There are five different methods used to turn-on a thyristor which are listed below.
- Gate Triggering
- Forward Voltage Triggering
- Temperature or Thermal Triggering
- Light or Radiation Triggering
- dv/dt Triggering
Gate Triggering
Gate triggering method is the most efficient, simple, reliable and common method to turn on a thyristor. A positive gate signal is used to turn-on the thyristor in this method even before the specific breakdown voltage. When the thyristor is forward biased, sufficient positive gate signal is applied between the gate and cathode terminals which injects electrons into the junction j2 resulting increases in reverse leakage current causing breakdown at the junction j2.
The conduction period of thyristor can be controlled by varying the gate current within the minimum and maximum values. There are three types of signal which can be used for gate triggering that are dc signal, pulse signal and ac signal.
Forward Voltage Triggering
When anode to cathode forward voltage is increased with gate circuit open, then the reverse biased junction j2 will have an avalanche breakdown at certain forward voltage called forward breakdown voltage. Then, the thyristor changes its states from off state to on state with sharply increase in forward current from anode to cathode. Now, the forward voltage drop across the thyristor increase with the load current passing through it.
This method is not practically used because here needs large anode to cathode voltage and once thyristor turns-on it instantly draws large current which can damage the thyristor. Hence, this method is mostly avoided to practice.
Temperature or Thermal Triggering
The reverse leakage current depends upon the temperature as the number of hole-electron pairs increases with the increase in temperature which increases the leakage current and current gain of the thyristor. The width of the depletion layer decreases with the increase in temperature at the junction of the thyristor. Hence, when the forward voltage across the anode and cathode of thyristor is near to forward breakdown value then, the thyristor can changes its states from off state to on state and the thyristor starts conducting.
Light or Radiation triggering
dv / dt Triggering
In forward blocking state the junctions j1 and j3 are forward biased and junction j2 is reverse biased. This reverse bias junction has characteristics of a capacitor i.e. the junction j2 behaves as a capacitor due to the charge existing across the junction. Here, junction j1 and j3 act as the conducting plates with the depletion layer of j2 as dielectric due to space charge in depletion region. Now, the charging current of capacitor is given by
Ic = dQ / dt
where, Q = CjV
Ic = d(CjV) / dt
where, junction capacitance (Cj) is constant
Ic = Cj * dV / dt
From the above equation, it is seen that if the rate of change in applied voltage is large then, the flowing current is increased which turns-on the thyristor. This method is also practically avoided because it can result sometimes false turn on and also high voltage spikes across the thyristor which may damage the thyristor.