Chopper is used to change the dc level of voltage, it is dc/dc converter.
In current commutated chopper , as the name suggests, chopper is commutated by current pulse.
Following events take place when the current commutated chopper
- At first the T2 is turned on - T2 is fired through gate and turned on
- Capacitor C is charged to -Edc - As T2 is on, the capacitor indutor series circuit is shorted and so the capacitor is charged to negative voltage -Edc
- The capacitor C is fully charged, and current reverses, and so T2 is turned off - When the capacitor is charged, it provides -Edc voltage reverse bias to T2 thyristor, so T2 is turned off. So we can say that T2 was turned on to charge thyristor to negative voltage
- Now the current flows opposite through the thyristor T1 - The capacitor, charged to negative Edc, pushes current ic opposite to T1.Why doesn't ic not flow through diode, because the forward voltage of diode is more than that of drop accross thyristor T1. Meaning, the current can reverse bias thyristor because thyristor provides lesser impedance than compared to diode.
- T1 is turned off- Now the ic turns off T1. So now the impedance offered by T1 is more than diode, and the current ic flows through diode.
- The D1 is reversed biased - The current ic slowly decrease, and when ic=io, the diode d1 is no longer forward biased. So the diode is turned off.
- C is charged to +Edc The diode being turned off, the capacitor is charged to +Edc.
- Now the commutation period is complete. The T1 is turned on by gate pulse. And again T2 is turned on, capacitor charges to negative potential and cycle repeats.
NOTE: The steps are explained according to diagram mentioned in page number 486 and 488 of below stated book.