How Do Turn Off SCR, Working, Waveform, Characteristic Explanation

TURN – OFF MECHANISM (TURN – OFF CHARACTERISTIC)

Once the thyristor starts conducting mode and corresponding forward current , the gate has no limit it and the device can be taken back to the blocking state only by decreasing the forward current to a level below that of the off stage(holding current). Operation of turn – off is also known commutation. Different methods used for turning off SCR thyristor.

A forward voltage is applied suddenly after decreasing the anode current zero. It will not block the forward voltage and will (turn on) start conducting again, even when it is not triggered by a trigger pulse. Accordingly required to keep the thyristor reverse biased for a finite period before a forward anode voltage can be retired.

The turn-off time of the thyristor is explained as the minimum time interval between the immediate at which the anode current becomes zero, and the immediate which the devices is accomplished of blocking the forward voltage. The whole value of turn-off time t_off is separated into two time intermission the reverse recovery time t_rr and the gate recovery time t_gr.

To immediate t_1, the anode forward current becomes zero. At reverse recovery time , t_1, t₃.the anode current passes in the reverse direction. The instant t_2, a reverse anode voltage is established and the reverse recovery current continues to decrease.

At t_3, junction J_1, J₃ are fit to block a reverse voltage. Anyway , the thyristor is not yet able to block a forward voltage because carriers, known trapped chargers, are still present at the junction J_2. During the interval t₃ to t_4, that carriers recombine. At t_4, the recombination is complete and consequently a forward voltage can be reapplied at this instant.

The thyristor turn-off is the interval between t_4, t₁. Thyristor , this time varies in the range 10 to 100 μs. Although, the total turn-off time (t_q) necessary for the devices is the sum of the duration for which the reverse recovery current flows after the application of reverse voltage, and time necessary for the recombination of all excess carriers in the inner two layer of the device.

Highly inductive load circuit is considered , the current cannot change precipitous at t₁. As well as , the fast change in current at t₂ may give rise to high voltages surges in the inductance , which will then appears across the terminals of the SCR.

Waveforms during SCR turn-on

An experimental applications , the turn off time necessary to the thyristor by the circuit, known as the circuit turn off time t_q, it should be greater than the device turn-off time t_off by a acceptable safe margin, apart from that device will turn-on an undesired instant a process called commutation failure. SCR having large turn-off time (50-100) are known as slow switching or phase control type thyristors, and have low turn-off time known fast switching (or) inverter classification thyristor. In high frequency used , the required circuit turn – off exhaust an distinguishable portion of the total cycle time and therefore inverter grade thyristor must be used