3235–3240.
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SolarMagic ICs in a Microinverter Application
Quasi-Resonant Flyback
Quasi-Resonant Flyback
Temperature
Sensor
QR MOSFET
Gate Drive
Flyback MOSFET
Gate Drive
Flyback Current
Sense Amp
Voltage
Reference
AC Current
Sense Amp
AC Voltage
Sense Amp
AC Zero
Crossing Amp
+12V
+5V
+3.3V
Figure 5. Block Diagram of SolarMagic ICs in a Microinverter Application
6
AN-2116 SolarMagic™ ICs in Microinverter Applications
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List of Devices
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9
List of Devices
Table 1. List of Devices
Gate Drives
Description
SM72295
Photovoltaic Full-Bridge Driver
SM72482
Dual 5A Compound Gate Driver
SM74101
Tiny 7A MOSFET Gate Driver (LLP-6 package)
SM74104
High Voltage Half-Bridge Gate Driver with Adaptive Delay
High Voltage Switching Regulators with Integrated Switch
SM72485
100V, 150mA Step-Down (Buck) Converter
SM74301
100V, 350 mA Constant On-Time Buck Switching Regulator
SM74304
80V, 500mA Step Down Swithching Regulator
Low Dropout Voltage Regulators
SM74501
50mA Low Dropout Voltage Regulator (3.3V, 5.0V), max Vin 40V
SM72238
100mA Low Dropout Voltage Regulator (3.3V, 5.0V), max Vin 30V
SM74503
800mA Low-Dropout Regulator (3.3V, 5.0V), max Vin 15V
Amplifiers for current sensing, voltage sensing, and buffering
SM72501
Precision, CMOS Input, Rail-to-Rail Input Output, Wide Supply Range Amplifier
SM73301
Rail-to-Rail Input Output, High Output Current & Unlimited Cap Load Op Amp
SM73302
88 MHz, Precision, Low Noise, 1.8V CMOS Input, Op Amp
SM73303
5 MHz, Low Noise, Rail-to-Rail Output, Dual Operational Amplifier with CMOS Input
SM73304
Dual 17 MHz, Low Noise, CMOS Input Amplifier
SM73305
17 MHz, Low Noise, CMOS Input Amplifier
SM73306
Dual CMOS Rail to Rail Input and Output Operational Amplifier
Comparators
SM72375
Dual Micro-Power CMOS Comparator
SM73402
Low Power Low Offset Quad Comparators
SM73403
Single General Purpose Voltage Comparator
Reset and Supervisory
SM72240
5-Pin Microprocessor Reset Circuit (3.08V, 4.63V thresholds)
SM74601
Precision Micropower Series Voltage Reference (2.5V)
Thermostats and Temperature Sensors
SM72480
125°C, 120°C, and 105°C Thermostat
SM73710
±4°C Accurate, Temperature Sensor
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