The internal structure of the thyristor

The internal structure of the thyristor

The internal structure of the thyristor is shown in Figure 1. The die is composed of P1N1P2N2 four-layer semiconductors, forming three PN junctions (J1, J2, J3). When reverse voltage is applied to the anode and cathode of the tube, J1 and J3 are in the reverse blocking state; when the forward voltage is applied, J2 is in the reverse blocking state, and the tube still does not conduct. If a forward current IG is added between the gate and the cathode at this time, the thyristor will be forward conducting like a diode. It can be seen that the thyristor has unidirectional conductivity characteristics. Unlike diodes, thyristors have forward blocking characteristics. When the positive anode voltage is applied, the tube cannot be turned on, and the gate voltage must be added at the same time. After enough gate current flows in, the thyristor can be turned on. Therefore, the thyristor has the controllable characteristic of forward conduction, and this kind of device that uses current input to control conduction is called a current-driven device.

The internal structure of the thyristor
Figure 1 – Internal structure, dual transistor model and equivalent circuit of thyristor

The process in which the gate electrode current IG is turned on by the thyristor is called triggering. Once the thyristor is triggered and turned on, the gate will lose its control. This kind of gate can trigger its conduction but cannot turn it off. The device is called a semi-controlled device.

To restore the blocking of the thyristor that has been turned on, the anode power supply voltage can be reduced or the anode loop resistance can be increased to reduce the cathode current IA flowing through the tube. When IA is reduced to a certain value (usually tens of milliamps), IA will suddenly drop to zero, and then even if the voltage is increased or the resistance is reduced, the current will not increase, indicating that the tube has resumed positive blocking; when the gate is off, the minimum anode current required to maintain the tube conduction is called the maintenance current IH, so the condition for the tube to turn off is IA<IH.

Now further analyze the above-mentioned characteristics of the thyristor from the internal structure. The thyristor is composed of four layers of semiconductors alternately stacked, which can be equivalent to two transistors V1 (P1-N1-P2) and V2 (N1-P2-N2), as shown in Figure 1b.

When the positive voltage is applied to the anode of the tube, the key to make the tube forward is to make the J2 junction lose its blocking effect in the reverse direction. It can be seen from Figure 1c that when S is turned on, the collector current IC1 of the V1 tube is the base current IB2 of the V2 tube; the collector current IC2 of the V2 tube is again the base current IB1 of the V1 tube. When S is closed, enough gate current IG flows in, and the current amplification through the two tubes immediately forms a strong positive feedback. The process is as follows:

The internal structure of the thyristor
(1-1)

The two transistors are saturated and turned on instantaneously, that is, the thyristor is turned on.

Suppose the current amplification factors of the common base connection of the V1 and V2 tubes are α1 and α2 respectively. According to the working principle of the transistor, the following equations can be listed:

IC1=α1IA+ICBO1                         (1-2)

IC2=α2IK+ICBO2                         (1-3)

  IK=IA+IG                              (1-4)

  IA=IC1+IC2                           (1-5)

In the formula, ICBO1 and ICBO2 are the common base leakage current of V1 and V2 respectively.

From formula (1-2) ~ formula (1-5), we can get

The internal structure of the thyristor
(1-6)

Knowing from the knowledge of transistors, the common base current amplification factor α gradually increases as the emitter current increases. When IG increases to a certain value, the emitter current of the two transistors also increases accordingly, so that when (α12) increases to close to 1, the tube anode current IA in formula (1-6) will increase dramatically and become uncontrollable. At this time, the IA value is determined by the power supply voltage EA and the load resistance R. Since the power supply voltage EA generally cannot be changed, the turn-off of the thyristor is determined by the load current, and the forward voltage drop of the thyristor is about 1V. Due to the effect of positive feedback, even if the gate current of the turned-on tube drops to zero or negative, the tube cannot be turned off. Only trying to reduce the anode current IA of the tube below the maintenance current IH. At this time, when α1 and α2 are also reduced correspondingly, the internal positive feedback cannot be maintained, and the thyristor resumes blocking.