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.
Classification of forced commutation methods
☞Class A – Self commutated by a resonating load
☞Class B – Self commutated by an L-C circuit
☞Class C – C or L-C switched by another load carrying SCR
☞Class D – C or L-C switched by an auxiliary SCR
☞Class E – An external pulse source for commutation
Class A, Self commutated by resonating the load
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
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.
Class C, C or L-C switched by another load–carrying SCR
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 SCR 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 SCR 2 via SCR 1 and the discharge current of C opposes the flow of load current in SCR2.
Class D, L-C or C switched by an auxiliary SCR
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, SCR A 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 SCR M 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.
Class E – External pulse source for commutation
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 SCR 1 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.