Working principle of thyristor

1. Structure of thyristor components
Regardless of the appearance of thyristors, their cores are all four-layer P1N1P2N2 structures composed of P-type silicon and N-type silicon. See Figure 1. It has three PN junctions (J1, J2, J3), with the anode A leading from the P1 layer of the J1 structure, the cathode K leading from the N2 layer, and the control electrode G leading from the P2 layer, so it is a four-layer three-terminal semiconductor device. 2. Working principle Thyristor is a P1N1P2N2 four-layer three-terminal structure element with a total of three PN junctions. When analyzing the principle, it can be regarded as consisting of a PNP tube and an NPN tube. Its equivalent diagram is shown in Figure 1 When a positive voltage is applied to the anode A, both BG1 and BG2 tubes are in the amplification state. At this time, if a positive trigger signal is input from the control electrode G, BG2 will have a base current ib2 flowing through it, which is amplified by BG2, and its collector current ic2=β2ib2. Because the collector of BG2 is directly connected to the base of BG1, ib1=ic2. At this time, the current ic2 is amplified by BG1, so the collector current of BG1 ic1=β1ib1=β1β2ib2. This current flows back to the base of BG2, forming a positive feedback, which makes ib2 increase continuously. As a result of this positive feedback cycle, the current of the two tubes increases sharply, and the thyristor is saturated and turned on. Due to the positive feedback effect formed by BG1 and BG2, once the thyristor is turned on, even if the current of the control electrode G disappears, the thyristor can still maintain the on state. Since the trigger signal only plays a triggering role and has no shut-off function, this thyristor cannot be turned off. Since the thyristor has only two working states, on and off, it has a switching characteristic. This characteristic requires certain conditions to be transformed. The conditions are shown in Table 1. The basic volt-ampere characteristics of the thyristor are shown in Figure 2. Figure 2 Basic volt-ampere characteristics of the thyristor (1) Reverse characteristics When the control electrode is open and the anode is applied with a reverse voltage (see Figure 3), the J2 junction is forward biased, but the J1 and J2 junctions are reverse biased. At this time, only a very small reverse saturation current can flow. When the voltage is further increased to the avalanche breakdown voltage of the J1 junction, the differential J3 junction also breaks down, and the current increases rapidly. The characteristic of Figure 3 begins to bend, as shown in the characteristic OR segment. The voltage URO at the bend is called the "reverse breakover voltage." At this time, the thyristor will have a permanent reverse (2) forward characteristics When the control electrode is open and a forward voltage is applied to the anode (see Figure 4), the J1 and J3 junctions are forward biased, but the J2 junction is reverse biased, which is similar to the reverse characteristics of an ordinary PN junction. Only a small current can flow through it. This is called the forward blocking state. When the voltage increases, the characteristic of Figure 3 bends, as shown in the characteristic OA segment. The bend is UBO, which is called: forward transition voltage Figure 4 Anode plus forward voltage As the voltage rises to the avalanche breakdown voltage of the J2 junction, the J2 junction undergoes an avalanche multiplication effect, generating a large number of electrons and holes in the junction area. Electrons enter the N1 area, and holes enter the P2 area. The electrons entering the N1 region recombine with the holes injected from the P1 region through the J1 junction into the N1 region. Similarly, the holes entering the P2 region recombine with the electrons injected from the N2 region through the J3 junction into the P2 region. The avalanche breakdown occurs, and the electrons entering the N1 region and the holes entering the P2 region cannot recombine completely. In this way, electrons accumulate in the N1 region and holes accumulate in the P2 region. As a result, the potential of the P2 region increases, the potential of the N1 region decreases, and the J2 junction becomes forward biased. As long as the current increases slightly, the voltage drops rapidly, and the so-called negative resistance characteristic appears, as shown in the dotted line AB section of Figure 3. At this time, the three junctions J1, J2, and J3 are all in forward bias, and the thyristor enters the forward conductive state---the on state. At this time, its characteristics are similar to the forward characteristics of the ordinary PN junction, as shown in the BC section of Figure 2. 2. Trigger conduction Figure 5 A positive voltage is applied to both the anode and the control electrode Figure 1. Schematic diagram and symbol diagram of thyristor structure

1. Structural principle of unidirectional thyristor (SCR): unidirectional thyristor is a controllable rectifier electronic component that can be turned on and off by external control signals. However, if it is turned on , external signals cannot turn it off. It can only be turned off by removing the load or reducing the voltage at both ends. The unidirectional thyristor is a four-layer three-terminal semiconductor device composed of three PN junctions PNPN and a diode with a PN junction . The forward conduction of the unidirectional thyristor is controlled by the control electrode current; unlike the triode with two PN junctions, the thyristor has no amplification effect on the control electrode current. Structural principle of

unidirectional thyristor

(thyristor): Bidirectional thyristor has the characteristics of turning on and off in two directions in turn. Bidirectional thyristor is essentially two anti-parallel unidirectional thyristors, which are semiconductor devices with three electrodes and four PN junctions formed by five layers of semiconductors NPNPN. The structure of the main electrode is symmetrical (both are drawn from the N layer). Its electrodes are not called anode and cathode respectively like unidirectional thyristors. The one close to the control electrode is called the first electrode A1, and the other is called the second electrode A2. The main disadvantage of bidirectional thyristors is that they have a low ability to withstand the voltage rise rate. This is because when the bidirectional thyristor ends the conduction in one direction, the carriers in the silicon wafer in each layer have not returned to the cutoff state . Take corresponding protective measures. Bidirectional thyristor components are mainly used in AC control circuits, such as temperature control, lighting control, explosion-proof AC switches, and DC motor speed regulation and commutation circuits.

Bidirectional thyristor structure and circuit symbol diagram