How to turn ON an SCR ?
As mentioned earlier, the SCR can be switched on either by increasing the forward voltage beyond forward break over voltage VFB0 or by applying a positive gate signal when the device is forward biased. Of these two methods, the latter, called the gate-control method, is used as it is more efficient and easy to implement for power control. The following points have to be noted when designing the gate-control circuit.
- Appropriate gate-to-cathode voltage must be applied for turn-on when the device is forward biased.
2. The gate signal must be removed after the device is turned-on.
3. No gate signal should be applied when the device is reverse-biased.
4. When the device is in the off-state, a negative voltage applied between the gate and the cathode will improve the characteristics of the device. In such an instance, a large positive voltage will be required to overcome this negative bias for turn-on.
There are three methods of triggering the device by gate control.
(a) Triggering By a DC Gate Signal.
In this method, a dc voltage of proper polarity and magnitude is applied between the gate and the cathode when the device is to be turned-on. It must, however, be noted that the SCR is a current-operated device and it is the gate current that turns-on the device. The drawback of this method is that gate signal has to be continuously applied resulting in increase in internal power dissipation and that there is no isolation of the gate-control circuit from the main power circuit.
(b) Triggering By an AC Gate Signal.
In many power-control circuits that use ac input, the gate-to-cathode voltage is obtained from a phase-shifted ac voltage derived from the main supply. The main advantage of this method is that proper isolation of power and control circuits can be provided. The firing angle control is obtained very conveniently by changing the phase angle of the control signal. However, the gate drive is maintained for 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.
(c) Triggering By a Pulsed-Gate Signal.
In this method, the gate-drive consists of a single pulse appearing periodically, or a sequence of high-frequency pluses. This is called the carrier frequency gating. A pulse transformer is used for isolation. The gate losses are very much reduced since the drive is discontinuous.
Trigger Current and Trigger Voltage.
An SCR, as shown in figure, has a gate voltage of VG. When this gate voltage is in the vicinity 6f 0.7 V, the SCR will turn-on and the output voltage will drop from + Vcc to a low value. When a gate resistor RG is used as shown, the input voltage required to trigger an SCR can be determined from the following equation:
VIN = VT + IT KG
where VT and IT are the trigger voltage and trigger current respectively, required for the device gate. This information is available on data sheets. For instance, the data sheet of a 2 N4441 gives VT = 0.75 V and IT = 10 m A. Sometimes a gate resistor is not used. In this case, Rr is the Thevenin’s resistance of the circuit driving the gate. Unless equation is satisfied, the SCR cannot turn-on. After the SCR has. turned on, it stays on even VIN is reduced to zero. In this case, the output voltage remains low indefinitely.