Fundamental Characteristics of Thyristors Application Note

Fundamental Characteristics of Thyristors Application Note
Teccor® brand Thyristors
AN1001
Fundamental Characteristics of Thyristors
The connections between the two transistors trigger
the occurrence of regenerative action when a proper
gate signal is applied to the base of the NPN transistor.
Normal leakage current is so low that the combined hFE
of the specially coupled two-transistor feedback amplifier
is less than unity, thus keeping the circuit in an off-state
condition. A momentary positive pulse applied to the gate
biases the NPN transistor into conduction which, in turn,
biases the PNP transistor into conduction. The effective
hFE momentarily becomes greater than unity so that the
specially coupled transistors saturate. Once saturated,
current through the transistors is enough to keep the
combined hFE greater than unity. The circuit remains “on”
until it is “turned off” by reducing the anode-to-cathode
current (IT) so that the combined hFE is less than unity and
regeneration ceases. This threshold anode current is the
holding current of the SCR.
Introduction
The Thyristor family of semiconductors consists of several
very useful devices. The most widely used of this family
are silicon controlled rectifiers (SCRs), Triacs, SIDACs, and
DIACs. In many applications these devices perform key
functions and are real assets in meeting environmental,
speed, and reliability specifications which their electromechanical counterparts cannot fulfill.
This application note presents the basic fundamentals
of SCR, Triac, SIDAC, and DIAC Thyristors so the user
understands how they differ in characteristics and
parameters from their electro-mechanical counterparts.
Also, Thyristor terminology is defined.
SCR
Geometric Construction
Basic Operation
Figure AN1001.3 shows cross-sectional views of an SCR
chip and illustrations of current flow and junction biasing in
both the blocking and triggering modes.
Figure AN1001.1 shows the simple block construction of an
SCR.
Anode
P
Forward
Blocking
Junction
Cathode
(-)
N
Gate
J3
N
N
P
J2
P
Cathode
(-)
Gate
(+) IGT
J1
N
Gate
Anode
P
Cathode
Cathode
Block Construction
Schematic Symbol
Figure AN1001.1
(+)
Anode
SCR Block Construction
Cathode
(+)
Gate
N
P
Anode
(-)
Anode
P
P
N
N
P
Gate
Gate
J2
J3
N
N
P
P
Cathode
(+)
Reverse Biased
Gate Junction
N
P
Load
Equivalent Diode
Relationship
Forward Bias and Current Flow
The operation of a PNPN device can best be visualized as
a specially coupled pair of transistors as shown in Figure
AN1001.2.
Anode
(+)
Anode
IT
Reverse Bias
J1
Reverse Biased
Junction
(-)
Anode
Equivalent Diode
Relationship
J2
Figure AN1001.3
N
Cross-sectional View of SCR Chip
N
Cathode
Two-transistor
Schematic
Figure AN1001.2
Cathode
Two-transistor Block
Construction Equivalent
Coupled Pair of Transistors as a SCR
Fundamental Characteristics of Thyristors
389
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
Teccor® brand Thyristors
AN1001
Triac
Basic Operation
Geometric Construction
Figure AN1001.4 shows the simple block construction of a
Triac. Its primary function is to control power bilaterally in
an AC circuit.
Figure AN1001.6 show simplified cross-sectional views of a
Triac chip in various gating quadrants and blocking modes.
Main
Terminal 2
(MT2)
P
N
P
N
N
N
Main
Terminal 1
(MT1)
MT1(-)
GATE(+)
IGT
Gate
N
N
P
MT1(-)
N
MT2
Block Construction
P
IT
N
MT2(+)
QUADRANT I
MT1(-)
GATE(-)
IGT
Gate
N
N
Blocking
Junction
P
MT2(+)
N
MT1
P
Schematic Symbol
Equivalent Diode
Relationship
N
MT2(+)
Figure AN1001.4
Triac Block Construction
QUADRANT II
Operation of a Triac can be related to two SCRs connected
in parallel in opposite directions as shown in Figure
AN1001.5.
MT1(+)
GATE(-)
IGT
N
Although the gates are shown separately for each SCR,
a Triac has a single gate and can be triggered by either
polarity.
N
P
N
MT1
P
N
MT2(-)
IT
MT1(+)
QUADRANT III
MT1(+)
GATE(+)
IGT
N
N
Blocking
Junction
P
N
P
MT2(-)
N
MT2(-)
IT
Equivalent Diode
Relationship
QUADRANT IV
Figure AN1001.6
Simplified Cross-sectional of Triac Chip
MT2
Figure AN1001.5
SCRs Connected as a Triac
Since a Triac operates in both directions, it behaves
essentially the same in either direction as an SCR would
behave in the forward direction (blocking or operating).
Fundamental Characteristics of Thyristors
390
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
Teccor® brand Thyristors
AN1001
SIDAC
DIAC
Basic Operation
Basic Operation
The construction of a DIAC is similar to an open base
NPN transistor. Figure AN1001.9 shows a simple block
construction of a DIAC and its schematic symbol.
The SIDAC is a multi-layer silicon semiconductor switch.
Figure AN1001.7 illustrates its equivalent block construction
using two Shockley diodes connected inverse parallel.
Figure AN1001.7 also shows the schematic symbol for the
SIDAC.
MT1
MT1
MT1
N
N
P
MT2
Block Construction
Schematic Symbol
P1
N2
Figure AN1001.9
N2
P3
P3
N4
MT2
MT2
Equivalent Diode Relationship
DIAC Block Construction
The bidirectional transistor-like structure exhibits a highimpedance blocking state up to a voltage breakover point
(VBO) above which the device enters a negative-resistance
region. These basic DIAC characteristics produce a
bidirectional pulsing oscillator in a resistor-capacitor AC
circuit. Since the DIAC is a bidirectional device, it makes
a good economical trigger for firing Triacs in phase control
circuits such as light dimmers and motor speed controls.
Figure AN1001.10 shows a simplified AC circuit using a
DIAC and a Triac in a phase control application.
N4
P5
Figure AN1001.7
MT2
MT1
Schematic Symbol
SIDAC Block Construction
The SIDAC operates as a bidirectional switch activated
by voltage. In the off state, the SIDAC exhibits leakage
currents (IDRM) less than 5 µA. As applied voltage exceeds
the SIDAC VBO, the device begins to enter a negative
resistance switching mode with characteristics similar to
an avalanche diode. When supplied with enough current
(IS), the SIDAC switches to an on state, allowing high
current to flow. When it switches to on state, the voltage
across the device drops to less than 5 V, depending on
magnitude of the current flow. When the SIDAC switches
on and drops into regeneration, it remains on as long as
holding current is less than maximum value (150 mA,
typical value of 30 mA to 65 mA). The switching current (IS)
is very near the holding current (IH) value. When the SIDAC
switches, currents of 10 A to 100 A are easily developed by
discharging small capacitor into primary or small, very highvoltage transformers for 10 µs to 20 µs.
Load
Figure AN1001.10
AC Phase Control Circuit
Geometric Construction
MT1
The main application for SIDACs is ignition circuits or
inexpensive high voltage power supplies.
MT1
N
P
Geometric Construction
N
MT1
MT2
Cross-section of Chip
P1
N2
P3
Figure AN1001.11
MT2
Equivalent Diode
Relationship
Cross-sectional View of DIAC Chip
N4
P5
MT2
Figure AN1001.8
Cross-sectional View of a Bidirectional SIDAC
Chip with Multi-layer Construction
Fundamental Characteristics of Thyristors
391
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
Teccor® brand Thyristors
AN1001
Electrical Characteristic Curves of Thyristors
+I
+I
Voltage Drop (VT) at
Specified Current (iT)
IT
Latching Current (IL)
IH
Reverse Leakage
Current - (IRRM) at
Specified VRRM
Off - State Leakage
Current - (IDRM) at
Specified VDRM
Minimum Holding
Current (IH)
-V
IS
Specified Minimum
Off - State
Blocking
Voltage (VDRM)
Reverse
Breakdown
Voltage
Figure AN1001.12
IDRM
-V
+V
Specified Minimum
Reverse Blocking
Voltage (VRRM)
RS =
IBO
+V
VT
(VBO - VS)
VBO
VS
VDRM
(IS - IBO)
Forward
Breakover
Voltage
-I
RS
-I
V-I Characteristics of SCR Device
Figure AN1001.15
+I
V-I Characteristics of a SIDAC Chip
Methods of Switching on Thyristors
Voltage Drop (vT) at
Specified Current (iT)
Three general methods are available for switching
Thyristors to on-state condition:
Latching Current (IL)
• Application of gate signal
• Static dv/dt turn-on
• Voltage breakover turn-on
Off-state Leakage
Current – (IDRM) at
Specified VDRM
Minimum Holding
Current (IH)
-V
+V
Application Of Gate Signal
Gate signal must exceed IGT and VGT requirements of the
Thyristor used. For an SCR (unilateral device), this signal
must be positive with respect to the cathode polarity. A
Triac (bilateral device) can be turned on with gate signal of
either polarity; however, different polarities have different
requirements of IGT and VGT which must be satisfied. Since
DIACs and SIDACs do not have a gate, this method of turnon is not applicable. In fact, the single major application of
DIACs is to switch on Triacs.
Specified Minimum
Off-state
Blocking
Voltage (VDRM)
Breakover
Voltage
-I
Figure AN1001.13
V-I Characteristics of Triac Device
Static dv/dt Turn-on
+I
10 mA
Static dv/dt turn-on comes from a fast-rising voltage
applied across the anode and cathode terminals of an
SCR or the main terminals of a Triac. Due to the nature of
Thyristor construction, a small junction capacitor is formed
across each PN junction. Figure AN1001.16 shows how
typical internal capacitors are linked in gated Thyristors.
∆V
Breakover
Current
IBO
-V
+V
Breakover
Voltage
VBO
-I
Figure AN1001.14
V-I Characteristics of Bilateral Trigger DIAC
Fundamental Characteristics of Thyristors
Figure AN1001.16
392
Internal Capacitors Linked in Gated Thyristors
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
Teccor® brand Thyristors
AN1001
When voltage is impressed suddenly across a PN junction,
a charging current flows, equal to:
Generally, Thyristor application circuits are designed with
static dv/dt snubber networks if fast-rising voltages are
anticipated.
The most common quadrants for Triac gating-on are
Quadrants I and III, where the gate supply is synchronized
with the main terminal supply (gate positive -- MT2 positive,
gate negative -- MT2 negative). Gate sensitivity of Triacs is
most optimum in Quadrants I and III due to the inherent
Thyristor chip construction. If Quadrants I and III cannot be
used, the next best operating modes are Quadrants II and
III where the gate has a negative polarity supply with an AC
main terminal supply. Typically, Quadrant II is approximately
equal in gate sensitivity to Quadrant I; however, latching
current sensitivity in Quadrant II is lowest. Therefore, it is
difficult for Triacs to latch on in Quadrant II when the main
terminal current supply is very low in value.
Voltage Breakover Turn-on
This method is used to switch on SIDACs and DIACs.
However, exceeding voltage breakover of SCRs and Triacs
is definitely not recommended as a turn-on method.
Special consideration should be given to gating circuit
design when Quadrants I and IV are used in actual
application, because Quadrant IV has the lowest gate
sensitivity of all four operating quadrants.
( )
( )
__
i = C dv
dt
dv
When C __ becomes greater or equal to Thyristor IGT,
dt
the Thyristor switches on. Normally, this type of turn-on
does not damage the device, providing the surge current is
limited.
In the case of SCRs and Triacs, leakage current increases
until it exceeds the gate current required to turn on these
gated Thyristors in a small localized point. When turn-on
occurs by this method, localized heating in a small area
may melt the silicon or damage the device if di/dt of the
increasing current is not sufficiently limited.
General Terminology
The following definitions of the most widely-used Thyristor
terms, symbols, and definitions conform to existing EIAJEDEC standards:
DIACs used in typical phase control circuits are basically
protected against excessive current at breakover as long
as the firing capacitor is not excessively large. When DIACs
are used in a zener function, current limiting is necessary.
Breakover Point − Any point on the principal voltage-current
characteristic for which the differential resistance is zero and
where the principal voltage reaches a maximum value
Principal Current − Generic term for the current through
the collector junction (the current through main terminal 1
and main terminal 2 of a Triac or anode and cathode of an
SCR)
SIDACs are typically pulse-firing, high-voltage transformers
and are current limited by the transformer primary. The
SIDAC should be operated so peak current amplitude,
current duration, and di/dt limits are not exceeded.
Principal Voltage − Voltage between the main terminals:
Triac Gating Modes Of Operation
(1) In the case of reverse blocking Thyristors, the principal
voltage is called positive when the anode potential is
higher than the cathode potential and negative when
the anode potential is lower than the cathode potential.
Triacs can be gated in four basic gating modes as shown in
Figure AN1001.17.
ALL POLARITIES ARE REFERENCED TO MT1
MT2
(-)
MT2 POSITIVE
(Positive Half Cycle)
+
IGT
GATE
(+)
IGT
GATE
MT1
IGT
REF
(-)
IGT
GATE
(+)
MT1
REF
Off State − Condition of the Thyristor corresponding to the
high-resistance, low-current portion of the principal voltagecurrent characteristic between the origin and the breakover
point(s) in the switching quadrant(s)
MT1
REF
QII QI
QIII QIV
MT2
(2) For bidirectional Thyristors, the principal voltage is called
positive when the potential of main terminal 2 is higher
than the potential of main terminal 1.
MT2
-
+
IGT
MT2
On State − Condition of the Thyristor corresponding to the
low-resistance, low-voltage portion of the principal voltagecurrent characteristic in the switching quadrant(s).
IGT
GATE
MT2 NEGATIVE
(Negative Half Cycle)
MT1
REF
NOTE: Alternistors will not operate in Q IV
Figure AN1001.17
Gating Modes
Fundamental Characteristics of Thyristors
393
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
Teccor® brand Thyristors
AN1001
Specific Terminology
Repetitive Peak Off-state Voltage (VDRM) − Maximum
instantaneous value of the off-state voltage which occurs
across a Thyristor, including all repetitive transient voltages
and excluding all non-repetitive transient voltages
Average Gate Power Dissipation [PG(AV)] − Value of gate
power which may be dissipated between the gate and main
terminal 1 (or cathode) averaged over a full cycle
Breakover Current (IBO) − Principal current at the breakover
point
Repetitive Peak Reverse Current of an SCR (IRRM)
− Maximum instantaneous value of the reverse current
resulting from the application of repetitive peak reverse
voltage
Breakover Voltage (VBO) − Principal voltage at the
breakover point
Circuit-commutated Turn-off Time (tq) − Time interval
between the instant when the principal current has
decreased to zero after external switching of the principal
voltage circuit and the instant when the Thyristor is capable
of supporting a specified principal voltage without turning on
Repetitive Peak Reverse Voltage of an SCR (VRRM)−
Maximum instantaneous value of the reverse voltage which
occurs across the Thyristor, including all repetitive transient
voltages and excluding all non-repetitive transient voltages
Surge (Non-repetitive) On-state Current (ITSM) − On-state
current of short-time duration and specified waveshape
Critical Rate-of-rise of Commutation Voltage of a Triac
(Commutating dv/dt) − Minimum value of the rate-of-rise
of principal voltage which will cause switching from the off
state to the on state immediately following on-state current
conduction in the opposite quadrant
Thermal Resistance, Junction to Ambient (RθJA)−
Temperature difference between the Thyristor junction
and ambient divided by the power dissipation causing
the temperature difference under conditions of thermal
equilibrium
Critical Rate-of-rise of Off-state Voltage or Static dv/
dt (dv/dt) − Minimum value of the rate-of-rise of principal
voltage which will cause switching from the off state to the
on state
Note: Ambient is the point at which temperature does not
change as the result of dissipation.
Thermal Resistance, Junction to Case (RθJC) −
Temperature difference between the Thyristor junction and
the Thyristor case divided by the power dissipation causing
the temperature difference under conditions of thermal
equilibrium
Critical Rate-of-rise of On-state Current (di/dt) −
Maximum value of the rate-of-rise of on-state current that a
Thyristor can withstand without harmful effect
Gate-controlled Turn-on Time (tgt) − Time interval
between a specified point at the beginning of the gate pulse
and the instant when the principal voltage (current) has
dropped to a specified low value (or risen to a specified high
value) during switching of a Thyristor from off state to the on
state by a gate pulse.
Gate Trigger Current (IGT) − Minimum gate current required
to maintain the Thyristor in the on state
Gate Trigger Voltage (VGT) − Gate voltage required to
produce the gate trigger current
Holding Current (IH) − Minimum principal current required
to maintain the Thyristor in the on state
Latching Current (IL) − Minimum principal current required
to maintain the Thyristor in the on state immediately after
the switching from off state to on state has occurred and the
triggering signal has been removed
On-state Current (IT) − Principal current when the Thyristor
is in the on state
On-state Voltage (VT) − Principal voltage when the Thyristor
is in the on state
Peak Gate Power Dissipation (PGM) − Maximum power
which may be dissipated between the gate and main
terminal 1 (or cathode) for a specified time duration
Repetitive Peak Off-state Current (IDRM) − Maximum
instantaneous value of the off-state current that results from
the application of repetitive peak off-state voltage
Fundamental Characteristics of Thyristors
394
©2013 Littelfuse, Inc
Specifications are subject to change without notice.
Revised: 09/23/13
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