Triggering (Turn on) Methods of Thyristor:
The turning on Process of the SCR is known as Triggering. In other words, turning the SCR from Forward-Blocking state to Forward-Conduction state is known as Triggering.The various methods of SCR triggering are discussed here.
It is recommended to read The Basics, Structure and VI characteristics of Power Thyristor before proceeding further.
The various SCR triggering methods are
- Forward Voltage Triggering
- Thermal or Temperature Triggering
- Radiation or Light triggering
- dv/dt Triggering
- Gate Triggering
(a) Forward Voltage Triggering:-
- In this mode, an additional forward voltage is applied between anode and cathode.
- When the anode terminal is positive with respect to cathode(VAK) , Junction J1 and J3 is forward biased and junction J2 is reverse biased.
- No current flows due to depletion region in J2 is reverse biased (except leakage current).
- As VAK is further increased, at a voltage VBO (Forward Break Over Voltage) the junction J2 undergoes avalanche breakdown and so a current flows and the device tends to turn ON(even when gate is open)
(b) Thermal (or) Temperature Triggering:-
- The width of depletion layer of SCR decreases with increase in junction temperature.
- Therefore in SCR when VAR is very near its breakdown voltage, the device is triggered by increasing the junction temperature.
- By increasing the junction temperature the reverse biased junction collapses thus the device starts to conduct.
(c) Radiation Triggering (or) Light Triggering:-
- For light triggered SCRs a special terminal niche is made inside the inner P layer instead of gate terminal.
- When light is allowed to strike this terminal, free charge carriers are generated.
- When intensity of light becomes more than a normal value, the thyristor starts conducting.
- This type of SCRs are called as LASCR
(d) dv/dt Triggering:-
- When the device is forward biased, J1 and J3 are forward biased, J2 is reverse biased.
- Junction J2 behaves as a capacitor, due to the charges existing across the junction.
- If voltage across the device is V, the charge by Q and capacitance by C then,
ic = dQ/dt
Q = CV
ic = d(CV) / dt
= C. dV/dt + V. dC/dt
as dC/dt = 0
ic = C.dV/dt
- Therefore when the rate of change of voltage across the device becomes large, the device may turn ON, even if the voltage across the device is small.
(e) Gate Triggering:-
- Applying a positive voltage between gate and cathode can Turn ON a forward biased thyristor.
- When a positive voltage is applied at the gate terminal, charge carriers are injected in the inner P-layer, thereby reducing the depletion layer thickness.
- As the applied voltage increases, the carrier injection increases, therefore the voltage at which forward break-over occurs decreases.
- Three types of signals are used for gate triggering.
1. DC gate triggering:-
- A DC voltage of proper polarity is applied between gate and cathode ( Gate terminal is positive with respect to Cathode).
- When applied voltage is sufficient to produce the required gate Current, the device starts conducting.
- One drawback of this scheme is that both power and control circuits are DC and there is no isolation between the two.
- Another disadvantages is that a continuous DC signal has to be applied. So gate power loss is high.
2. AC Gate Triggering:-
- Here AC source is used for gate signals.
- This scheme provides proper isolation between power and control circuit.
- Drawback of this scheme is that a separate transformer is required to step down ac supply.
3. Pulse Gate Triggering:-
- In this method the gate drive consists of a single pulse appearing periodically (or) a sequence of high frequency pulses.
- This is known as carrier frequency gating.
- A pulse transformer is used for isolation.
- The main advantage is that there is no need of applying continuous signals, so the gate losses are reduced.