The Class D if the load current is carried by only one of the SCR’s, the other acting as an auxiliary turn-off SCR. The auxiliary SCR would have a resistor in its anode lead of say ten times the load resistance. Class D turn-off. Class D commutation by a C (or LC) switched by an Auxiliary SCR. MOSFET CHOPPER: CLICK HERE.
SCR must be triggered first in order to charge the upper terminal of the capacitor as positive. As soon as C is charged to the supply voltage, SCRwill turn off. If there is substantial inductance in the input lines, the capacitor may charge to voltages in excess of the supply voltage. This extra voltage would discharge through the diode-inductor-load circuit.
When SCR is triggered the current flows in two paths: Load current flows through the load and the commutating current flows through C- SCR2 -L-D network. The charge on C is reversed and held at that level by the diode D. When SCR1 is re-triggered, the voltage across C appears across SCR2 via SCR1 and SCR2 is turned off. If the load carries a constant current as, the capacitor again charges linearly to the dot as positive.