Power Electronics Circuits
In a thyristor phase-control circuit connected to an AC supply, which of the following correctly explains why the thyristor turns off without any action to turn it off from the gate?
- AThe supply voltage reverses every half cycle, and the thyristor turns off naturally when its current falls below the holding current
- BRemoving the gate signal stops the anode current at that instant, and the thyristor stays off until the gate signal is applied again
- CAfter a fixed time an internal timer in the thyristor turns it off
Show answer and explanation
Answer: A. The supply voltage reverses every half cycle, and the thyristor turns off naturally when its current falls below the holding current
Once a thyristor conducts, it keeps conducting even after the gate signal is removed and does not turn off until its current falls below the holding current. In an AC circuit the supply voltage reverses every half cycle and the current falls to zero each time, so the thyristor turns off naturally. This is called line (natural) commutation, and it is why thyristors that cannot be turned off from the gate can still be used for AC phase control. Turning a thyristor off in a DC circuit requires a commutation circuit that applies a reverse voltage, which is why devices that can be turned off from the gate, such as GTOs and IGBTs, later came into use.
- A. Correct. It relies on the AC current falling to zero, so there is no need to turn it off from the gate.
- B. The thyristor keeps conducting even when the gate signal is removed. Devices such as GTO thyristors can be turned off from the gate.
- C. There is no timer inside a thyristor. It turns off when its current falls below the holding current.