- di/dt is the rate of change of current in a device.
- When SCR is forward biased and is turned ON by the gate signal, the anode current flows.
- The anode current requires some time to spread inside the device. (Spreading of charge carriers)
- But if the rate of rise of anode current(di/dt) is greater than the spread velocity of charge carriers then local hot spots is created near the gate due to increased current density. This localised heating may damage the device.
- Local spot heating is avoided by ensuring that the conduction spreads to the whole area very rapidly. (OR) The di/dt value must be maintained below a threshold (limiting) value.
- This is done by means of connecting an inductor in series with the thyristor.
- The inductance L opposes the high di/dt variations.
- When the current variation is high, the inductor smooths it and protects the SCR from damage. (Though di/dt variation is high, the inductor ‘L’ smooths it because it takes some time to charge). L ≥ [Vs / (di/dt)]
dv/dt Protection:-
- dv/dt is the rate of charge of voltage in SCR.
- We know that iC=C.dv/dt. ie, when dv/dt is high, iC is high.
- This high current(iC) may turn ON SCR even when gate current is zero. This is called as dv/dt turn ON or false turn ON of SCR.
- To protect the thyristor against false turn ON or against high dv/dt a “Snubber Circuit” is used.