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 of the SCR 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.

POWER RATING OF THE SCR.PDF

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