About the working principle of thyristor

About the working principle of thyristor

Analysis of the working principle of thyristor:

thyristor has three electrodes - anode (A), cathode (C), and control electrode (G). The tube core is a four layer structure composed of P-type and N-type conductors, with a total of three PN junctions, which is completely different from silicon rectifier diodes with only one PN junction in structure. The four layer structure of controllable silicon and the introduction of control electrodes have laid the foundation for its excellent control characteristics of "controlling large with small". When using thyristor, as long as a small current or voltage is applied to the control electrode, a large anode current or voltage can be controlled. At present, controllable silicon components with a current capacity of several hundred amperes to thousands of amperes can be manufactured. Generally, thyristors below 5 amperes are called low-power thyristors, while thyristor above 50 amperes are called high-power thyristors.

Another PNP transistor. The second and third layers are shared by two overlapping pipes. An equivalent circuit diagram of Figure 1 can be drawn. When a forward voltage E is applied between the anode and cathode, and a positive trigger signal is input between the control electrode G and cathode C (equivalent to the base beam of BG2), BG2 will generate the base current Ib2. After amplification, BG2 will have an amplification β Double the collector current IC2. Because the BG2 collector is connected to the BG1 base, IC2 is the base current Ib1 of BG1. BG1 has magnified Ib1 (Ib2) again β The collector current IC1 of 1 is sent back to the base of BG2 for amplification. Repeat the amplification process until BG1 and BG2 are fully conductive. In fact, this process is "instantaneous". For the thyristor, when the trigger signal is applied to the control electrode, the thyristor immediately conducts. The conduction time mainly depends on the performance of the thyristor.

After triggering the conduction of the thyristor, due to cyclic feedback, the current flowing into the BG2 base is no longer just the initial Ib2, but the current amplified by BG1 and BG2（ β 1* β 2 * Ib2), this current is much greater than Ib2, which is sufficient to maintain the continuous continuity of BG2. At this point, even if the trigger signal disappears, the thyristor remains in a conductive state. 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 for maintaining conduction. Of course, if the polarity of E is reversed, BG1 and BG2 will be in a cut-off state under the action of reverse voltage. At this point, even if a trigger signal is input, the thyristor cannot operate. On the contrary, E is connected in a positive direction, while the triggering signal is negative, and the thyristor cannot conduct. In addition, if no trigger signal is added and the forward anode voltage exceeds a certain value, the thyristor will also conduct, but it is already an abnormal working condition.

The controllable characteristic of thyristor, which controls conduction through triggering signals (small triggering currents) and large currents in thyristor, is an important feature that distinguishes it from ordinary silicon rectifier diodes.