Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Basic Electronics
Turning Off the SCR
Turning off the SCR:
- Turning off the SCR—changing it from the conducting state to the blocking state—can be accomplished by reducing the current i below a critical value (called the holding current) required to maintain the α1 α2 = 1 condition.
- In some SCR devices, gate turnoff can be used to reduce the alpha sum below unity.
- For example, if the gate voltage is reversed in Fig. given below, holes are extracted from the p2 base region.
- If the rate of hole extraction by the gate is sufficient to remove the n-p-n transistor from saturation, the device turns off.
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However, there are often problems involving the lateral flow of current in p2 to the gate; nonuniform biasing of j3 can result from the fact that the bias on
this emitter junction varies with position when a lateral current flows.
- Therefore, SCR devices must be specifically designed for turn-off control, at best, this turn-off capability can be utilized only over a limited range for a given device.