About how thyristors work

Analysis of the working principle of thyristor:

The thyristor has three poles---- anode (A), cathode (C) and control electrode (G), and the die is a four-layer structure composed of overlapping P-type conductors and N-type conductors, with a total of three PN junctions, which is very different from the silicon rectifier diode with only one PN junction. The four-layer structure of the thyristor and the introduction of the control pole have laid the foundation for it to exert its excellent control characteristics of "controlling large with small". In thyristor applications, a large anode current or voltage can be controlled by adding a small current or voltage to the control pole. At present, thyristor components with a current capacity of several hundred amperes and thousands of amperes can be manufactured. Generally, thyristors below 5 amperes are called low-power thyristors, and thyristors above 50 amps are called high-power thyristors.

Another PNP-type transistor. Among them, the second and third layers are shared by two overlapping pipes. The equivalent circuit diagram of Figure 1 can be drawn. When a forward voltage E is added between the anode and the cathode, and a positive trigger signal is input between the control pole G and the cathode C (equivalent to the base shot of BG2), BG2 will generate the base current Ib2, and after amplification, BG2 will have a collector current IC2 amplified by β2. Because the collector of BG2 is connected to the base of BG1, IC2 is the base current of BG1. BG1 sends the collector current of Ib1 (Ib2) amplified β1 back to the base of BG2 for amplification. This cycle amplifies until BG1 and BG2 are fully on. In fact, this process is "touch-and-go", for the thyristor, the trigger signal is added to the control pole, and the thyristor is immediately turned on. The turn-on time is mainly determined by the performance of the thyristor.

Once the thyristor is triggered to turn on, due to the cyclic feedback, the current flowing into the base of BG2 is not only the initial Ib2, but the amplified current (β1*β2*Ib2) after BG1 and BG2, which is much larger than Ib2 and is enough to keep BG2 continuously on. At this time, even if the trigger signal disappears, the thyristor will still remain in the conduction state, and the thyristor can only be turned off when the power supply E is disconnected or the output voltage of E is reduced, so that the collector current of BG1 and BG2 is less than the minimum value of maintaining conduction. Of course, if the E polarity is reversed, BG1 and BG2 will be cut off when they are subjected to reverse voltage. In this case, even if a trigger signal is input, the thyristor will not work. Conversely, E is positive, while the trigger signal is negative, and the thyristor cannot be conducted. In addition, if there is no trigger signal, and the forward anode voltage is large enough to exceed a certain value, the thyristor will also be turned on, but it is already an abnormal working condition.

The controllable characteristic of the thyristor to control the conduction through a trigger signal (small trigger current) (high current in the thyristor) is an important feature that distinguishes it from ordinary silicon rectifier diodes.