Structure and working principle of thyristor

Thyristors are Crystal Short for thyratron , Also known as Thyristor Rectifier tube （SCR）, Previously referred to as thyristor . Thyristor can control its conduction through signal , But it cannot be controlled to turn off , So it is called semi controlled device . The name of thyristor often refers to a basic type of thyristor - ordinary thyristor . But in a broad sense , Thyristors also include many derivative devices , Such as bidirectional thyristor （TRIAC）、 Fast thyristor （FST）、 Reverse conducting thyristor （RCT） And optically controlled thyristors （LTT） etc. .

One 、 Structure of thyristor

 

At present, the common shape structures of high-power thyristors are bolt type and flat plate , It has three PN junction Four story structure of , Its shape 、 The structure and graphics The symbol is shown in the figure .

Inside the thyristor is PNPN Four layer semiconductor structure , Named as P1、N1、P2、N2 Four districts . From the outermost P Layer and the N Two electrodes from the layer , They are anodes A And cathode K, From the middle P2 The electrode from the layer is the gate G（ Also known as control pole ）. Four zones form J1、J2、J3 Three PN junction . therefore , Three thyristors can be used PN The knot is connected in series to be equivalent , Pictured 8-2 The international common name of the thyristor shown is Thyristor, Shorthand for VT.

Thyristors are power electronic devices , In the working process, it will be heated due to loss , Therefore, the radiator must be installed . For bolted packages , Usually the bolt is its anode , It is made into a bolt shape in order to be closely connected with the radiator and easy to install , Through the anode （ Bolt ） Screw on the aluminum radiator , For natural cooling ; Flat thyristor is clamped by two mutually insulated radiators , Cooled by cold air .

The flat type has good heat dissipation effect on both sides , rated electric current Greater than 200A All thyristors adopt flat plate structure . In addition, it can also be cooled by water 、 Oil cooling and other cooling methods .

Two 、 Working principle of thyristor

As shown in the figure circuit Do a simple experiment , Explain the working principle of thyristor . By the power supply Us、 Incandescent lamp 、 Anode of thyristor 、 The cathode forms the main circuit of thyristor . Power Supply Uc、 switch S、 The control circuit composed of thyristor gate and cathode is also called trigger circuit .

When the anode of the thyristor A Connect the power supply Us Positive end of . The cathode K Connect the negative end of the power supply through the incandescent lamp , At this time, the thyristor bears the forward voltage . When the switch in the control circuit S When disconnected , The incandescent lamp doesn't work , It means that the thyristor is not conductive .

When the anode and cathode of the thyristor bear positive voltage , Switch in control circuit S closed , Make the control pole also apply forward voltage ,（ The voltage between the control pole and the cathode ）. At this time, the incandescent lamp is on , It indicates that the thyristor is on .

When the quality control tube is connected , Remove the voltage on the control pole （ About to open a beautiful S To break off ）, The incandescent lamp is still on . Explain that once the thyristor is turned on , The control pole loses control .

When a reverse voltage is applied between the anode and cathode of the thyristor , Whether the control pole is applied with or without voltage , The lights are not on , At this time, the thyristor is cut off . If the control pole is applied with reverse voltage , Whether the main circuit of the quality control is applied with forward voltage or reverse voltage , Thyristors are not conductive .

The following conclusions are drawn from the above experimental results .

（1） To make the thyristor turn on, two conditions must be met at the same time ：

① Anode of thyristor A And cathode K Add forward voltage between ,② The gate pole of the quality control G And cathode K Appropriate forward voltage and current are also added between .

（2） Once the thyristor is turned on , The gate loses control , Therefore, the thyristor is a semi controlled device .

（3） To turn off the thyristor , It must be anodised A The current drops to zero or below a certain value , This can be achieved by placing the anode A Reduce the voltage to zero or apply a reverse voltage to the anode .