6 May 2017

Power Rating of the SCR

There are following power loss when SCR is in turned on condition.

Forward conduction loss

  • The forward conduction loss in the SCR is product of average anode current and forward voltage drop
  • The average current and average power loss for different conduction angle is shown in the figure A.


  Gate power loss

  • There is gate power loss when gate signal applies. 
  • The gate power loss reduces when pulse triggering circuit is used for gate circuit. 
  • The power loss increases if the duty cycle of the gate is high.

Turn on loss

  • There will some time takes before SCR is turned on when gate signal is applied. 
  • The voltage across the SCR is higher and current flows through it during that condition resulting there is power loss in it.
  • The anode current reaches 90% of its maximum value and anode – cathode voltage reaches 10% of its maximum value during turned on condition of the SCR. 
  • The turned on power loss is product of anode current and anode – cathode voltage during SCR on condition.

Turn off loss

  • The turn off power loss in the SCR occurs due to instantaneous reverse voltage and reverse current. 
  • The impedance start to increase when SCR is turned off resulting current decreases and reverse voltage built up.
  • The inductor is used in the circuit to limit di/dt during turned off process but there is transient voltage generate due to inductance.

Forward and reverse blocking losses

  • As the anode of the SCR is more positive than cathode in the forward blocking mode, the forward leakage current flows. 
  • Therefore the forward power loss is product of forward blocking voltage and forward leakage current. 
  • Similarly the reverse power loss occurs in the reverse blocking mode.


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