Engineering Tutorial

SCR Triggering Methods

Triggering (Turn on) Methods of Thyristor:

Triggering:-
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.

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(V_AK) , 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 V_AK is further increased, at a voltage V_BO (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 V_AR 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,
i_c = dQ/dt
Q = CV
i_c = d(CV) / dt
= C. dV/dt + V. dC/dt
as dC/dt = 0

i_c = 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:-

This is most widely used SCR triggering method.
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.
There are two methods of AC voltage triggering namely (i) R Triggering (ii) RC triggering

(i) Resistance triggering:
The following circuit shows the resistance triggering.

In this method, the variable resistance R is used to control the gate current.
Depending upon the value of R, when the magnitude of the gate current reaches the sufficient value(latching current of the device) the SCR starts to conduct.
The diode D is called as blocking diode. It prevents the gate cathode junction from getting damaged in the negative half cycle.
By considering that the gate circuit is purely resistive, the gate current is in phase with the applied voltage.
By using this method we can achieve maximum firing angle up to 90°.

(ii) RC Triggering
The following circuit shows the resistance-capacitance triggering.

By using this method we can achieve firing angle more than 90°.
In the positive half cycle, the capacitor is charged through the variable resistance R up to the peak value of the applied voltage.
The variable resistor R controls the charging time of the capacitor.
Depends upon the voltage across the capacitor, when sufficient amount of gate current will flow in the circuit, the SCR starts to conduct.
In the negative half cycle, the capacitor C is charged up to the negative peak value through the diode D2.
Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half cycle.

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.

Advantages of pulse train triggering:

Low gate dissipation at higher gate current.
Small gate isolating pulse transformer
Low dissipation in reverse biased condition is possible.So simple trigger circuits are possible in some cases
When the first trigger pulse fails to trigger the SCR, the following pulses can succeed in latching SCR. This important while
Triggering inductive circuits and circuits having back emf’s.

Also Read: Turning OFF SCR – Commutation