By: Engr. Irfan Ahmed Halepoto

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Title: By: Engr. Irfan Ahmed Halepoto

1
Instrumentation Power Electronic Systems
LECTURE11
THYRISTOR FUNDAMENTALS
By Engr. Irfan Ahmed Halepoto
2
Limitation of power semiconductor devices

Majority carrier devices, like Schottky diode,
MOSFET exhibit very fast switching responses,
controlled essentially by the charging of the
device capacitances.
However, forward voltage drops of these devices
increases quickly with increasing breakdown
voltage.
Minority carrier devices, like BJT, IGBT can
exhibit high breakdown voltages with relatively
low forward voltage drop.
But they can have longer switching times due to
stored minority charges.
Energy is lost during switching transitions, due
to a variety of mechanisms.
The resulting average power loss, or switching
loss, is equal to this energy loss multiplied by
the switching frequency.
So need of a mechanism to have a compensation
between these issues.

3
THYRISTOR

Thyristor, a three terminal, four layers solid
state semiconductor device, each layer consisting
of alternately N-type or P-type material, i.e
P-N-P-N, that can handle high currents and high
voltages, with better switching speed and
improved breakdown voltage .
Name thyristor, is derived by a combination of
the capital letters from THYRatron and
transISTOR.
Thyristor has characteristics similar to a
thyratron tube which is a type of gas filled
tube used as a high energy electrical switch and
controlled rectifier.
But from the construction view point, a thyristor
(pnpn device) belongs to transistor (pnp or npn
device) family.
This means that thyristor is a solid state device
like a transistor and has characteristics similar
to that of a thyratron tube.

4
THYRISTORS

Thyristor (famous as Silicon Control
Rectifier-SCR) can handle high currents and high
voltages.
Typical rating are 1.5kA 10kV which responds to
15MW power handling capacity.
This power can be controlled by a gate current of
about 1A only.
Thyristor a three terminal (Anode, Cathode and
Gate), three junctions and four layers
solid-state semiconductor device, with silicon
doped alternate material with P-N-P-N structure.
Thyristor act as bistable switches.
It conducts when gate receives a current pulse,
and continue to conduct as long as forward biased
(till device voltage is not reversed).
They stay ON once they are triggered, and will go
OFF only if current is too low or when triggered
off.

5
Thyristor Schematic Representation
6
Two-Transistor Model of Thyristors
7
Two-Transistor Model of Thyristors

Two-transistor model is obtained by bisecting the
two middle layers in two separate halves.
Junctions J1J2 J2-J3 constitute pnp npn
transistors separately.
In transistors off-state, IC is related to IE as
IC aIE ICBO
where a is the common-base current gain and ICB0
is collector-base leakage current of transistor.
For transistor Q1, IC1 a1 Ia ICBO1
...(01)
Similarly, for transistor Q2, the collector
current IC2 is given by
IC2 a2 Ik ICBO2 .. ( 02)
Sum of two collector currents given by Eqs. (01)
(02) is equal to the external circuit current
Ia entering at anode terminal A. There fore
Ia IC1 IC2
Ia a1 Ia ICBO1 a2 Ik ICBO2 ...
(03)
When gate current is applied, then Ik Ia Ig .
Substituting this value of Ik in Eq. (03) gives
Ia a1 Ia ICBO1 a2 (Ia Ig ) ICBO2
Or Ia a2 Ig ICBO1 ICBO2 /1-( a1 a2)

8
Thyristor Internal constructional view
9
Thyristor- Operation Principle

Thyristor has three p-n junctions (J1, J2, J3
from the anode).
When anode is at a positive potential (VAK) w.r.t
cathode with no voltage applied at the gate,
junctions J1 J3 are forward biased, while
junction J2 is reverse biased.
As J2 is reverse biased, no conduction takes
place, so thyristor is in forward blocking state
(OFF state).
Now if VAK (forward voltage) is increased w.r.t
cathode, forward leakage current will flow
through the device.
When this forward voltage reaches a value of
breakdown voltage (VBO) of the thyristor, forward
leakage current will reach saturation and reverse
biased junction (J2) will have avalanche
breakdown and thyristor starts conducting (ON
state), known as forward conducting state .
If Cathode is made more positive w.r.t anode,
Junction J1 J3 will be reverse biased and
junction J2 will be forward biased.
A small reverse leakage current flows, this state
is known as reverse blocking state.
As cathode is made more and more positive, stage
is reached when both junctions A C will be
breakdown, this voltage is referd as reverse
breakdown voltage (OFF state), and device is in
reverse blocking state

10
Characteristics of Thyristors
11
Thyristor Operating modes

Thyristors have three modes
Forward blocking mode Anode is positive w.r.t
cathode, but the anode voltage is less than the
break over voltage (VBO) .
only leakage current flows, so thyristor is not
conducting .
Forward conducting mode When anode voltage
becomes greater than VBO, thyristor switches from
forward blocking to forward conduction state, a
large forward current flows.
If the IGIG1, thyristor can be turned ON even
when anode voltage is less than VBO.
The current must be more than the latching
current (IL).
If the current reduced less than the holding
current (IH), thyristor switches back to forward
blocking state.
Reverse blocking mode When cathode is more
positive than anode , small reverse leakage
current flows.
However if cathode voltage is increased to
reverse breakdown voltage , Avalanche breakdown
occurs and large current flows.

12
Thyristor turn-ON methods

Thyristor turning ON is also known as Triggering.
With anode positive with respect to cathode, a
thyristor can be turned ON by any one of the
following techniques
Forward voltage triggering
Gate triggering
dv/dt triggering
Temperature triggering
Light triggering

13
Forward Voltage Triggering

When breakover voltage (VBO) across a thyristor
is exceeded than the rated maximum voltage of
the device, thyristor turns ON.
At the breakover voltage the value of the
thyristor anode current is called the latching
current (IL) .
Breakover voltage triggering is not normally used
as a triggering method, and most circuit designs
attempt to avoid its occurrence.
When a thyristor is triggered by exceeding VBO,
the fall time of the forward voltage is quite low
(about 1/20th of the time taken when the
thyristor is gate-triggered).
However, a thyristor switches faster with VBO
turn-ON than with gate turn-ON, so permitted
di/dt for breakover voltage turn-on is lower.

14
Gate Triggering

Turning ON of thyristors by gate triggering is
simple and efficient method of firing the forward
biased SCRs.
In Gate Triggering, thyristor with forward
breakover voltage (VBO), higher than the normal
working voltage is chosen.
This means that thyristor will remain in forward
blocking state with normal working voltage across
anode and cathode with gate open.
Whenever thyristors turn-ON is required, a
positive gate voltage b/w gate and cathode is
applied.
With gate current established, charges are
injected into the inner p layer and voltage at
which forward breakover occurs is reduced.
Forward voltage at which device switches to
on-state depends upon the magnitude of gate
current.
Higher the gate current, lower is the forward
breakover voltage .
When positive gate current is applied, gate P
layer is flooded with electrons from cathode, as
cathode N layer is heavily doped as compared to
gate P layer.
As the thyristor is forward biased, some of these
electrons reach junction J2.
As a result, width of depletion layer around
junction J2 is reduced.
This causes junction J2 to breakdown at an
applied voltage lower than forward breakover
voltage VB0.
If magnitude of gate current is increased, more
electrons will reach junction J2, thus thyristor
will get turned ON at a much lower forward
applied voltage.

15
dv/dt triggering

With forward voltage across anode cathode of a
thyristor, two outer junctions (A C) are
forward biased but the inner junction (J2) is
reverse biased.
The reversed biased junction J2 behaves like a
capacitor because of the space-charge present
there.
As p-n junction has capacitance, so larger the
junction area the larger the capacitance.
If a voltage ramp is applied across the
anode-to-cathode, a current will flow in the
device to charge the device capacitance according
to the relation
If the charging current becomes large enough,
density of moving current carriers in the device
induces switch-on.
This method of triggering is not desirable
because high charging current (Ic) may damage
the thyristor.

16
Temperature Triggering

During forward blocking, most of the applied
voltage appears across reverse biased junction
J2.
This voltage across junction J2 associated with
leakage current may raise the temperature of this
junction.
With increase in temperature, leakage current
through junction J2 further increases.
This cumulative process may turn on the SCR at
some high temperature.
High temperature triggering may cause Thermal
runaway and is generally avoided.

17
Light Triggering

In this method light particles (photons) are made
to strike the reverse biased junction, which
causes an increase in the number of electron hole
pairs and triggering of the thyristor.
For light-triggered SCRs, a slot (niche) is made
in the inner p-layer.
When it is irradiated, free charge carriers are
generated just like when gate signal is applied
b/w gate and cathode.
Pulse light of appropriate wavelength is guided
by optical fibers for irradiation.
If the intensity of this light thrown on the
recess exceeds a certain value, forward-biased
SCR is turned on. Such a thyristor is known as
light-activated SCR (LASCR).
Light-triggered thyristors is mostly used in
high-voltage direct current (HVDC) transmission
systems.

18
Thyristor Gate Control Methods

An easy method to switch ON a SCR into conduction
is to apply a proper positive signal to the gate.
This signal should be applied when the thyristor
is forward biased and should be removed after the
device has been switched ON.
Thyristor turn ON time should be in range of 1-4
micro seconds, while turn-OFF time must be
between 8-50 micro seconds.
Thyristor gate signal can be of three
varieties.
D.C Gate signal
A.c Gate Signal
Pulse

19
Thyristor Gate Control Methods

D.C Gate signal Application of a d.c gate signal
causes the flow of gate current which triggers
the SCR.
Disadvantage is that the gate signal has to be
continuously applied, resulting in power loss.
Gate control circuit is also not isolated from
the main power circuit.
A.C Gate Signal In this method a phase - shifted
a.c voltage derived from the mains supplies the
gate signal.
Instant of firing can be controlled by phase
angle control of the gate signal.
Pulse Here the SCR is triggered by the
application of a positive pulse of correct
magnitude.
For Thyristors it is important to switched ON at
proper instants in a certain sequence.
This can be done by train of the high frequency
pulses at proper instants through a logic
circuit.
A pulse transformer is used for circuit
isolation.
Here, the gate looses are very low because the
drive is discontinuous.

20
Thyristor Commutation

Commutation Process of turning off a conducting
thyristor
Current Commutation
Voltage Commutation
A thyristor can be turned ON by applying a
positive voltage of about a volt or a current of
a few tens of milliamps at the gate-cathode
terminals.
But SCR cannot be turned OFF via the gate
terminal.
It will turn-off only after the anode current is
negated either naturally or using forced
commutation techniques.
These methods of turn-off do not refer to those
cases where the anode current is gradually
reduced below Holding Current level manually or
through a slow process.
Once the SCR is turned ON, it remains ON even
after removal of the gate signal, as long as a
minimum current, the Holding Current (IH), is
maintained in the main or rectifier circuit.

21
Thyristor Turn-off Mechanism

In all practical cases, a negative current flows
through the device.
This current returns to zero only after the
reverse recovery time (trr) , when the SCR is
said to have regained its reverse blocking
capability.
The device can block a forward voltage only after
a further tfr, the forward recovery time has
elapsed.
Consequently, the SCR must continue to be
reverse-biased for a minimum of tfr trr tq,
the rated turn-off time of the device.
The external circuit must therefore reverse bias
the SCR for a time toff gt tq.
Subsequently, the reapplied forward biasing
voltage must rise at a dv/dt lt dv/dt (reapplied)
rated. This dv/dt is less than the static
counterpart.

22
Thyristor Commutation Classification

Commutation can be classified as
Natural commutation
Forced commutation

23
Line Commutation (Natural Commutation)

Occurs only in AC circuits.
Natural Commutation of thyristor takes place in
AC Voltage Regulators
Phase controlled rectifiers
Cycloconverters

24
Thyristor Turn-Off Line-Commutated Thyristor
Circuit
25
Forced Commutation

Applied to d.c circuits.
If a thyristor is used in a DC circuit, when
first turned on, it will stay on until the
current goes to zero. To turn off the thyristor
it is possible to use a Forced commutation
circuit. The circuit creates a reverse voltage
over the thyristor (and a small reverse current)
for a short time, but long enough to turn off the
thyristor.
A simple circuit consist of a precharged
capacitor and a switch (e.g. another thyristor)
parallel to the thyristor. When the switch is
closed, the current is supplied by the capacitor
for a short while. This cause a reversed voltage
over the thyristor, and the thyristor is turned
off.
Commutation is achieved by reverse biasing
thyristor or reducing the thysristor current
below the holding current value.
Commutating elements such as inductor, capacitors
are used for commutation purpose.
Force commutation is applied to choppers and
inverters.
Force Commutation methods
Class A- Resonant Load
Class B- Self commutation
Class C- Auxiliary commutation
Class D- Complimentary commutation
Class E- External pulse commutation

26
Thyristor Turn-Off Forced- Commutated Thyristor
Circuit
27
THYRISTOR SWITCHING CHARACTERISTICS
Thyristor Turn-ON time for a resistive Load
Thyristor Turn -OFF time for a resistive Load
28
THYRISTOR turn-ON turn-OFF Characteristics
29
Thyristor protection circuits

Reliable operation of a thyristor demands that
its specified ratings are not exceeded.
In practice, a thyristor may be subjected to
overvoltages or overcurrents. During SCR turn-on,
di/dt may be prohibitively large.
There may be false triggering of SCR by high
value of dv/dt.
A spurious signal across gate-cathode terminals
may lead to unwanted turn-on.
A thyristor must be protected against all such
abnormal conditions for satisfactory and reliable
operation of SCR circuit and the equipment.
SCRs are very delicate devices, their protection
against abnormal operating conditions is,
therefore, essential.
The object of this section is to discuss various
techniques adopted for the protection of SCRs.
di/dt protection.
dv/dtprotection.

30
di/dt protection

When a thyristor is forward biased and is turned
on by a gate pulse, conduction of anode current
begins in the immediate neighbourhood of the
gate-cathode junction.
Thereafter, the current spreads across the whole
area of junction.
The thyristor design permits the spread of
conduction to the whole junction area as rapidly
as possible.
However, if the rate of rise of anode current,
i.e. di/dt, is large as compared to the spread
velocity of carriers, local hot spots will be
formed near the gate connection on account of
high current density.
This localized heating may destroy the thyristor.
Therefore, the rate of rise of anode current at
the time of turn-on must be kept below the
specified limiting value.
The value of di/dt can be maintained below
acceptable limit by using a small inductor,
called di/dt inductor, in series with the anode
circuit. Typical di/dt limit values of SCRs are
20-500 A/µ sec.
Local spot heating can also be avoided by
ensuring that the conduction spreads to the whole
area as rapidly as possible.
This can be achieved by applying a gate current
nearer to (but never greater than) the maximum
specified gate current.

31
di/dt Protection

A thyristor requires a minimum time to spread the
current conduction uniformly throughout the
junctions
Otherwise, a localized hot-spot heating may
occur due to high current density.

32
dv/dt protection

With forward voltage across the anode cathode
of a thyristor, the two outer junctions (A C)
are forward biased but the inner junction (J2) is
reverse biased.
The reversed biased junction J2 behaves like a
capacitor because of the space-charge present
there.
Let the capacitance of this junction be Cj. For
any capacitor, i C dv/dt.
In case it is assumed that entire forward voltage
va appears across reverse biased junction J2 then
charging current across the junction is given by
i dQ/dt d(Cj Va )/dt
iCj (d Va /dt) Va(d Cj /dt)
i Cj dva /dt
This charging or displacement current across
junction J2 is collector currents of Q2 and Q1
Currents IC2, IC1 will induce emitter current in
Q2, Q1.
In case rate of rise of anode voltage is large,
the emitter currents will be large and as a
result, a1 a2 will approach unity leading to
eventual switching action of the thyristor.
If the rate of rise of forward voltage dVa/dt is
high, the charging current i will be more. This
charging current plays the role of gate current
and turns on the SCR even when gate signal is
zero.
Such phenomena of turning-on a thyristor, called
dv/dt turn-on must be avoided as it leads to
false operation of the thyristor circuit.
For controllable operation of the thyristor, the
rate of rise of forward anode to cathode voltage
dVa/dt must be kept below the specified rated
limit.
Typical values of dv/dt are 20 500 V/µsec.
False turn-on of a thyristor by large dv/dt can
be prevented by using a snubber circuit in
parallel with the device.

33
Snubber circuit

A snubber circuit consists of a series
combination of resistance Rs and capacitance Cs
in parallel with the thyristor as shown in Fig.
Strictly speaking, a capacitor Cs in parallel
with the device is sufficient to prevent unwanted
dv/dt triggering of the SCR.
When switch S is closed, a sudden voltage appears
across the circuit. Capacitor Cs behaves like a
short circuit, therefore voltage across SCR is
zero.
With the passage of time, voltage across Cs
builds up at a slow rate such that dv/dt across
Cs and therefore across SCR is less than the
specified maximum dv/dt rating of the device.
Here the question arises that if Cs is enough to
prevent accidental turn-on of the device by
dv/dt, what is the need of putting Rs in series
with Cs ? The answer to this is as under.

34
snubber circuit (continue)

Before SCR is fired by gate pulse, Cs charges to
full voltage Vs. When the SCR is turned on,
capacitor discharges through the SCR and sends a
current equal to Vs / (resistance of local path
formed by Cs and SCR).
As this resistance is quite low, the turn-on
di/dt will tend to be excessive and as a result,
SCR may be destroyed. In order to limit the
magnitude of discharge current, a resistance Rs
is inserted in series with Cs as shown in Fig.
Now when SCR is turned on, initial discharge
current Vs/Rs is relatively small and turn-on
di/dt is reduced.
In actual practice Rs, Cs and the load circuit
parameters should be such that dv/dt across Cs
during its charging is less than the specified
dv/dt rating of the SCR and discharge current at
the turn-on of SCR is within reasonable limits.
Normally, Rs Cs and load circuit parameters form
an underdamped circuit so that dv/dt is limited
to acceptable values.

35
Thyristor Family Members

SCR Silicon Controlled Rectifier
DIAC Diode on Alternating Current
TRIAC Triode for Alternating Current
SCS Silicon Control Switch
SUS Silicon Unilateral Switch
SBS Silicon Bidirectional Switch
SIS Silicon Induction Switch
LASCS Light Activated Silicon Control Switch
LASCR Light Activated Silicon Control Rectifier
SITh Static Induction Thyristor
RCT Reverse Conducting Thyristor
GTO Gate Turn-Off thyristor
MCT MOSFET Controlled Thyristor
ETOs Emitter Turn ON thyristor

36
Silicon-Controlled Rectifier (SCR)

SCR is a synonym of thyristor
SCR is a four-layer pnpn device.
Has 3 terminals anode, cathode, and gate.
In off state, it has a very high resistance.
In on state, there is a small on (forward)
resistance.
Applications motor controls, time-delay
circuits, heater controls, phase controls, etc.

37
Turning the SCR ON Method and its Characteristics

The SCR can be turned on by exceeding the forward
breakover voltage or by gate current.
Notice that the gate current controls the amount
of forward breakover voltage required for turning
it on.
VBR(F) decreases as IG is increased.

The positive pulse of current at the gate turns
on Q2 providing a path for IB1.
Q1 then turns on providing more base current for
Q2 even after the trigger is removed.
Thus, the device stays on (latches).

38
Turning SCR Off

The SCR will conduct as long as forward current
exceeds IH.
There are two ways to drop the SCR out of
conduction
Anode Current Interruption
Forced Commutation.

39
Turning SCR Off Anode Current Interruption

Anode current can be interrupted by breaking the
anode current path , providing a path around the
SCR, or dropping the anode voltage to the point
that IA lt IH.

40
Turning The SCR Off Force Commutation

Force commutation uses an external circuit to
momentarily force current in the opposite
direction to forward conduction.
SCRs are commonly used in ac circuits, which
forces the SCR out of conduction when the ac
reverses.

41
SCR Characteristics Ratings

Forward- breakover voltage, VBR(F) voltage at
which SCR enters forward-conduction (ON) region.
Holding current, IH value of anode current for
SCR to remain in on region.
Gate trigger current, IGT value of gate current
to switch SCR on.
Average forward current, IF (avg) maximum
continuous anode current (dc) that the SCR can
withstand.
Reverse-breakdown voltage, VBR(R) maximum
reverse voltage before SCR breaks into avalanche.

42
SCR Applications - dc motor control

SCRs are used in a variety of power control
applications.
One of the most common applications is to use it
in ac circuits to control a dc motor or appliance
because the SCR can both rectify and control.
The SCR is triggered on the positive cycle and
turns off on the negative cycle.
A circuit like this is useful for speed control
for fans or power tools and other related
applications.

43
SCR Applications- crowbar circuits

Another application for SCRs is in crowbar
circuits (which get their name from the idea of
putting a crowbar across a voltage source and
shorting it out!)
The purpose of a crowbar circuit is to shut down
a power supply in case of over-voltage.
Once triggered, the SCR latches on.
The SCR can handle a large current, which causes
the fuse (or circuit breaker) to open.

44
DIAC (diode for alternating current)

The DIAC is a five-layer device trigger diode
that conducts current only after its breakdown
voltage has been exceeded momentarily.
When this occurs, the resistance of the diode
abruptly decreases, leading to a sharp decrease
in the voltage drop across the diode and,
usually, a sharp increase in current flow through
the diode.
The diode remains "in conduction" until the
current flow through it drops below a value
characteristic for the device, called the holding
current.
Below this value, the diode switches back to its
high-resistance (non-conducting) state.
This behavior is bidirectional, meaning typically
the same for both directions of current flow .
their terminals are not labeled as anode and
cathode but as A1 and A2 or MT1 ("Main Terminal")
and MT2.
Most DIACs have a breakdown voltage around 30 V.
DIACs have no gate electrode, unlike some other
thyristors they are commonly used to trigger,
such as TRIACs.
diac is normally used in ac circuits
The drawback of the diac is that it cannot be
triggered at just any point in the ac power
cycle it triggers at its preset breakover
voltage only. If we could add a gate to the diac,
we could have variable control of the trigger
point, and therefore a greater degree of control
over just how much power will be applied to the
line-powered device.

45
DIAC (diode for alternating current)
46
TRIAC (Triode for Alternating Current)

Triac is five layer device that is able to pass
current bidirectionally and is therefore behaves
as an a.c. power control device.
In triac , the main connections are simply named
main terminal 1 (MT1) and main terminal 2 (MT2).
The gate designation still applies, and is still
used as it was with the SCR.
The useful feature of the triac is that it not
only carries current in either direction, but the
gate trigger pulse can be either polarity
regardless of the polarity of the main applied
voltage.
The gate can inject either free electrons or
holes into the body of the triac to trigger
conduction either way.
So triac is referred to as a "four-quadrant"
device.
Triac is used in an ac environment, so it will
always turn off when the applied voltage reaches
zero at the end of the current half-cycle.
If we apply a turn-on pulse at some controllable
point after the start of each half cycle, we can
directly control what percentage of that
half-cycle gets applied to the load, which is
typically connected in series with MT2.
This makes the triac an ideal candidate for light
dimmer controls and motor speed controls. This is
a common application for triacs.

47
Triac operation

The triac can be considered as two thyristors
connected in antiparallel as shown in Fig .
The single gate terminal is common to both
thyristors.
The main terminals MT1 and MT2 are connected to
both p and n regions of the device and the
current path through the layers of the device
depends upon the polarity of the applied voltage
between the main terminals.
The device polarity is usually described with
reference to MT1, where the term MT2 denotes
that terminal MT2 is positive with respect to
terminal MT1.

48
The Gate Turn-Off Thyristor (GTO)
49
GTOs Schematic representation
50
GTO Turn-on and Turn-off Pulses
51
Thyristor Summary

A thyristor is a latching device and it can be
turned on with a small gate pulse, typically
100µs .
Thyristors are generally off by line commutation
due to the natural behavior of the input ac line
supply.
During the turn-off process, thyristors must be
subjected to a reverse voltage for a certain
minimum time known the turn-off.

52
Summary Thyristors

The thyristor family
double injection yields lowest forward voltage
drop in high voltage devices.
More difficult to parallel than MOSFETs and IGBTs
The SCR
highest voltage and current ratings, low cost,
passive turn-off transition
The GTO
intermediate ratings (less than SCR, somewhat
more than IGBT). Slower than IGBT.
Slower than MCT.
Difficult to drive.
The MCT
So far, ratings lower than IGBT.
Slower than IGBT.
Easy to drive.
Still emerging devices?

53
Thyristor (SCR)
v-i characteristics

If the forward breakover voltage (Vbo) is
exceeded, the SCR self-triggers into the
conducting state.
The presence of gate current will reduce Vbo.
Normal conditions for thyristors to turn on
the device is in forward blocking state (i.e Vak
is positive)
a positive gate current (Ig) is applied at the
gate
Once conducting, the anode current is latched.
Vak collapses to normal forward volt-drop,
typically 1.5-3V.
In reverse -biased mode, the SCR behaves like a
diode.

54
Thyristor Conduction
vo _

Thyristor cannot be turned off by applying
negative gate current. It can only be turned off
if Ia goes negative (reverse)
This happens when negative portion of the of
sine-wave occurs (natural commutation).
Another method of turning off is known as forced
commutation,
The anode current is diverted to another
circuitry.

55
Types of thyristors