In this script, we will discuss  Power Electronics, In this blog, we will discuss the common turning-on method of thyristors and Mathematical terms.

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The common turning-on is the method of thyristors.

• Light triggering 

• Gate triggering

 • Forward voltage triggering

 • Temperature triggering 

• dV/dT triggering 


Light Triggering
  • If the light is allowed to strike the junctions of a thyristor, the electron-hole pairs increase, and the thyristor may be turned on.
  • The light-activated thyristor is turned on by allowing light to strike the silicon wafers.
Gate Triggering
  • In the gate triggering method, a positive voltage is applied between the gate and the cathode terminals. As the gate current is increased, the forward blocking voltage is decreased.
  • When the thyristor is forward biased, and a gate signal is injected by applying positive gate voltage is applied between gate and cathode terminals, then the thyristor is turned on.

Forward Voltage Triggering 

  • By increasing the forward anode to cathode voltage, the depletion layer width is also increased at junction J2. 
  • Which causes an increase in minority charge carriers.
  • This further leads to an avalanche breakdown of the junction J2 at a forward break-over voltage VBO.
  • In practice, this method is not employed because it needs a very large anode to cathode voltage.
Temperature Triggering 
  • The reverse leakage current depends on the temperature.
  • If the temperature is increased to a certain value, the number of hole-pairs also increases.
  • This starts the regenerative action inside the thyristors since the (α1 + α2) value approaches unity.
  • This type of triggering is practically not employed because it causes thermal runaway and hence the device or thyristors may be damaged.
dv/dt Triggering

  •  In forwarding blocking state junctions, J1 and J3 are forward biased and J2 is reverse biased. 
  •  So the junction J2 behaves as a capacitor due to the space charges in the depletion region. The charging current of the capacitor is given as                              I = C dv/ dt 
  • dv/dt is the rate of change of applied voltage and C is the junction capacitance.
  • From the above equation, if the rate of change of the applied voltage is large that leads to an increase in the charging current which is enough to increase the value of alpha.
  • So the thyristors evolve turned ON without a gate signal.

Mathematical terms 

Problem: 1
The capacitance of reverse-biased junction J2 in a thyristor is CJ2 =20 pF and can be assumed to be independent of the off-state voltage. The limiting value of the charging current to turn on the thyristor is 16 mA. Determine the critical value of dv/dt.

Solution:
Given, 
CJ2 = 20pF,
dv(C12)/dt=0 = 16 m

dv/dt = (16 x 10^-3) / ( 20 x 10^-12) = 800V/µs