A thyristor can be turned ON by applying a positive voltage of about a volt or a current of a few tens of milliamps at the gate-cathode terminals. However, the amplifying gain of this regenerative device being in the order of the 108, the SCR cannot be turned OFF via the gate terminal. It will turn-off only after the anode current is annulled either naturally or using forced commutation techniques. These methods of turn-off do not refer to those cases where the anode current is gradually reduced below Holding Current level manually or through a slow process. Once the SCR is turned ON, it remains ON even after removal of the gate signal, as long as a minimum current, the Holding Current, Ih, is maintained in the main or rectifier circuit.
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.
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.
This configuration has two SCRs. One of them may be the main SCR and the other auxiliary. Both may be load current carrying main SCRs. The configuration may have four SCRs with the load across the capacitor, with the integral converter supplied from a current source. Assume SCR2 is conducting. C then charges up in the polarity shown. When SCR1 is triggered, C is switched across SCR2 via SCR1 and the discharge current of C opposes the flow of load current in SCR2.
SCRA 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, SCRA will 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 SCRM is triggered the current flows in two paths: Load current flows through the load and the commutating current flows through C- SCRM -L-D network. The charge on C is reversed and held at that level by the diode D. When SCRA is re-triggered, the voltage across C appears across SCRM via SCRA and SCRM is turned off. If the load carries a constant current as in (Fig.Class c) the capacitor again charges linearly to the dot as positive.
The transformer is designed with sufficient iron and air gap so as not to saturate. It is capable of carrying the load current with a small voltage drop compared with the supply voltage.
When SCR1 is triggered, current flows through the load and pulse transformer. To turn SCR1 off a positive pulse is applied to the cathode of the SCR from an external pulse generator via the pulse transformer. The capacitor C is only charged to about 1 volt and for the duration of the turn-off pulse it can be considered to have zero impedance. Thus the pulse from the transformer reverses the voltage across the SCR, and it supplies the reverse recovery current and holds the voltage negative for the required turn-off time.
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