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Class D commutation
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Class D commutation: This is also called as auxiliary commutation because it uses an auxiliary SCR to switch the charged capacitor across conducting SCR to turn it off. In this scheme, the main SCR is commutated by the auxiliary SCR. The main SCR with load resistance RL forms the power circuit while the diode D, inductor L, capacitor C and SCR2 forms the commutation circuit.

When the supply voltage Vdc is applied, both SCRs are in OFF state and hence the capacitor voltage is zero. In order to charge the capacitor, SCR2 must be triggered first. So the capacitor charges through the path Vdc-C-SCR2-RL-Vdc. 

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When the capacitor is fully charged, the charging current becomes zero and the SCR2 is turned-off naturally. The supply voltage Vdc as well as the charged capacitor C holds the SCR1 in forward bias condition. If the SCR1 is triggered, it is turned-on and two currents flow through it: one is the load current supplied by source, through path VdcSCR1 – RL – Vdc and another one is capacitor discharge current through path C – SCR1 – L – D - C. The capacitor while discharging supplies its energy to the inductor L. When the capacitor fully discharges, its voltage becomes zero at peak discharge current instant. Then the inductor L utilizes its energy to maintain the current through the same path and the capacitor charges with reversed polarity. When the inductor gives out its energy to the capacitor, the current naturally falls to zero and the capacitor charges fully with reversed polarity. Due to the presence of diode the reverse discharge is not possible. Thus after reverse charging of C, the SCR1 continues to carry only load current. The capacitor voltage maintains forward bias across SCR1, thereby it can be triggered at any instant. Now when it is desired to turn-off SCR1 for load voltage control, the SCR2 is triggered. The charged capacitor (lower plate positive) then placed across conducting SCR1, applying reverse bias to SCR1. Also, the capacitor discharging starts through path C - SCR2- RL- Vdc - C. The load current is shifted from SCR1 to C – SCR2 path. When this discharging current becomes more than the load current the SCR1 is turned OFF. After turning off of SCR1, the reverse bias is maintained across it by capacitor voltage, which ensures the proper turn-off. The capacitor discharges fully first and then starts charging with polarity of upper plate positive, through the SCR2 – RL to a supply voltage Vdc. When the capacitor fully charges, the charging current falls to zero and SCR2 is naturally turned off. The capacitor voltage as well as supply voltage make SCR1 forward biased and keep ready for next triggering. The above cyclic process is repeated.

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