Schematic diagram of thyristor (thyristor) and analysis of working principle of thyristor

During the working process of the thyristor T, its anode A and cathode K are connected with the power source and the load to form a main circuit of the thyristor, and the gate G and the cathode K of the thyristor are connected with the device for controlling the thyristor. Control circuit for thyristors.

From the internal analysis of the thyristor work process:

The thyristor is a four-layer three-terminal device, which has three PN junctions J1, J2, and J3. The NP in the middle can be divided into two parts to form a composite tube of a PNP type triode and an NPN type triode.

When the thyristor is subjected to the forward anode voltage, in order for the thyristor to conduct copper, the PN junction J2 subjected to the reverse voltage must be prevented from blocking. The collector current of each transistor in Figure 2 is the base current of the other transistor. Therefore, when two mutually complex transistor circuits have sufficient gate current Ig flowing in, a strong positive feedback is formed, causing the two transistors to be saturated and the transistor is saturated.

Let the collector currents of PNP and NPN be corresponding to Ic1 and Ic2; the emitter currents should be Ia and Ik respectively; the current amplification factors should be a1=Ic1/Ia and a2=Ic2/Ik, set to reverse the J2 junction. The leakage current is Ic0,

The anode current of the thyristor is equal to the sum of the collector current and the leakage current of the two tubes:

Ia=Ic1+Ic2+Ic0 or Ia=a1Ia+a2Ik+Ic0

If the gate current is Ig, the thyristor cathode current is Ik=Ia+Ig

It can be concluded that the thyristor anode current is: I=(Ic0+Iga2)/(1-(a1+a2))(1-1)

The corresponding current amplification factors a1 and a2 of the silicon PNP tube and the silicon NPN tube vary abruptly as the emitter current changes as shown in FIG.

When the thyristor is subjected to the forward anode voltage and the gate is not subjected to the voltage, in the formula (1-1), Ig=0, (a1+a2) is small, so the anode current Ia≈Ic0 of the thyristor The gatekeeper is in a positive blocking state. When the thyristor flows into the current Ig from the gate G at the forward anode voltage, since the sufficiently large Ig flows through the emitter junction of the NPN tube, the starting point amplification factor a2 is increased, and a sufficiently large pole electrode current Ic2 flows. The emitter junction of the PNP tube increases the current amplification factor a1 of the PNP tube, resulting in a larger pole electrode current Ic1 flowing through the emitter junction of the NPN tube. This strong positive feedback process proceeds quickly. From Fig. 3, when a1 and a2 increase with the emitter current and (a1+a2) ≈1, the denominator 1-(a1+a2) ≈0 in equation (1-1) increases the anode of the thyristor. Current Ia. At this point, the current flowing through the thyristor is completely determined by the voltage and loop resistance of the main circuit. The thyristor is already in a forward conduction state.

In the formula (1-1), after the thyristor is turned on, 1-(a1+a2)≈0, even if the gate current Ig=0, the thyristor can maintain the original anode current Ia and continue to conduct. through. After the thyristor is turned on, the gate has lost its effect.

After the thyristor is turned on, if the power supply voltage is continuously reduced or the loop resistance is increased to reduce the anode current Ia below the sustain current IH, since a1 and a1 decrease rapidly, when 1-(a1+a2)≈ At 0, the thyristor resumes the blocking state.