This article gives brief overview on the different commutation techniques of SCR.

1. THYRISTORS
-COMMUTATION-
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
1. Commutation
2. Natural Commutation
3. Forced Commutation
4. Class A Commutation: Self Commutation by Resonating Load
5. Class B Commutation : Self Commutation by L – C Circuit
6. Class C Commutation : Complementary Commutation
7. Class D Commutation : Impulse Commutation
8. Class E Commutation : External Pulse Commutation
9. Class F Commutation : Line Commutation
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3. Commutation – Turning Off SCR
 To turn on a thyristor, a low voltage, short duration pulse is applied to the gate (typically
4V, 100µs).
 Once the thyristor is turned-on, the gate loses control and the thyristor will only turn off
when the load current falls virtually to zero, or the thyristor is reverse biased.
 The thyristor will turn off naturally with A.C. supplies as the voltage reverses (which is
called as Natural Commutation).
 No such reversal occurs with D.C. supplies and it is necessary to force a voltage
reversal if tum-off is to occur. This process is called Forced Commutation.
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4. Commutation
 The process of turning OFF SCR is defined as "Commutation".
 In all commutation techniques, a reverse voltage is applied across the thyristor during the
turn OFF process.
 By turning OFF a thyristor we bring it from forward conducting to the forward blocking
mode.
 The conditions to be satisfied in order to turn OFF an SCR are:
 IA < IH ( Anode current must be less than holding current)
 A reverse voltage is applied to SCR for sufficient time enabling it to recover its blocking
state.
 There are two methods by which a thyristor can be turned OFF.
 Natural Commutation
 Forced Commutation
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5. Natural Commutation
 In AC circuit, the current always passes through zero for every half cycle.
 As the current passes through natural zero, a reverse Voltage will simultaneously appear
across the device.
 This will turn OFF the device immediately.
 This process is called as natural commutation, since no external circuit is required for this
purpose.
 This method is only applicable for A.C supply.
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6. Forced Commutation
 To turn OFF a thyristor, the forward anode current should be brought to zero for
sufficient time to allow the removal of charged carriers.
 In case of DC circuits, the forward current should be forced to zero by means of some
external circuits.
 This process is called as forced commutation.
 With D.C. supply, we use external circuit and active/passive components to reduce
passing current’s value below holding current.
 The circuit involved in this procedure is called “commutation circuit”.
 The components used are called “commutating components”
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7. Natural Commutation vs Forced Commutation
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8. Class A: Self Commutated by a Resonating Load
 Class A is one of frequently used thyristor commutation techniques.
 If thyristor is triggered or turned on, then anode current will flow by charging capacitor C
with dot as positive.
 The second order under-damped circuit is formed by the inductor or AC resistor, capacitor
and resistor.
 If the current builds up through SCR and completes the half cycle, then the inductor
current will flow through the SCR in the reverse direction which will turn off thyristor.
 The capacitor voltage is at its peak when the SCR turns off and the capacitor discharges
into the resistance in an exponential manner.
 The SCR is reverse-biased till the capacitor voltage returns to the level of the supply
voltage V.
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9. Class A: Self Commutated by a Resonating Load
 After the thyristor commutation or turning off the thyristor, the capacitor will start
discharging from its peak value through the resistor is an exponential manner.
 The thyristor will be in reverse bias condition until the capacitor voltage returns to the
supply voltage level.
 The time for switching OFF the SCR depends on the resonant frequency which is a factor
of L and C components.
 The method is reliable, simple and for high frequency application above 1000 Hz.
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10. Class A: Self Commutated by a Resonating Load
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11. Class B: Self Commutated by an L-C Circuit
 The major difference between the class A and class B thyristor commutation techniques
is that the LC is connected in series with thyristor in class A, whereas in parallel with
thyristor in class B.
 Before triggering on the SCR, the capacitor is charged up (dot indicates positive).
 If the SCR is triggered or given triggering pulse, then the resulting current has two
components.
 The constant load current flowing through the R-L load is ensured by the large reactance
connected in series with the load which is clamped with freewheeling diode.
 If sinusoidal current flows through the resonant L-C circuit, then the capacitor C is
charged up with dot as negative at the end of the half cycle.
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12. Class B: Self Commutated by an L-C Circuit
 The total current flowing through the SCR becomes zero with the reverse current
flowing through the SCR opposing the load current for a small fraction of the negative
swing.
 If the resonant circuit current or reverse current becomes just greater than the load
current, then the SCR will be turned OFF.
 This commutation technique is mostly used for chopper circuits.
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13. Class B: Self Commutated by an L-C Circuit
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14. Class C: C or L-C Switched by another Load
Carrying SCR
 In the above thyristor commutation techniques we observed only one SCR but in these
class C commutation techniques of thyristor there will be two SCRs.
 One SCR is considered as main thyristor and the other as auxiliary thyristor.
 In this classification both may act as main SCRs carrying load current and they can be
designed with four SCRs with load across the capacitor by using a current source for
supplying an integral converter.
 If the thyristor T2 is triggered, then the capacitor will be charged up.
 If the thyristor T1 is triggered, then the capacitor will discharge and this discharge
current of C will oppose the flow of load current in T2 as the capacitor is switched
across T2 via T1.
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15. Class C: C or L-C Switched by another Load
Carrying SCR
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 Mainly used in single – phase inverters with centre tapped transformers.
 Viz. Mc Murray Bedford inverter.
 It is useful even at frequencies below 1000 Hz.

16. Class C: C or L-C Switched by another Load
Carrying SCR
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17. Class D: L-C or C Switched by an Auxiliary SCR
 The class C and class D thyristor commutation techniques can be differentiated with the
load current in class D:
 Class D: Only one of the SCR’s will carry the load current while the other acts as an
auxiliary thyristor whereas in class C both SCRs will carry load current.
 The auxiliary thyristor consists of resistor in its anode which is having resistance of
approximately ten times the load resistance.
 By triggering the T a (auxiliary thyristor) the capacitor is charged up to supply voltage and
then the Ta will turn OFF.
 The extra voltage if any, due to substantial inductance in the input lines will be discharged
through the diode-inductor-load circuit.
 If the T m (main thyristor) is triggered, then the current will flow in two paths:
commutating current will flow through the C-Tm-L-D path and load current will flow
through the load.
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18. Class D: L-C or C Switched by an Auxiliary SCR
 If the charge on the capacitor is reversed and held at that level using the diode and if Ta is
re-triggered, then the voltage across the capacitor will appear across the Tm via Ta.
 Thus, the main thyristor Tm will be turned off.
 Used in inverters and Jones choppeer circuit.
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19. Class D: L-C or C Switched by an Auxiliary SCR
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20. Class E: External Pulse Source for Commutation
 For the class E thyristor commutation techniques, a transformer which can not saturate (as
it is having a sufficient iron and air gap) and is capable to carry the load current with small
voltage drop compared with the supply voltage.
 If the thyristor T is triggered, then the current will flow through the load and pulse
transformer.
 An external pulse generator is used to generate a positive pulse which is supplied to the
cathode of the thyristor through pulse transformer.
 The capacitor C is charged to around 1 V and it is considered to have zero impedance for
the turn off pulse duration.
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21. Class E: External Pulse Source for Commutation
 The voltage across the thyristor is reversed by the pulse from the electrical transformer
which supplies the reverse recovery current, and for the required turn off time it holds the
negative voltage.
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22. Class E: External Pulse Source for Commutation
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23. Class F: AC Line Commutated
 In class F thyristor commutation techniques, an alternating voltage is used for supply and,
during the positive half cycle of this supply, load current will flow.
 If the load is highly inductive, then the current will remain until the energy stored in the
inductive load is dissipated.
 During the negative half cycle as the load current becomes zero, then thyristor will turn
off.
 If voltage exists for a period of rated turn off time of the device, then the negative polarity
of the voltage across the outgoing thyristor will turn it off.
 Here, the duration of the half cycle must be greater than the turn off time of thyristor.
 This commutation process is similar to the concept of three phase converter.
 Let us consider, primarily T1 and T11 are conducting with the triggering angle of the
converter, which is equal to 60 degrees, and is operating in continuous conduction mode
with highly inductive load.
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24. Class F: AC Line Commutated
 If the thyristors T2 and T22 are triggered, then instantaneously the current through the
incoming devices will not rise to the load current level.
 If the current through the incoming thyristors reaches the load current level, then the
commutation process of outgoing thyristors will be initiated.
 This reverse biasing voltage of thyristor should be continued until the forward blocking
state is reached.
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25. Dynamic Turn OFF Switching Characteristics
 The transition of an SCR from forward conduction state to forward blocking state is called
as turn OFF or commutation of SCR. As we know that once the SCR starts conducting, the
gate has no control over it to bring back to forward blocking or OFF state.
 To turn OFF the SCR, the current must be reduced to a level below the holding current of
SCR.
 Various methods are discussed to turn OFF the SCR in which SCR turn OFF is achieved
by reducing the forward current to zero.
 But if the forward voltage applied immediately after the current zero of SCR, it starts
conducting again even without gate triggering.
 This is due to the presence of charge carriers in the four layers.
 Therefore, it is necessary to apply the reverse voltage, over a finite time across the SCR to
remove the charge carriers.
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26. Dynamic Turn OFF Switching Characteristics
 Hence the turn OFF time is defined as the time between the instant the anode current
becomes zero and the instant at which the SCR retains the forward blocking capability.
 The excess charge carriers from the four layers must be removed to bring back the SCR to
forward conduction mode.
 This process takes place in two stages.
 In a first stage excess carriers from outer layers are removed and in second stage excess
carriers in the inner two layers are to be recombined.
 Hence, the total turn OFF time t q is divided into two intervals; reverse recovery time t rr
and gate recovery time t gr.
t q = t rr + t gr
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27. Dynamic Turn OFF Switching Characteristics
 The figure below shows the switching characteristics of SCR during turn ON and OFF.
 The time t1 to t3 is called as reverse recovery time; at the instant t1 the anode current is
zero and builds up in the reverse direction which is called as reverse recovery current. This
current removes the excess charge carriers from outer layers during the time t1 to t3.
 At instant t3, junctions J1 and J3 are able to block the reverse voltage but, the SCR is not
yet able to block the forward voltage due to the presence of excess charge carriers in
junction J2.
 These carriers can be disappeared only by the way of recombination and this could be
achieved by maintaining a reverse voltage across the SCR.
 Hence , during the time t3 to t4, the recombination of charges takes place and at the instant
t4, junction J2 completely recovers.
 This time is called gate recovery time t gr.
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28. Dynamic Turn OFF Switching Characteristics
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29. Dynamic Turn OFF Switching Characteristics
 From the figure the turn OFF time is the time interval between the t4 and t1.
 Generally, this time varies from 10 to 100 microseconds. This turn OFF time t q is
applicable to the individual SCR.
 The time required by the commutation circuit to apply the reverse voltage to commutate
the SCR is called the circuit turn OFF time (t c).
 For a safety margin or reliable commutation, this t c must be greater than the t q otherwise
commutation failure occurs.
 The SCRs which have slow turn OFF time as in between 50 to 100 microseconds are
called as converter grade SCRs.
 These are used in phase controlled rectifiers, cyclo converters, AC voltage regulators, etc.
 The SCRs which have fast turn OFF time as in between 3 to 50 microseconds are inverter
grade SCRs.
 These are costlier compared to converter grade and are used in choppers, force
commutated converters and inverters.
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30. References:
1. Power Electronics A to Z: Turning Off SCR (Commutation)
2. http://www.electronicshub.org/scr-turn-off-methods/
3. www.pantechsolutions.net/powerelectronics-tutorials
4. www.elprocus.com/classification-of-thyristor-commutation-methods/
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