Thyristor is also called silicon controlled rectifier (SCR), the following is described as SCR
(1) Working conditions of SCR. Because SCR has only two working states of on and off, it has switching characteristics. This characteristic requires certain conditions to be transformed. Such conditions are shown in Table 1.
① When the SCR bears the reverse anode voltage, no matter what voltage the gate bears, the SCR is in the off state.
② When the SCR bears the positive anode voltage, the SCR can only be turned on when the gate is subjected to the positive voltage.
③When the SCR is on, as long as there is a certain positive anode voltage, the SCR remains on regardless of the gate voltage. That is to say, after the SCR is turned on, the gate loses its function.
④ When the SCR is turned on, when the main circuit voltage (or current) decreases to close to zero, the SCR is turned off.
(2) The volt-ampere characteristics of SCR. The relationship between the voltage between the anode A and cathode K of the SCR and the anode current of the SCR is called the SCR volt-ampere characteristic, as shown in Figure 1. The forward characteristic is located in the first quadrant, and the reverse characteristic is located in the third quadrant.
①Reverse characteristics: When the gate G is open and the anode A is applied with a reverse voltage (as shown in Figure 2(a)), the J2 junction is forward biased, but the J1 and J3 junctions are reverse biased. Only a small reverse saturation current can flow at this time. When the voltage is further increased to the avalanche breakdown voltage of the J1 junction, the breakdown current of the J3 junction also increases rapidly. The OR section of the characteristic curve in Figure 1 begins to bend, and the voltage URO at the bend is called the reverse bending voltage. After that, SCR will undergo permanent reverse breakdown.
②Forward characteristics: When the gate G is open and the positive voltage is applied to the anode A (as shown in Figure 2(b)), the J1 and J3 junctions are forward biased, but the J2 junctions are reverse biased, which is the opposite of ordinary PN junctions. The characteristics are similar, and only a small current can flow. This is called the forward blocking state. When the voltage increases, the OA section of the characteristic curve in Figure 1 begins to bend, and the voltage UBO at the bend is called the forward turning voltage.
After the voltage rises to the avalanche breakdown voltage of the J2 junction, the J2 junction will have an avalanche multiplication effect, and a large number of electrons and holes will be generated in the junction area. The electrons enter the N1 area and the holes enter the P2 area. The electrons entering the N1 region recombine with the holes injected into the N1 region from the P1 region through the J1 junction. Similarly, the holes entering the P2 region recombine with the electrons injected into the P2 region from the N2 region through the J3 junction. After avalanche breakdown, the electrons entering the N1 area and the holes entering the P2 area cannot all recombine. In this way, electrons accumulate in the N1 area and holes accumulate in the P2 area. As a result, the potential in the P2 area rises, the potential in the N1 area drops, and the J2 junction becomes forward biased. As long as the current increases slightly, the voltage will be rapid. Decrease, the so-called negative resistance characteristic appears, as shown in the dashed line AB in Figure 1. At this time, the three junctions of J1, J2, and J3 are all in the forward bias state, and the SCR enters the forward conduction state-the on state. At this time, its characteristics are similar to the normal PN junction forward characteristics, as shown in the BC section in Figure 1.
③ Trigger on: When a forward voltage is added to the gate G (as shown in Figure 2(c)), the holes in the P2 area enter the N2 area due to the positive bias of J3, and the electrons in the N2 area enter the P2 area to form a trigger Current IGT. On the basis of the internal positive feedback effect of SCR (as shown in Figure 1), coupled with the effect of IGT, the SCR is turned on in advance, resulting in the OA segment of the volt-ampere characteristic in Figure 1 shifting to the left. The larger the IGT, the characteristic shifting to the left. Faster.
Figure 2 Forward characteristics, forward characteristics and trigger conduction