There are two conditions for the thyristor to conduct. One is to apply a forward voltage between the anode and the cathode, and the other is to apply a trigger voltage to the control pole. The thyristor can be turned on when the two conditions are met at the same time. According to the description of the subject, the thyristor is not simply two diodes connected in series. Although it looks like two PN junctions are connected in series from the structural diagram, the unidirectional thyristor is actually composed of three PN junctions. For the details of the four-layer semiconductor, please see below.

SCR principle

The internal structure of the thyristor is composed of three PN junctions, the first is the uppermost PN junction, the second is the middle reverse PN junction NP, and the third is the lowermost PN junction, and the control pole It is drawn in the P-type region of the third layer, and the purpose is to provide a bias voltage to the second PN junction. After it is turned on, a forward voltage is applied to the anode and cathode at both ends of the thyristor to conduct the entire The overall structure is equivalent to the connection of two transistors of different tube types, as shown in the figure below, instead of two diodes connected in series.

The thyristor conduction requires two conditions: 1. A positive voltage is applied between the anode and the cathode. 2. Simultaneously add a trigger voltage to the control pole. Two conditions are indispensable for the SCR to be in a conducting state, but once it is turned on, even if the control electrode voltage is removed, as long as there is a positive voltage between the anode and the cathode, the SCR is still conducting. That is to say, the control electrode loses its function after the SCR is turned on.

So how to turn off the thyristor after it is turned on? There are also two conditions for turning off the thyristor: 1. Make the forward conduction current of the thyristor lower than its working maintenance current, and turn off the power supply. 2. Reverse the voltage between the anode and the cathode. As long as one of the two conditions is met, for example, after the alternating current passes through the unidirectional thyristor, after the AC voltage crosses the zero point, the anode and the cathode of the thyristor will be closed due to the reverse voltage. During the reverse period, Even if a trigger signal is applied to the gate, the SCR will not be turned on. Therefore, the one-way SCR also has a rectifying effect.

To sum up, the one-way SCR is equivalent to two triodes connected together, and will never be equivalent to two diodes pieced together, and the two-way SCR is equivalent to two one-way SCRs connected together in reverse. The principle is similar to that of one-way thyristor, but it can be turned on in both positive and negative directions.

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