Class B Commutation of Thyristor | Resonant Pulse Commutation

Class B commutation or Resonant-Pulse commutation is one of the types of forced commutation. In this type of forced commutation, a resonant pulse generated by an LC circuit is used to turn OFF the SCR. Fig. 1 (a) shows the circuit diagram of a class-B (or resonant pulse) commutation of SCR and Fig 1 (b) shows its waveforms.

Fig. 1: Resonant pulse (class-B) commutation Circuit Diagram.

Waveforms of Class B Commutation

The waveforms of class-B (resonant pulse) commutation are as shown in Fig. 1 (b). This method of commutation is called as “Resonant-Pulse Commutation” method or Current Commutation.

(b) Waveforms

Fig. 2: Resonant pulse (class-B) commutation waveforms  

Working of Class B Commutation

Initially, it is assumed that the capacitor is charged to voltage V_S with left plate positive. As V_C = V_S; the charging current i_C = 0. At t = 0; main SCR (T₁) is fired or triggered by i_g hence the main SCR turns ON due to which i_o = I_o and i_T1 = I_o

To turn OFF the main SCR at desired instant t₁,  applied to auxiliary SCR (T_A). Hence, the capacitor begins to discharge through T_A and i_C becomes negative and is resonant due to which V_C starts decreasing. At the half cycle of t₁ from instant t₁ i.e. at time t₂, the current through capacitor i_C becomes zero causing T₁ to turn OFF. Therefore, V_C = – V_S and also i_o = i_T1 = I_o because T₁ (main SCR) is still conducting . As T_A turns OFF, the right plate of the capacitor gets positive V_S. Here the resonant current i_C builds up through C, L, D and T₁ which flows in opposite direction of i_T1 due to which i_T1 begins to drop and hence the voltage across the capacitor begins to reverse once again i.e. it increases.

At time t₃; i_C becomes equal to I_o. The current through T₁ reduces to zero and the main SCR turns OFF; whereas by time t₄, the voltage across the capacitor becomes zero. The output current will continue to flow till the capacitor charges to + V_S i.e. its left plate becomes positive. After this, T₁ and T_A are OFF and the current through the capacitor is zero. Therefore, i_o = 0.

Advantages of Class B Commutation

The advantages of class-B commutation method of an SCR are as under:

1. It is simple.
2. The commutating components L and C do not carry load current.

Applications of Class B Commutation

The application of class-B commutation method is as under

1. It is used in d.c. chopper circuit.