In class A and class B commutation methods discussed in the earlier articles, there is only one SCR used, coming to the class-c commutation method there will be two SCRs. One SCR acts as the main thyristor and the other as the auxiliary. It is a type of forced commutation technique that uses additional external components in the circuit for the commutation of the thyristor.

The class-C commutation of the thyristor is also known as complementary impulse commutation or complementary commutation. The name complementary commutation comes after its operation that, the auxiliary thyristor is triggered in order to turn-OFF the main thyristor, while the auxiliary thyristor is turned-OFF when the main thyristor is triggered.

In this method of commutation, a reverse voltage is applied across the thyristor by discharging a capacitor such that it creates reverse bias and helps in commutating the thyristor. Let us see the circuit design and working of class-C or complementary commutation of thyristor or SCR.

## Circuit of Class C Commutation :

The circuit for class c or complementary commutation is shown in the below figure. The circuit consists of the main thyristor T_{1} along with the commutating components that compress capacitor C and complementary thyristor T_{2}. The load resistance R_{1} is connected in series with the main thyristor T_{1}.

## Working of Class C or Complementary Commutation :

### Mode-1 :

Initially, the main thyristor T_{1}, as well as the complementary thyristor T_{2}, are in the OFF state and the voltage across the capacitor, E_{C} is zero. In mode-1, the condition of T_{1}, T_{2}, and capacitor can be represented as,

*Both T*

_{1}and T_{2}are in OFF state and E_{C}= 0### Mode-2 :

At ωt = t_{0}, main thyristor T_{1} is triggered due to which the load current starts flowing and also the capacitor C starts charging with the polarities as shown in the figure above. The load current flows through the path,

*E*

_{dc}→ R_{1}→ T_{1}→ E_{dc}And the charging capacitor current flows through the path,

*E*

_{dc}→ R_{2}→ C → T_{1}→ E_{dc}In this mode the capacitor is fully charged to the supply voltage i.e., it continues to charge until it gains a voltage equal to the input voltage E_{dc}. Thus the condition of T_{1}, T_{2}, and capacitor in mode-2 will be changed and is represented as,

*T*

_{1}is in ON state, T_{2}is in OFF state, and E_{C}= E_{dc}### Mode-3 :

At ωt = t_{1}, thyristor T_{2} is triggered and it starts conducting. The main thyristor T_{1} turns OFF immediately after triggering the complementary thyristor T_{2} since it will be reverse biased.

When the thyristor T_{2} is triggered, the negative polarity of the capacitor C is applied across the anode of T_{1} and the positive polarity of capacitor C is applied across the cathode of T_{1}. This causes to reverse bias the T_{1} and thus, thyristor T_{2} will conduct and T_{1} will be turned OFF i.e., commutation of T_{1} is done by turning ON T_{2}.

#### Now, the capacitor charges to a voltage equal to -E_{dc} (from t_{1} to t_{2}) through the load which will be in reverse polarity. The charging path of capacitor C is,

*E*

_{dc}→ R_{1}→ C → T_{2}→ E_{dc}At the end of mode-3, the state of T_{1}, T_{2} and capacitor becomes,

*T*

_{1}is in OFF state, T_{2}is in ON state, and E_{C}= -E_{dc}### Mode-4 :

At ωt = t_{3}, thyristor T_{1} is again triggered and it starts conducting. Immediately, thyristor T_{2} gets commutated since it gets reverse bias voltage due to discharge of capacitor in the reverse direction. Again the capacitor starts to charge as in the case mode-2 and the process continues. By the end of this mode, the condition of T_{1}, T_{2}, and capacitor is represented as,

*T*

_{1}is in ON state, T_{2}is in OFF state, and E_{C}= E_{dc}## Waveform of Class-C or Complementary Commutation :

This type of commutation is very useful for frequencies less than 1000 Hz. The waveforms for complementary impulse commutation are shown in the below figure.

In the above waveforms, i_{g1} and i_{g2} are the gate triggering pulses given to thyristors T_{1} and T_{2}. V_{T1} and i_{T1} are the voltage and current across thyristor T_{1}, similarly, V_{T2} and i_{T2} are the voltage and current across thyristor T_{2}, and V_{C} and i_{C} are the voltage and current of capacitor C.

We can see that, at instant t_{1} when triggering pulse i_{g1} is given, the thyristor T_{1} turns-ON. Hence the voltage drop, V_{T1} across T_{1} will be zero and there will be the flow of load current i_{T1} through T_{1}. Since the triggering pulse is not given to T_{2}, the current i_{T2} will be zero and the voltage across T_{2} will be maximum. Also at the same time, the capacitor charges to the voltage V_{C} from t_{0} to t_{1}.

Now in order to turn-OFF T_{1}, thyristor T_{2} is triggered at t_{2} by applying pulse i_{g2}. When T_{2} turns ON, current i_{T2} flows and voltage V_{T2} becomes zero. Due to this, capacitor voltage makes the thyristor T_{1} reverse bais (from t_{1} to t_{2}) and thus T_{1} gets turned OFF. At this condition the supply voltage appears across T_{1} as V_{T1}, current i_{T1} becomes zero and the capacitor will charge in reverse direction i.e., to -V_{s} from t_{1} to t_{3}.

## Circuit Design of Class-C Commutation :

#### To find the current through and the voltage across commutating capacitor, applying KVL to the closed-loop of the capacitor at instant ωt = t_{1}, i.e., for loop E_{dc} → R_{1} → C → T_{2} → E_{dc}.

Now, applying Inverse Laplace transform for the above equation, we get,

The voltage across the capacitor must be equal to the voltage across the main thyristor T_{1} for turning OFF the main thyristor T_{1}.

The maximum permissible dV/dt across thyristor T_{1} using commutation components is given by (dV/dt)_{max} > 2V/R_{L}C. Since the commutation of SCR in this method is due to the application of reverse voltage, the class-C commutation is also known as Voltage Commutation. The single-phase inverter circuit that employs centre-tapped transformer uses the class-C method of commutation of SCR.