Principle of operation of SCR (Silicon Controlled Rectifier)

Silicon Controlled Rectifier
Silicon Controlled Rectifier
When the anode voltage voltage is made positive with respect to the cathode, the junctions J1 and J3 are forward biased but the middle junction J2 is reverse biased and only a small leakage current flows from anode to cathode due to the mobile charges. The junction J2, because of the presence of depletion layer does not allow any current to flow through the device. The leakage current is insufficient to make the device conduct. The depletion layer mostly of immovable charges does not constitute any flow of current. The SCR is then said to be in the forward blocking or OFF sate condition and the leakage current is known as OFF state current ID.
Silicon Controlled Rectifier
Silicon Controlled Rectifier
When the cathode voltage is positive with respect to the anode, the middle junction J2 becomes forward biased but the two outer junctions J1 and J3 becomes reverse biased. This is like two series connected diodes with reverse voltage across them. The junction J1 and J3 do not allow any current to flow through the device. Only a very small leakage current may flow because of the drift the charges. This leakage current is again insufficient to make the device conduct. The SCR is in the reverse blocking state or OFF state and a reverse leakage current known as reverse current IR flows through the device. The width of the depletion layer at the junction J2 decreases with increase in anode to cathode voltage (since the width is inversely proportional to the voltage). If the anode to cathode voltage VAK is kept on increasing sufficiently to a large value, a stage comes when the depletion layer at J2 vanishes. The reverse biased junction J2 will breakdown due to the large voltage gradient across its depletion layer. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage VBO.
Silicon Controlled Rectifier
Silicon Controlled Rectifier
Because the other junctions J1 and J3 are already forward biased, there will be a free carrier movement across all three junctions resulting in a large forward anode to cathode current through the device. Due to the flow of this anode to cathode forward current, the device is said to be in conducting state or ON state. The voltage drop would be due to the ohmic drop in the four layers and is small typically, 1V.
The anode to cathode forward current must be more than latching current IL to maintain the required amount of carrier flow across the junction; otherwise, the device reverts to blocking state as the anode to cathode voltage is reduced.

Latching Current (IL):- 

It is the minimum anode to cathode current that must flow through SCR to maintain the device in the ON state immediately after it has been turned ON and the gate signal has been removed.

Once an SCR conducts, it behaves like a conducting diode and there is no control over the device. The device continues to conduct because there is no depletion layer on the junction J2 due to free movements of carriers. However, if the forward anode current is reduced below a level known as holding current IH, a depletion region develops around junction J2 due to the reduced number of carriers and SCR is in the blocking state.

Holding Current (IH):-

It represents the minimum current that can flow through SCR and still "hold" it in the ON state. The accompanying voltage is termed as VH. If the forward anode current is reduced below holding current, SCR will be turned OFF. The holding current is defined for zero gate current (IG = 0).

Note:- The ON state of SCR is known as firing or triggering.

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