How thyristors work

In the intermediate frequency furnace, the rectifier side turn-off time is within KP-60 microseconds, and the inverter side turn-off time is within KK-30 microseconds. This is also the main difference between KP tubes and KK tubes. During the working process of thyristor T, its anode A and cathode K are connected to the power supply and load to form the main circuit of the thyristor. The gate G and cathode K of the thyristor are connected to the device that controls the thyristor to form the control circuit of the thyristor. Working conditions of thyristor: 1. When the thyristor is subjected to reverse anode voltage, the thyristor is in a short-off state regardless of the gate voltage. 2. When the thyristor is subjected to a forward anode voltage, the thyristor will only conduct when the gate is subjected to a forward voltage. 3. When the thyristor is turned on, as long as there is a certain forward anode voltage, the thyristor will remain turned on regardless of the gate voltage. That is, after the thyristor is turned on, the gate will lose its function. 4. When the thyristor is on, when the main circuit voltage (or current) decreases to close to zero, the thyristor turns off. Analyze the working process from the inside of the thyristor: The thyristor is a four-layer three-terminal device. It has three PN junctions J1, J2, and J3 (Figure 1). The NP in the middle can be divided into two parts to form a PNP type transistor and an NPN type transistor. Composite tube Figure 2 When the thyristor is subjected to the forward anode voltage, in order for the thyristor to conduct copper, the PN junction J2, which is subject to the reverse voltage, must lose its blocking effect. The collector current of each transistor in Figure 2 is simultaneously the base current of the other transistor. Therefore, when enough gate current Ig flows into the two transistor circuits that are combined with each other, a strong positive feedback will be formed, causing the two transistors to be saturated and conductive, and the transistor to be saturated and conductive. Assume that the collector currents of the PNP tube and the NPN tube are Ic1 and Ic2; the emitter currents are Ia and Ik; the current amplification coefficients are a1=Ic1/Ia and a2=Ic2/Ik. Assume that the reverse phase flowing through the J2 junction The leakage current is Ic0, and the anode current of the thyristor is equal to the sum of the collector current and leakage current of the two tubes: Ia=Ic1+Ic2+Ic0 or Ia=a1Ia+a2Ik+Ic0. If the gate current is Ig, then the cathode current of the thyristor is Ik =Ia+Ig It can be concluded that the anode current of the thyristor is: I=(Ic0+Iga2)/(1-(a1+a2)) (1-1) The corresponding current amplification coefficients a1 and silicon NPN tubes of the silicon PNP tube and silicon NPN tube a2 changes sharply as its emitter current changes, as shown in Figure 3. When the thyristor is subjected to the forward anode voltage but the gate is not subject to voltage, in formula (1-1), Ig=0, (a1+a2) is very small, so the anode current of the thyristor Ia≈Ic0, the thyristor switch is at Forward blocking state. When the thyristor is under the forward anode voltage, the current Ig flows from the gate G. Since a large enough Ig flows through the emitter junction of the NPN tube, the starting point current amplification coefficient a2 is increased, and a sufficiently large electrode current Ic2 flows through the PNP tube. The emitter junction of the PNP tube is increased, and the current amplification factor a1 of the PNP tube is increased, resulting in a larger electrode current Ic1 flowing through the emitter junction of the NPN tube. Such a strong positive feedback process proceeds rapidly. From Figure 3, when a1 and a2 increase with the emitter current and (a1+a2) ≈ 1, the denominator 1-(a1 + a2) ≈ 0 in the formula (1-1), thus increasing the anode current Ia of the thyristor. .At this time, the current flowing through the thyristor is completely determined by the voltage of the main circuit and the circuit resistance. The thyristor is already in forward conducting state. In formula (1-1), after the thyristor is turned on, 1-(a1+a2)≈0. Even if the gate current Ig=0 at this time, the thyristor can still maintain the original anode current Ia and continue to turn on. After the thyristor is turned on, the gate has lost its function. After the thyristor is turned on, if the power supply voltage is continuously reduced or the loop resistance is increased, and the anode current Ia is reduced to below the holding current IH, due to the rapid decline of a1 and a1, when 1-(a1+a2)≈0 , the thyristor returns to the blocking state.