SOLUTION

SCR Turn on Method

To initiate conduction in SCR the anode voltage must be positive with respect to the cathode. Under this condition, an SCR may be caused to conduct by any of several techniques. Various members of the thyristor family of devices are associated with particular triggering methods described below.

Gate Triggering

This is the most commonly used method for triggering SCRs. In laboratories, almost all the SCR devices are triggered by this process. By applying a positive signal at the gate terminal of the device, it can be triggered much before the specified breakover voltage. The conduction period of the SCR can be controlled by varying the gate signal within the specified values of the maximum and minimum gate currents. For gate triggering, a signal is applied between the gate and the cathode of the device.

Three types of signals can be used for this purpose.

1. DC Gate Triggering
2. AC Gate Triggering
3. Pulse Gate Triggering

DC Gate Triggering

In this method, a dc voltage of proper magnitude and polarity is applied between the gate and the cathode in such a way that the gate is positive with respect to the cathode. When the applied voltage is sufficient to produce the necessary gate current, the SCR switches on and starts conducting.

One drawback of this is that the power circuit (anode-cathode circuit) is also dc there is no isolation between the power circuit and the triggering circuit. This method requires a continuous DC signal which causes more gate power loss.

AC gate triggering

AC source is most commonly used for the gate signal in all applications of thyristor control adopted for a.c. applications. This scheme provides the proper isolation between the power and control circuits. The firing angle control is obtained very conveniently by changing the phase angle of the control signal. However, the gate drive is maintained for the one-half cycle after the device is turned ON, and a reverse voltage is applied between the gate and the cathode during the negative half cycle.

Here the SCR is turned on with the positive cycle of an ac gate voltage source. During the positive half-cycle, when the gate current I_G reaches IGT. where I_GT is the minimum gate current to trigger, the SCR turns on. This method of triggering is used in SCR circuits for ac applications.

The drawback of this scheme is that a separate transformer is required to step down the a.c. supply, which adds to the cost.

Pulse Gate triggering

This is the most popular method for triggering an SCR. In this method, the gate signal consists of a single short-duration pulse appearing Periodically or a sequence of high-frequency pulses. The main advantage of this method is that there is no need to apply continuous signals and therefore the gate power losses are very much reduced.

Note:- The other TURN-ON method of SCR is

Forward voltage triggering:-  In this method, the forward voltage is applied between anode and cathode by keeping the gate open. In this case, the junction j₂ of SCR is reverse biased. If the applied voltage is increased up to the breakover voltage (V_BO) the device becomes turned on.

Thermal triggering (temperature triggering):- The width of the depletion layer of an SCR decreases with increasing the junction temperature. Steps in thermal triggering are as follows:

(i) Voltage is applied anode to cathode near the breakdown voltage.

(ii) Increase the junction temperature to a certain value so theist the reverse-biased junction gets collapsed and the device becomes turned ON.

Radiation triggering:- If the light is allowed to bombard on the junction, there is the generation of electron-hole pairs and by increasing charge carriers (electron-hole pairs), the device becomes turned on, eg. LASCR.