Electrical Revolution: 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.

Electrical Revolution