Current Commutated Chopper Circuit:

Figure 11.34 shows a Current Commutated Chopper Circuit. The main thyristor Th₁ of the chopper is commutated by a current pulse generated in the commutation circuitry. The sequence of operation is given below, and various significant waveforms are shown in Fig. 11.35. It is to be noted that this charging of the capacitor C takes place from the source V through resistance R and the oscillation of the LC combination is set to be far faster than the RC charging transient.

The capacitor C is already charged via R through the source V. The main thyristor Th₁ is fired at t = t₀. The load terminals are connected to the supply, and the load current flows through Th_1. At t = t₁, commutation of Th₁ is initiated by firing the auxiliary thyristor Th₂. An oscillatory current flows in the circuit consisting of C, Th₂ and L. When i_C reverses at t₂, thyristor Th₂ turns off (natural commutation), and the oscillatory current i_C flows through D₂ and Th₁. At t₃, i_C = i_{Th1 ,} and therefore zero current flows through Th₁ and it turns off, and diode D₁ begins to conduct the current i_c– i_a and keeps the main thyristor Th₁ reverse-biased.

At t₄, i_C = i_a and i_D1 = 0. At this point D_FW is not forward-biased, the load current will flow through V, C, L, D₂ and load.

At t₅, the D_FW becomes forward biased and starts to conduct the load current i_a. The oscillatory circuit consisting of C and L is now subjected to new circuit conditions through V, C, L, D₂ and D_FW and i_C decays to zero at t₆ leaving ν_c less than V. During t₅ to t₆, i_a=i_C+i_FW and therefore, as i_C decays, i_Fw builds up. From t₆ onward, load current free-wheels through D_FW and decays. The capacitor voltage ν_c also decays because of discharge through R.

At t = T, the main thyristor Th₁ is fired again, and the cycle repeats.

Advantages:

This Current Commutated Chopper Circuit has several advantages. Commutation is reliable as long as load current is less than the peak commutation current i_C. The capacitor always remains Charged with right polarity.