- The class D commutation is also called as impulse commutation.
- The SCR T1 is main SCR whereas SCR T2 is auxiliary SCR.
- The auxiliary SCR T2 is used to turn off main SCR T1 therefore this method is also called as auxiliary commutation.
Stage 1
- The SCR T1 and SCR T2 are in off condition during initial condition and voltage across capacitor is zero.
SCR T1 – OFF
Stage 2
- The SCR T2 is turned on in order to charge capacitor C.
- The charging of capacitor is done through path (+)Vdc – (+) C – C (-) – SCR T2 – RL –Vdc(-).
- As the voltage across capacitor increases, the current through SCR T2 decreases.
- The SCR T2 is turned off when the voltage across capacitor reaches Vdc and charging current reduces below holding current.
SCR T1 – OFF
SCR T2 – OFF
Voltage across capacitor = Vdc
Stage 3
- When SCR T1 is turned on, the load current flows through path (+)Vdc – SCR T1 – RL –Vdc(-).
- The commutating current at the same time flows through path (+) C – SCR T1 – L – D – C (-).
- Once the capacitor discharges, it will start to charge in the reverse direction. ( It means that its upper plate becomes negative and lower plate becomes positive).
- The capacitor does not discharge due to diode D after charging in the reverse direction.
SCR T1 – ON
SCR T2 – OFF
Voltage across capacitor =
- Vdc
Stage 4
- The discharging of capacitor is done through path (+) C – SCR T2 – SCR T1 – C (-) when SCR T2 is turned on.
- The SCR T1 is turned off when the discharging current of capacitor increases beyond load current.
SCR T1 – OFF
SCR T2 – ON
Stage 5
- The capacitor C once again start to charge through path (+)Vdc – (+) C – C (-) – SCR T2 – RL – Vdc (-).
- The whole cycle repeats in this way. The efficiency of Class D commutation is high because the commutation energy transfers in the load.
- The application of the Class D Commutation is in the Jones chopper.
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