Class D commutation
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.
Power circuit diagram of class D commutation
Working of class D 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
SCR T2 – 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.
Waveform of class D commutation
Application of class D commutation
The application of the Class D Commutation is in the Jones chopper.
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