Description
Thyristor is also a silicon controllable element, which is a high-power semiconductor device composed of three PN structures, and is mostly used for controlled rectifier, inverter, voltage regulation and other circuits, and also as a contactless switch. Thyristors can be divided into unidirectional thyristors and bidirectional thyristors as well as gate level turn-off thyristors (GTO), and their symbols are shown below:
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1.1. One-way thyristor
When the positive voltage is applied between the anode A and cathode C of unidirectional thyrior, and the control pole G does not apply voltage, the corresponding PN junction in the middle is in reverse bias (the direction of the electric field and the direction of the PN junction are opposite) – J2 is reverse biased, the tube is not conductive, and the group is in a blocked state.
When A positive voltage is applied between anode A and cathode C, and a positive voltage is also applied between the control pole G and cathode C, as shown in the figure below:
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By equivalent circuit analysis, the T2 tube is open. If the base current of the T2 tube is iB2
The collector current is β2iB2
; Further, iB1=iC2=β2iB2
Thus iC1=β1iB1=β1β2iB2
And since iB2=iC1
The above amplification process is carried out again to form a positive feedback. Because in a very short time (no more than a few microseconds) both tubes enter a saturated state, the transistor is fully switched on. Once the transistor is switched on, the control pole loses its function, and the tube always maintains its on-state by relying on internal positive feedback (this is because after the transistor is switched on, the base of BG2 always has the trigger current of the BG1 collector, so the control pole only “triggers” the action when the trigger signal (voltage) is just connected). After unidirectional thyristor is switched on, the voltage between the anode and cathode is generally 0.6 to 1.2V.
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