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.