Figure 1 shows the power-circuit of a load commutated chopper.This chopper circuit consist of four thyristors $T_1,T_2,T_3,T_4$ and one commutating Capacitor C.Here,thyristors $T_1, T_2$ from one pair and thyristors $T_3, T_4$ form another pair which conduct the load current alternatively.When one thyristor pair $T_1,T2$ functions as main thyristors,at the same time other thyristor pair $T_3,T_4$ functions auxillary thyristors,and vice-versa.Again,operation of the chopper circuit is divided into different modes.These modes are described with associated waveforms as shown in figure.2.

The waveforms shown in Fig.2 starts at the instant $t=t_{0}$ . It is assumed that prior to this instant $t_{0}$ , capacitor C was charged to the reverse voltage $\left(-E_{\mathrm{dc}}\right)$ due to the conduction of thyristors $T_{3}$ and $T_{4}$ . Therefore, before the instant $t_{0},$ the capacitor upper plate becomes negative and lower plate positive.

Mode 1 Operation: As shown in Fig.2 at $t=t_{0}$ both thyristors $T_{1}$ and $T_{2}$ are triggered. Therefore, load current flows through the path $E_{\mathrm{dc}}-T_{1}, C, T_{2}$ and the load. Load voltage $e_{0}$ now becomes, $e_{0}=E_{\mathrm{dc}}-V_{c},$ i.e., 2$E_{\mathrm{dc}}$ . The capacitor C is charged linearly by a constant load current $i_{0}$ from $\left( -E_{\mathrm{dc}}\right)$ at $t=0$ to $E_{\mathrm{dc}}$ at $t_{1}$ .

When the capacitor is charged fully positive at $t=t_{1},$ the current through the conducting thyristors $T_{1}, T_{2}$ becomes zero and these go into the blocking mode. The load voltage $e_{0}$ falls linearly. The freewheeling diode $D_{f}$ becomes forward biased, and the load current is transferred from $T_{1}$ and $T_{2}$ to $D_{f}$

Mode 2 Operation: As shown in Fig.2 from $t_{1}$ to T , the freewheeling diode $D_{F}$ conducts the load current. For the period $t_{1}$ to $T, V_{c}=E_{\mathrm{dc}}, i_{c}=0, i_{f}=i_{0}$ and the load voltage $e_{0}=0 .$ Now, at $t=T,$ the second pair of thyristors $T_{3}, T_{4}$ is triggered. This places the fully charged capacitor across thyristors $T_{1} T_{2},$ reverse biasing them and turning them off. The cycle now repeats.

Advantage:

1.This chopper is capable of commutating any amount of current.

2.No commutating inductor is required in this chopper circuit which is normally costly, bulky and noisy.

3.This circuit can operate at high frequencies of the order of $\mathrm{kHz}$ , and therefore filtering requirements to smooth out load current are minimal.

Disadvantage:

1.The peak load voltage is twice the supply voltage. However, this peak can be reduced by filtering.

2.Because of higher switching losses at high frequencies and losses in the two conducting thyristors in series with the load, efficiency may become low for high power applications.

3.Since freewheeling diode $D_{f}$ is subjected to twice the supply voltage $\left(2 E_{\mathrm{dc}}\right)$ in a short time, a fast recovery type diode must be used.

4.The commutating capacitor has to carry the load current at a frequency half the chopping period.

5.One thyristor pair should be turned-on only when the other pair is commutated. This can be realised by sensing the capacitor current that is alternating.