Silicon Controlled Rectifier (SCR) is a high-power electrical component, also known as a thyristor. It has the advantages of small size, high efficiency and long life. In the automatic control system, it can be used as a high power driving device to realize the control of high power equipment with low power control. It has been widely used in AC/DC motor speed regulating system, power regulating system and servo system.
Thyristor is divided into two kinds: one-way thyristor and two-way thyristor. Bidirectional thyristor is also called TRIAC, or Triac for short. Bidirectional thyristor is equivalent in structure to two one-way thyristor reverse connection, this thyristor has the function of double guide pass. Its on-off state is determined by the control pole G. Positive (or negative) pulse is added to the control pole G to make it positive (or reverse) conduction. The advantage of this device is that the control circuit is simple, no reverse voltage problem, so it is particularly suitable for AC contactless switch use.
We use the unidirectional thyristor, also known as the common thyristor, which is composed of four layers of semiconductor material, with three PN junction, external three electrodes (Figure 2(a)) : the first layer of P-type semiconductor led electrode called anode A, the third layer of P-type semiconductor led electrode called control pole G, the fourth layer of N-type semiconductor led electrode called cathode K. As can be seen from the circuit symbol of the thyristor (FIG. 2(b)), it is a single-direction conducting device like the diode. The key is that it has a control pole G, which makes it have completely different operating characteristics from the diode.
Based on silicon single crystal as the basic material of P1N1P2N2 four-layer three-terminal device, started in 1957, because of its characteristics similar to vacuum thyristor, so the international known as silicon thyristor, thyristor T for short, and because the thyristor initially in static rectifier, so also known as silicon controlled rectifier element, referred to as SCR.
In terms of performance, thyristor not only has unidirectional conductivity, but also has more valuable controllability than silicon rectifier element (commonly known as “dead silicon “). It can only be on and off.
Thyristor can control high-power electromechanical equipment with milliampere-level current. If the power exceeds this level, the average current phase allowed through will decrease due to the significant increase in component switching loss. At this time, the nominal current should be degraded.
Thyristor has many advantages, such as: small power control high power, power amplification up to hundreds of thousands of times; The response is very fast, in the microsecond level of opening and closing; No contact operation, no spark, no noise; High efficiency, low cost and so on.
The weakness of thyristor: static and dynamic overload ability is poor; Susceptible to interference and misleading.
Thyristor from the appearance of the main classification: bolt shape, plate shape and bottom shape.
Structure of thyristor element
Regardless of the thyristor’s appearance, their cores are a four-layer P1N1P2N2 structure made up of P-type and N-type silicon. See Figure 1. It has three PN junctions (J1, J2, J3), leading from the P1 layer of the J1 structure to the anode A, from the N2 layer to the negative stage K, from the P2 layer to the control pole G, so it is a four-layer three-terminal semiconductor device.
