Class C Commutation of Thyristor (SCR) | Complementary Commutation | Voltage Commutation

In complementary (or class C) commutation, the SCR carrying load current is commutated (turned-OFF) by transferring its load current to another incoming SCR. The SCR to be commutated is called main SCR and the additional SCR used for commutating the conducting SCR is called complementary SCR. In this technique of commutation, both the SCRs conduct the load current alternately.

Fig. 1: Circuit diagram of complementary voltage commutation (class C commutation).

The circuit diagram for the complementary voltage commutation is as shown in Fig. 1. Both the thyristors SCR₁ and SCR₂ are capable of handling the load current. C is the commutating capacitor and the voltage across it is used to commutate the conducting SCR. In this way, class C commutation is voltage commutation.

The resistors R₁ and R₂ are load resistors and let I_R1 and I_R2 be the corresponding load currents such that

\[{{\text{I}}_{\text{R1}}}=\text{ }\frac{{{\text{V}}_{\text{S}}}}{{{\text{R}}_{\text{1}}}}\]

And

\[{{\text{I}}_{\text{R2}}}=\text{ }\frac{{{\text{V}}_{\text{S}}}}{{{\text{R}}_{\text{2}}}}\]

We assume that both the SCRs are ideal. Therefore the on state voltage drop across them will be zero.

Class C Commutation Operation of the circuit

The operation of complementary commutation circuit can be explained by dividing one cycle of operation into four intervals or modes of operation.

(a) Equivalent circuit for mode I

(b) Equivalent circuit for mode II

Fig 2: Equivalent circuit

Before SCR₁ is triggered at instant t₀ assume that SCR₂ is conducting. The voltage across the commutating capacitor is equal to V_S with the polarities of voltage as shown in Fig. 2(a). The load current I_R2 flows through SCR₂.

Mode I (t₀ to t₁):

At the instant t = t₀, SCR₁ is triggered. This connects the commutating capacitor C across the conducting SCR₂. The voltage across C reverse biases the conducting SCR₂ and it is turned off due to voltage commutation. The load current I_R1 which is equal to V_S / R₁ starts flowing through SCR₁. The charging current of the capacitor I_C also flows through SCR₁ as shown in Fig. 2(a). At t = t₁ the capacitor is fully charged with polarities as shown in Fig. 2(b), and I_C = 0. SCR₁ continues to conduct.

Mode II (t₁ to t₂):

At t = t₁, the charging current of the capacitor C goes to zero. The voltage on C is equal to V_C with the polarities as shown in Fig. 1.26.6(b). SCR₁ continues to conduct the load current I_R1 = V_S / R₁. At the end of this mode i.e. at t = t₂ the other thyristor SCR₂ is triggered to turn off SCR₁.

Mode III (t₂ to t₃):

(a) Equivalent circuit for mode III

(b) Equivalent circuit for mode IV

Fig 3: Equivalent circuit

At instant t = t₂, SCR₂ is turned on. This will connect the commutating capacitor C across SCR₁ via SCR₂. The voltage on C reverse biases SCR₁ and turns it off, due to voltage commutation. The current through SCR₂ i.e. I_SCR2 is the sum of the load current I_R2 and the charging current of capacitor I_C as shown in Fig. 3 (a). The voltage across SCR₁ will follow the variations in the capacitor voltage. At the end of this mode (t = t₃), the capacitor is fully charged, I_C = 0 and V_SCR1 = V_S.

Mode IV (t₃ to t₄):

In this mode of operation, SCR₂ continues to carry the load current I_R2 = V_S / R₂. The voltage on C is held constant equal to V with the polarities as shown in Fig. 3 (b). The charging current of the capacitor I_C is equal to zero. At instant t₄, SCR₁ is triggered again to turn off SCR₂ and the cycle of operation repeats itself.

For successful commutation, the time for which the outgoing SCR is reverse biased (t_c) as shown in Fig. 4, must be greater than the turn off time of the SCR i.e. (t_q).

Class C Commutation Waveforms

The input and output waveforms are as shown in Fig. 4.

Fig. 4: Waveforms of class-C commutation.

Class C Commutation Advantages

The advantage of class-C commutation method of an SCR is as under:

1. The charging current flowing through the capacitor does not flow through the load.

Class C Commutation Drawbacks

The drawbacks of class-C commutation method of an SCR are as under:

1. The capacitor C can be charged to the voltage limited to the supply voltage and not more than that of it.
2. The turn-ON time of the main SCR is limited due to limit on supply voltage. In high current applications, it requires high value of turn-ON time.