Class A Commutation of Thyristor (SCR) | Current Commutation of Thyristor (SCR)

Load in Parallel with the SCR

The class A commutation is also called as self commutation. Fig. 1 shows the SCR circuit that uses a class A commutation. In Fig. 1, L and C are the commutating components and R is the load resistance. The commutation components L and C form an underdamped resonant circuit. Various voltage and current waveforms and equivalent circuits are shown in Fig. 2.

Fig. 1: Self commutation circuit (class A)

Operation of the circuit:

Refer to Fig. 2 (a). At t = t₀ the SCR₁ is turned on, the dc supply voltage is applied to the resonant circuit.

 

(a)

(b)

(c)

Fig. 2: Waveforms and equivalent circuits for self commutation (class A) circuit.

Due to the presence of underdamped LC resonant circuit, the anode current of SCR₁ is sinusoidal in nature (see Fig. 2(b)). This current will charge the capacitor C with its upper plate positive with respect to its lower plate.

At t = t₁ the voltage on capacitor C reaches its maximum and the anode current goes to zero. The capacitor will then attempt to force a reverse current through SCR₁ which turns off due to net anode current going to zero. In this way, the class A commutation is “current commutation”. As soon as the SCR₁ is turned off, the capacitor C starts discharging through R (see Fig. 2(c)). The voltage across SCR₁ at the instant t₁ is negative as the voltage on the capacitor C is higher than the input supply voltage V_S. The voltage across SCR₁ then gradually increases towards zero as the capacitor discharges and finally reaches the positive supply voltage level + V_S when C is completely discharged. The voltage across SCR₁ is nearly zero from t₀ to t₁ when it is conducting. The time period t_C shown by shaded portion in Fig. 2(b) is called as circuit turn off time. It is the time for which the commutated SCR is reverse biased. For successful commutation, t_C should be greater than or equal to the SCR turn off t_q. At instant t₂, the SCR₁ is turned on again and the operation repeats itself.

Load in Series with the SCR

This is another arrangement of class A type commutation. It is shown in Fig. 3 and the load is connected in series with L and C.

Fig. 3: Class A commutation with load in series with the capacitor

Operation

When the SCR is turned on by applying a gate pulse, the charging current i(t) of the capacitor starts flowing. As the LC components form an under damped circuit, the charging current will be sinusoidal in nature.  As soon as the capacitor is fully charged, the charging current reduces to zero and the SCR is turned off due to natural commutation