SCR Triggering Methods


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.

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:-

  • 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.VI Characteristics of SCR
  • 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.Resistance Triggering of SCR

  • 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.RC Triggering of SCR

  • 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

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