SCR Medium

Study Notes

Supply voltage is generally less than breakover voltage value at IG=0.
SCR is turned on by passing an appropriate amount of gate current (a few mA).
SCR must not exceed the reverse breakdown voltage when operated from an AC supply.
To turn off an SCR, anode current should be reduced to holding current.
Increasing gate current above the required value can cause the SCR to close at a much reduced supply voltage.

Advantages over mechanical or electromechanical switches:
- No moving parts, resulting in noiseless operation at high efficiency.
- High switching speed, up to 10^9 operations per second.
- Control over large current (30-100 A) in the load by means of a small gate current (a few mA).
- Small size and trouble-free service.

Anode current interruption: suppressing anode current from going through the anode to the cathode terminal.
Methods: series switch and parallel switch.
Forced commutation: momentarily forcing current through the SCR in the opposite direction to reduce net forward current below holding value.

Forward Breakover Voltage (VBR(F)): voltage at which SCR enters forward-conduction region.
Holding Current (IH): anode current value below which SCR switches from forward-conducting to forward-blocking region.
Gate Trigger Current (IGT): gate current value necessary to switch SCR from forward-blocking to forward-conducting region.
Gate Trigger Voltage (VGT): voltage across gate-cathode junction that turns SCR on.
Average Forward Current (IF(AVG)): maximum continuous anode current SCR can withstand in forward-conducting region.
Peak Reverse Voltage (PRV): maximum reverse voltage that can be applied to an SCR without conducting in a reverse direction.
Circuit Fusing Rating: product of square of forward surge current and time of duration of the surge.