Working of SCR
Important points to be remembered:
- In SCR, load is connected in series with anode.
- The anode is always kept at a potential higher than the cathode, i.e., A is always at a positive potential with respect to K.
The working of SCR is to be studied under two different conditions:
- When Gate (G) is open
- When Gate (G) is positive with respect to Cathode (K)
- When Gate is Open
Here no voltage is applied to the gate. Under this condition, the junction J2 is reverse biased while J1 & J3 are forward biased.
( For example: In junction J1, the p-type material is kept at a positive potential than the n-type material of the pn-junction due to the voltage source connected through the RL. Hence J1 is forward biased.)
Due to the reverse bias, no current will flow through the device => SCR is cut-off.
So can SCR never be turned ON with its gate open?
Yes it can. There is a way to turn ON the SCR with the gate remianing 'open'.
You can see that the supply voltage V apears as the reverse bias voltage across the junction J2 since J1 & J3 are in forward bias condition (conduct current).When the supply voltage 'V' is increased beyond a threshold, breakdown occurs at the junction J2. The SCR now conducts heavily and is in ON state.
This increased voltage at which the SCR conducts heavily without gate voltage is called 'breakover voltage'.
- When Gate is positive with respect to cathode
The device can be made to conduct heavily by applying a small positive potential to the Gate terminal, as shown in the below fig.
In this case,
- J3 is forward biased, J2 is reverse biased.
- Electrons from n-type material start moving acoss J3 towards left
- Similarly, holes move from p-type material towards the right.
- Eventually, the electrons that moved across J3 are now attracted across J2. This initiates the Gate current.
- As the J2 is now conducting, anode current flows through the SCR as soon gate current was initiated.
(in layman terms, J2 had been reverse biased - no charge flowed across J2 till now. Now that electrons move across J2, this initiates the current flow through the device)
Note that the Gate is used only to trigger the conduction. As soon as the SCR is triggered, the Gate loses all control as the breakdown of J2 has already been achieved. Hence the anode current does not decrease at all on removing the positive Gate voltage.
The only way to stop conduction is to reduce the applied voltage to zero.