Thiagarajar College of Engineering
Department of EEE

Power Electronics Virtual Laboratory

SCR stands for Silicon Controlled Rectifier.
Like a diode, it is a uni-directional device (it allows forward current and block reverse current)
It is a three terminal device (Anode, Cathode and Gate)
The SCR can be turned on by applying suitable pulse to the gate terminal.
A commutation circuit is needed to turn off a conducting SCR.

Modes of Operation of SCR

SCR only conducts in Forward Conduction Mode.
SCR never conducts in Reverse blocking mode regardless of whether the gate pulse is applied or not.

Commutation of SCR

The process of turning off the SCR is called commutation.
SCR cannot be turned OFF using the gate terminal.
To turn off the SCR, the anode current must be reduced below the holding current of the SCR or commutation voltage must be applied across the SCR.

The source of commutation voltage is the AC voltage source itself.
This method is also called as Source Commutation or AC Line Commutation.
As the current in the AC circuit goes through the natural zero, a reverse voltage is applied immediately across the SCR
For successful natural commutation, the turn OFF time must be less than the duration of half cycle of the supply.

Forced Commutation

In case of DC circuits, there is no natural current zero to turn OFF the SCR.
In this method, forward current must be reduced below holding current or reverse voltage voltage must be applied across the SCR with an external commutation circuit.

This circuit utilises two SCR and two diodes.
SCRs are controlled rectifers and diodes are uncontrolled rectifiers. That is why semi converter provides limited control over output voltage and is known as semi-converter.
Semi converters are single quadrant converters
When inductor in the load discharges, negative output voltage will occur. The diodes in the circuit provide flywheeling path for the voltage stored in the inductor. Hence negative output voltage is not possible.
Diodes and SCRs will not allow reverse current, so negative current is not possible in semi-converter circuit
Since semi converter can only provide positive voltage and positive current, it always operates in First Quadrant