Structure and characteristics of controllable silicon

Structure and characteristics of controllable silicon diode

Thyristors are mainly divided into three types in terms of appearance: spiral type, flat type, and flat bottom type. There are many applications of spiral type.

Thyristor has three electrodes - anode (A) cathode (C) and control electrode (G). It has a four layer structure consisting of a P-type conductor and an N-type conductor overlapping, with a total of three PN junctions.

The structure of thyristor and silicon rectifier diode with only one PN junction is completely different. The four layer structure of controllable silicon and the use of control electrodes lay the foundation for its excellent control characteristics of "controlling large with small". When applying 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 thyristors above 50 amperes are called high-power thyristors.

Why does thyristor have the controllability of "controlling large with small"?

Firstly, we can view the first, second, and third layers from the cathode upwards as one NPN type transistor, while the second, third, and fourth layers form another PNP type transistor. The second and third layers are shared by two overlapping pipes. When a forward voltage Ea 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 BG1), BG1 will generate the base current Ib1. After amplification, BG1 will have an amplification β Double the collector current IC1. Because the BG1 collector is connected to the BG2 base, IC1 is the base current Ib2 of BG2. BG2 has magnified it again compared to Ib2 (Ib1) β The collector current IC2 of 2 is sent back to the base of BG1 for amplification. Repeat the amplification process until BG1 and BG2 are fully conductive. In reality, this process is a "trigger triggered" process. For the thyristor, the trigger signal is added to the control electrode, and 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 BG1 base is no longer just the initial Ib1, but the current amplified by BG1 and BG2（ β 1* β 2 * Ib1) This current is much greater than Ib1, which is sufficient to maintain the continuous continuity of BG1. At this point, even if the trigger signal disappears, the thyristor still remains in a conductive state. Only by disconnecting the power supply Ea or reducing Ea, can the thyristor be turned off when the collector current in BG1 and BG2 is less than the minimum value for maintaining conduction. Of course, if Ea polarity is reversed, BG1 and BG2 will be in a cut-off state due to reverse voltage. At this point, even if a trigger signal is input, the thyristor cannot operate. On the contrary, Ea 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), is precisely the important feature that distinguishes it from ordinary silicon rectifier diode.