SCR Hard

Questions and Answers

What is the main difference between an SCR and an SUS?

The SCR has three terminals, whereas the SUS has two.

What is the primary method of turning on an SCR?

By passing an appropriate amount of gate current.

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

It has a very high switching speed.

What is the purpose of reducing the anode current to the holding current?

To turn the SCR off. Signup and view all the answers

What is the purpose of forced commutation?

To turn the SCR off. Signup and view all the answers

What is the significance of the Holding Current (IH) in SCR characteristics?

It is the value of the anode current below which the SCR switches from the forward-conducting region to the forward-blocking region Signup and view all the answers

What is the purpose of the Gate Trigger Voltage (VGT)?

To switch the SCR from the forward-blocking region to the forward-conducting region Signup and view all the answers

What is the Peak Reverse Voltage (PRV) in SCR characteristics?

The maximum reverse voltage that can be applied to an SCR without conducting in a reverse direction Signup and view all the answers

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.