SCR Medium

SCR Basics

• SCR is a four-layer PNPN device with three terminals: anode, cathode, and gate.
• Similar to a four-layer diode, but with three terminals.

SCR Normal Operation

• 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.

SCR as a Switch

• 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.

Turning off an SCR

• 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.

SCR Characteristics and Ratings

• 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.