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Turn-On of Thyristors


A thyristor is turned on by increasing the anode current. This can be accomplished in the following ways.
  1. Thermals: If the temperature of a thyristor is high, there will be an increase in the number of electron-hole pairs. This would increase the leakage current and the thyristor may be turned on. This type of turn-on may cause thermal runaway and should be avoided.
  2. High Voltage: If the forward anode to cathode voltage VAK is increased beyond the forward breakdown voltage VBO, high enough leakage currents will flow, causing regenerative turn-on. This type of turn-on is destructive and should be avoided.
  3. dv/dt: From the last equation, if the rate or rise of the anode to cathode voltage is high, (for example, when there is a voltage spike), the charging current of the capacitive junctions may be high enough to turn on the thyristor. A high value of charging current may cause damage to the thyristor and must be avoided. Hence, thyristors must be protected against high dv/dt and must be operated within the manufacturer's dv/dt specifications.
  4. Light: If light is allowed to strike the junction of a thyristor, the electron-hole pairs will increase and this may cause the thyristor to be turned on. This is the principle of operation of light activated thyristors (LASCR).Light-fired thyristor used for HVDC transmission Figure: Light-fired thyristor used for HVDC transmission.
  5. Gate Current: The injection of gate current into a forward biased thyristor would turn-on the device. This is the most desirable turn on process. As the gate current is increased, the forward voltage required to turn-on the device decreases. This is shown in figure below.
The injection of gate current into a forward biased thyristor would turn-on the device
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