1. The structure of the thyristor

The thyristor is a 4-layer power semiconductor device with 3 PN junctions. Its internal structure, shape and circuit symbols are shown in Figure 6-1. Among them, the outermost P area and N area respectively lead to two electrodes, called anode A and cathode K, and the middle P area leads to the control electrode (or gate).

2. The working principle of thyristor

The actual circuit composed of thyristors is shown in Figure 6-2.

In order to explain the working principle of the thyristor, it can be regarded as two transistors connected between NPN and PNP, and the symbol of the transistor is used to represent the equivalent circuit of the thyristor, as shown in Figure 6-3.

Its working process is shown in Figure 6-4.

When the positive voltage UZ is applied between the anode A and the cathode K of the thyristor and no voltage is applied to the control electrode K, the middle PN junction is in reverse bias, the tube is not conducting and is in an off state.

When the positive voltage UA is applied between the anode A and the cathode K of the thyristor, the M28F101-200K6 and the positive voltage UG is also applied between the control electrode G and the cathode K, the lower outer PN junction is in a conducting state. If the base current of the V2 tube is IB2, the collector current Ic2 is β2IB2, and the base current IB1 of the V1 tube is equal to the collector current of the Vz tube, so the collector current Icl of the V2 tube is βlβ2. The above-mentioned amplification process is performed once again to form a positive feedback. In a short time (usually a few microseconds), both diodes enter the saturation state, so that the thyristor is completely turned on. When the thyristor is completely turned on, the control pole loses its control function, and the tube always maintains the on state by virtue of internal positive feedback. The voltage drop of the pair of tubes is very small, generally 0.6 to 1.2 V, and the power supply voltage is almost all added to the load resistance R, and there is current flowing in the thyristor, which can reach tens to thousands of amperes. To turn off the thyristor, the anode current must be reduced to the point where the positive feedback process cannot be maintained. Of course, the anode power supply can also be disconnected or a reverse voltage can be applied between the anode and the cathode of the thyristor.

In summary, the following conclusions can be drawn:

① The thyristor is similar to the silicon rectifier diode, both have reverse blocking ability, but the thyristor also has the forward blocking ability, that is, the thyristor must have certain conditions for the forward conduction: the anode plus the forward voltage, and the control pole also plus the forward Trigger voltage (in actual work, positive trigger pulse signal is added to the control pole).

②Once the thyristor is turned on, the control pole loses its control function. To make the thyristor turn off again, one of the following two points must be achieved: one is to reduce the anode current to be less than the holding current IH; the other is to reduce the anode voltage to zero or reverse it.