SCR Medium

11 Questions

What is the term for the voltage at which the SCR enters the forward-conduction region?

Forward Breakover Voltage

What is the minimum current required to switch the SCR from forward-blocking to forward-conducting region?

Gate Trigger Current

What is the maximum continuous anode current that an SCR can withstand?

Average Forward Current

What is the product of the square of forward surge current and the time of duration of the surge?

Circuit Fusing Rating

What is the typical value of gate current required to turn on an SCR?

A few milliampere

When an SCR is operated from an AC supply, what should be ensured?

The peak reverse voltage does not exceed the reverse breakdown voltage

What is the advantage of using an SCR over a mechanical switch?

It has no moving parts

How is the SCR turned off from the ON state?

By reducing the anode current to holding current

What is the purpose of forced commutation?

To reduce the forward current below the holding value

What is the typical current handling capacity of an SCR?

30-100 A

What is the effect of increasing the gate current above the required value?

The SCR will turn on at a much reduced supply voltage

Study Notes

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.