Class A, Self commutated by resonating the load & Class B, Self commutated by an L-C circuit
Class A, Self commutated by resonating the load:
Fig: A resonant load commutated SCR and the corresponding waveforms
- When the SCR is triggered, anode current flows and charges up C with the dot as positive. The L-C-R forms a second order under-damped circuit.
- The current through the SCR builds up and completes a half cycle. The inductor current will then attempt to flow through the SCR in the reverse direction and the SCR will be turned off.
- The capacitor voltage is at its peak when the SCR turns off and the capacitor discharges into the resistance in an exponential manner. The SCR is reverse-biased till the capacitor voltages returns to the level of the supply voltage V.
Class B Self commutated by an L-C Circuit:
Fig: Class B, L-C turn off
- The Capacitor C charges up in the dot as positive before a gate pulse is applied to the SCR. When SCR is triggered, the resulting current has two components.
- The constant load current Iload flows through R - L load. This is ensured by the large reactance in series with the load and the freewheeling diode clamping it.
- A sinusoidal current flows through the resonant L-C circuit to charge-up C with the dot as negative at the end of the half cycle. This current will then reverse and flow through the SCR in opposition to the load current for a small fraction of the negative swing till the total current through the SCR becomes zero.
- The SCR will turn off when the resonant–circuit (reverse) current is just greater than the load current.
- The SCR is turned off if the SCR remains reversed biased for tq > toff, and the rate of rise of the reapplied voltage < the rated value.