Thyristor working principle
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 to form a control circuit of the thyristor.
Thyristor working conditions:
1. When the thyristor is subjected to the forward anode voltage, the thyristor is turned on only when the gate is subjected to the forward voltage. At this time, the thyristor is in a forward conduction state, which is the thyristor thyristor characteristic, which can be controlled.
2. When the thyristor is turned on, as long as there is a certain positive anode voltage, regardless of the gate voltage, the thyristor remains turned on, that is, after the thyristor is turned on, the gate loses its effect. The gate only acts as a trigger
3. When the thyristor is turned on, the thyristor turns off when the main loop voltage (or current) decreases to near zero.
4. When the thyristor is subjected to the reverse anode voltage, the thyristor is in the reverse blocking state regardless of the voltage applied to the gate.
In the intermediate frequency furnace, the rectification side turn-off time is within KP-60 microseconds, and the inverter side off time is within KK-30 microseconds. This is also the main difference between the KP tube and the KK tube. The thyristor T is in operation, its anode A and cathode K are connected to 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 to the device for controlling the thyristor to form a control circuit of the thyristor.
From the internal analysis of the thyristor: The thyristor is a four-layer three-terminal device with three PN junctions J1, J2, and J3. The NP in the middle can be divided into two parts to form a PNP transistor and an NPN transistor. Composite tube Figure 2 When the thyristor is subjected to a positive 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 the figure is also 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 transistors are saturated. Let the collector currents of the PNP and NPN tubes be Ic1 and Ic2 respectively; the emitter currents should be Ia and Ik respectively; the current amplification factors should be a1=Ic1/Ia and a2=Ic2/Ik, and set the inversion of 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 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) silicon PNP tube and silicon NPN tube corresponding to the current amplification factors a1 and a2 with their emitter current Change and change dramatically.
When the thyristor is subjected to the forward anode voltage and the gate is not subjected to voltage, in the formula (1-1), Ig=0, (a1 a2) is small, so the thyristor anode current Ia≈Ic0 is closed. To the blocking state. When the thyristor is at the forward anode voltage, the current Ig flows from the gate G, and since the Ig flows through the emitter junction of the NPN tube, the starting point amplification factor a2 is increased, and a sufficiently large electrode current Ic2 flows through the PNP tube. The emitter junction and the current amplification factor a1 of the PNP tube are increased, resulting in a larger pole electrode current Ic1 flowing through the emitter junction of the NPN tube.
This intense positive feedback process proceeds quickly.
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 anodic current Ia of the thyristor. The thyristor current is completely determined by the voltage and loop resistance of the main circuit. The thyristor is already in a forward conduction state. In the equation (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 maintain the original anode current Ia and continue to conduct.
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, so that the anode current Ia is reduced below the sustain current IH, since a1 and a1 decrease rapidly, when 1-(a1 a2) ≈ 0, The thyristor resumes its blocking state.
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