In a thyristor, what causes the limit when the current has a big increase?

Question

I know in a thyristor, you have a PNPN junction. So if separate it into 3 junctions where call first PN junction as J12, second NP as J23, and third PN as J34. So is it correct to say when apply increasing Vak voltage across thyristor, that J12 gets more forward biased, J23 gets more reverse biased, and J34 gets more forward biased.

So when does the limit occur?

Is it 1) the J23 voltage gets so reverse biased that break-down occurs and a lot of current is let through?

2) there is a major charge build-up on J23 junction, and forward-biasing the J23 junction. I read in a book that by forward-biasing the J23 junction is when the thyristor conducts a lot of current, but doesn't make sense. The forward-bias would be in the opposite-direction of current, and how would having a large charge-build-up decrease the reverse-bias. i don't see how this could cause conduction, unless possibly this charge-buildup caused a sudden increase in the applied voltage across the J12 junction, causing it to act like a BJT.

So it makes more sense to me that the breakdown situation in 1) causes the sudden increase in the current, but the book may say otherwise, and it would be great if someone could tell me the actual situation.

Answer 1

When a silicon-controlled rectifier (SCR) is forward biased, what you have called J12 and J34 are also forward biased but J34 is reverse biased.  In your traditional SCR, a small charge is injected into P region 3, temporarily starting conduction through the device as if layers 2-4 were a junction NPN transistor.  This in turn turns on what is effectively the PNP transistor consisting of layers 1 and 3, with layers 2 and 3 common to both transistors.  Thepositive feedback between both transistors keeps the SCR conducting as long as sufficient current is flowing through the device. in the original direction.

Sometimes the voltage across the device builds up to the point at which the junction breaks down as in your case 1, which is one reason these devices have voltage ratings.  A sudden increase in the voltage across the device can also result in enough capacitive charge to turn on these devices in the absence of a signal on the gate, which is why they also have dv/dt ratings.  Although turning on by such a mechanism does not by itself injure the SCR itself, this is still considered a fault mode because such a mechanism may result in malfunction of the equipment in which the device is installed.

Another thing to watch for is the di/dt rating, especially in the larger devices.  When these devices turn on, they do not turn on all at once.  Rather, injected charge spreads from the injection site.  Until the injected charge propagates through the entire device, only part of the device conducts, though an increasing fraction of it as the charge progresses.  If the current density in any part gets too high, the device may be damaged.

Answer 2

Thyristors form a family of device such as SCR, TRIAC and other switching device.  The avalanche breakdown occurs when one injects control current at the gate to activate the device and you have correctly stated that the device can be represented as a couple of BJT transistors (see fig 2.1 page 11 of ref 1). Check ref 2  "Function of the gate terminal"  fro more information.