Thyristor is the abbreviation of thyristor rectifier element. It is a high-power semiconductor device with three PN junctions and four layers. It is generally formed by the reverse connection of two thyristors. Its function is not only rectification, but also can be used as fast on or off of contactless switch; Realize the inversion of converting DC into AC; Change AC of one frequency into AC of another frequency, etc. Like other semiconductor devices, thyristor has the advantages of small volume, high efficiency, good stability and reliable operation. With its appearance, semiconductor technology has entered the field of strong current from the field of weak current, and has become a competitive component in industry, agriculture, transportation, military scientific research, as well as commercial and civil electrical appliances. At present, thyristors are widely used in automatic control, electromechanical applications, industrial electrical and household appliances.
There are three types of thyristors: spiral type, flat type and flat bottom type. Spiral type is widely used.
The thyristor has three poles -- anode (a), cathode (c) and control pole (g). The tube core is a four layer structure composed of overlapping p-type conductors and n-type conductors. It has three PN junctions, which is very different from the silicon rectifier diode with only one PN junction. The four layer structure of thyristor and the introduction of control electrode have laid a foundation for its excellent control characteristics of "controlling the large with the small". When thyristor is applied, 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, thyristor elements with current capacity of hundreds of amperes to thousands of amperes can be manufactured. Generally, thyristors below 5 amps are called low-power thyristors, and thyristors above 50 amps are called high-power thyristors.
We can see that the first, second and third layers from the cathode upward are an NPN transistor, and the second, third and fourth layers form another PNP transistor. The second and third layers are overlapped and shared by two tubes. The equivalent circuit diagram of Fig. 1 can be drawn. When a forward voltage e is added between the anode and cathode, and a positive trigger signal is input between the control electrode G and the cathode C (equivalent to the base emitter room of BG2), BG2 will generate the base current IB2. After amplification, BG2 will have an amplification β 2 times the collector current IC2. Because the BG2 collector is connected to the BG1 base, IC2 is the base current IB1 of BG1. BG1 magnifies IB1 (IB2) again β The collector current IC1 of BG1 is sent back to the base of BG2 for amplification. Repeat the amplification until BG1 and BG2 are completely turned on. In fact, this process is "hair trigger". For the thyristor, the trigger signal is added to the control pole and the thyristor is turned on immediately. The conduction time mainly depends on the performance of thyristor.
Once the thyristor is triggered and turned on, due to the cyclic feedback, the current flowing into the BG2 base is not only the initial IB2, but the current amplified by BG1 and BG2（ β 1* β 2 * IB2), which is much larger than IB2, enough to maintain the continuous conduction of BG2. At this time, even if the trigger signal disappears, the thyristor remains on. The thyristor can be turned off only when the power supply e is disconnected or the output voltage of E is reduced so that the collector current of BG1 and BG2 is less than the minimum value of maintaining on. Of course, if the e polarity is reversed, BG1 and BG2 will be cut off under the action of reverse voltage. At this time, even if the trigger signal is input, the thyristor cannot work. Conversely, e is connected in a positive direction, while the trigger signal is negative, and the thyristor cannot be turned on. In addition, if no trigger signal is added and the forward anode voltage exceeds a certain value, the thyristor will also be turned on, but it is an abnormal working condition.
Thyristor, which controls the conduction (large current in thyristor) through trigger signal (small trigger current), is an important feature different from ordinary silicon rectifier diode.