Q2Power Electronics
Question
2 marks
Name the methods to turn on the thyristor.
Answer
A thyristor can be turned on by: forward voltage breakover, gate triggering (most common), thermal triggering, light triggering, and dv/dt triggering.
- Gate Triggering (most common and controllable): A positive current pulse applied to the gate terminal when VAK > 0 turns the SCR on. This is the standard method used in power electronics circuits.
- Forward Voltage Breakover: If the forward anode-cathode voltage exceeds the breakover voltage VBO, the SCR switches on. This is generally avoided in practice as it can damage the device.
- dv/dt Triggering: A rapidly rising forward voltage causes displacement current through the junction capacitance; if large enough, it can trigger the SCR. Snubber circuits prevent this.
- Thermal Triggering: At high temperatures, thermally generated carriers increase leakage current, which can trigger the SCR. Avoided by keeping TJ within limits.
- Light Triggering: In LASCRs (Light-Activated SCRs), incident light on the junction generates carriers that trigger the device. Used in HVDC systems for isolation.