SCR
Construction and working
· When anode (A) is made positive with respect to cathode (K), junction J1 & J2 are forward biased.
· Leakage current can flow only through reverse biased junction J2.
· SCR is in OFF state (Forward blocking state) when junctions J1 & J3 are forward biased and J2 is reverse biased.
· If supply voltage is increased further, SCR turns ON at a particular voltage known as the breakover voltage (Vbo).
· When cathode is made positive with respect to anode, J1 & J3 are reverse biased and J2 is forward biased.
· Her, small reverse leakage current flows through the SCR.
· In this mode, SCR can support entire supply voltage across it.
· SCR is in ON state (reverse blocking state).
· When Vak is increased, J2 which is reverse biased will break down due to large voltage gradient across the depletion layer.
· This is the reverse breakdown which can damage the SCR if the SCR does not have a proper dimension, critical doping and good cooling solution.
· At the same time, if a current is injected into the gate, SCR turns ON.
· When SCR is turned on, SCR dos not support any forward voltage drop across it. This is true for an ideal SCR.
· Practically, in ON state, SCR has a forward voltage drop of 1-2 V.
· Device is now said to be in Forward Conduction Mode (saturation).
· Current flows from anode to cathode.
· Once SCR is turned ON, gate current loses control over the device.
· The only way to turn the SCR OFF is by reducing the current below the holding current value. This method of turning OFF SCR is known as SCR commutation. It is done by commutation circuits.
· Latching current – The minimum anode current required to maintain SCR in ON state immediately after the SCR has turned ON and gate signal has been removed is known as latching current.
· Holding current – The minimum anode current required to hold SCR in ON state is known as holding current.
· When anode (A) is made positive with respect to cathode (K), junction J1 & J2 are forward biased.
· Leakage current can flow only through reverse biased junction J2.
· SCR is in OFF state (Forward blocking state) when junctions J1 & J3 are forward biased and J2 is reverse biased.
· If supply voltage is increased further, SCR turns ON at a particular voltage known as the breakover voltage (Vbo).
· When cathode is made positive with respect to anode, J1 & J3 are reverse biased and J2 is forward biased.
· Her, small reverse leakage current flows through the SCR.
· In this mode, SCR can support entire supply voltage across it.
· SCR is in ON state (reverse blocking state).
· When Vak is increased, J2 which is reverse biased will break down due to large voltage gradient across the depletion layer.
· This is the reverse breakdown which can damage the SCR if the SCR does not have a proper dimension, critical doping and good cooling solution.
· At the same time, if a current is injected into the gate, SCR turns ON.
· When SCR is turned on, SCR dos not support any forward voltage drop across it. This is true for an ideal SCR.
· Practically, in ON state, SCR has a forward voltage drop of 1-2 V.
· Device is now said to be in Forward Conduction Mode (saturation).
· Current flows from anode to cathode.
· Once SCR is turned ON, gate current loses control over the device.
· The only way to turn the SCR OFF is by reducing the current below the holding current value. This method of turning OFF SCR is known as SCR commutation. It is done by commutation circuits.
· Latching current – The minimum anode current required to maintain SCR in ON state immediately after the SCR has turned ON and gate signal has been removed is known as latching current.
· Holding current – The minimum anode current required to hold SCR in ON state is known as holding current.
Symbol
Characteristics
Two transistor analogy of a SCR