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Thyristor is a four-layer three-terminal device. It has three PN junctions: J1, J2 and J3. Fig. 1. It can divide the NP in the middle into two parts and form a PNP-type triode and a NPN-type triode compound tube. Fig.
2 When thyristor withstands forward anode voltage, in order to make thyristor turn on, it is necessary to make PN junction J2 withstanding reverse voltage lose its blocking effect. The collector current of each transistor in Figure 2 is the base current of another transistor at the same time. Therefore, when there is enough gate current Ig flowing into the two composite transistor circuits, strong positive feedback will be formed, resulting in saturated conduction of the two transistors and saturated conduction of the transistors.
The collector currents of PNP and NPN are Ic1 and Ic2, emitter currents Ia and Ik, current amplification coefficients A1 = Ic1/Ia and A2 = Ic2/Ik, and reverse leakage current through J2 junction is Ic0.
The anode current of thyristor is equal to the sum of collector current and leakage current of two thyristors.
Ia = Ic1 + Ic2 + Ic0 or Ia = a1Ia + a2Ik + Ic0
If the gate current is Ig, the cathode current of thyristor is Ik=Ia+Ig.
It can be concluded that the anode current of thyristor is I=(Ic0+Iga2)/(1-(a1+a2))(1-1)
The corresponding current amplification coefficients A1 and A2 of silicon PNP and silicon NPN tubes vary sharply with the change of emitter current as shown in Figure 3.
When the thyristor is subjected to positive anode voltage and the gate is not subjected to voltage, in formula (1-1), Ig = 0, (a1+a2) is very small, so the anode current of thyristor Ia_Ic0 thyristor is in a positive blocking state. When thyristor flows current Ig from gate G under forward anode voltage, the current amplification coefficient A2 is increased because enough Ig flows through the emission junction of NPN tube, and the current amplification coefficient A1 of PNP tube is increased, and the current amplification coefficient A1 of PNP tube is increased, and the emission junction of NPN tube is generated because enough large Ig flows through the emission junction of NPN tube. Such a strong positive feedback process proceeds rapidly. From Fig. 3, when A1 and A2 increase with emitter current (a1+a2)1, denominator 1-(a1+a2)0 in formula (1-1) increases the anode current Ia of thyristor. At this time, the current flowing through thyristor is completely determined by the voltage and resistance of main circuit. The thyristor is in the forward conduction state.
In 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 turn on. After the thyristor is turned on, the gate has lost its function.
After the thyristor is turned on, if the supply voltage is continuously reduced or the circuit resistance is increased, the anode current Ia is reduced to below the maintenance current IH. Because A1 and A1 decrease rapidly, when 1-(a1+a2)0, the thyristor restores the blocking state.