di/dt Rating of an SCR :
The di/dt rating indicates the maximum rate of rise of current from anode to cathode without any damage to the thyristor. During the ON-state or conduction period of SCR, the maximum rate of change of current that the SCR can withstand is called the critical rate of rise of current.
When a thyristor is switched ON by applying a positive gate signal, the conduction begins at the location which is near the gate. After a fraction of second, the conduction area increases, and its spreads throughout the junction completely.
Now, if the rise of anode current (di/dt) is higher when compared with the spreading velocity of carriers across the cathode junction, then it leads to the formation of hot spots near the gate connection. This occurs due to the high current density in the junction region. The formation of hot spots increases the junction temperature of SCR beyond the maximum permissible limit. As a result, SCR suffers from a permanent failure.
Hence, during the turn-ON process of SCR, for reliable operation, a limit on di/dt value is always specified in terms of amperes/microsecond. The typical value of di/dt varies from 50 A/µsec to 800 A/µsec. In order to keep the di/dt values within the specified limits during the turn ON process of SCR, an inductor is connected in series with the SCR as shown above.
dv/dt Rating of an SCR :
The dv/dt rating or the forward dv/dt rating of a thyristor is also called the rate of rise of OFF-state voltage. The dv/dt rating of an SCR is an important parameter since it indicates the maximum rate of rise of anode voltage that does not bring SCR into conduction when no gate signal is applied.
If the rate of rise of forward anode voltage exceeds the specified maximum limit, then it causes switching from OFF state to ON state. It can also be said that if dv/dt increases beyond the specified value, then it leads to false triggering of SCR which is undesired.
The increase in the forward dv/dt can be explained by means of an internal capacitance to be acting inside on SCR. Upon increasing the voltage across the anode and cathode of the SCR, there will be a flow of charges similar to the charging current of a capacitor [ic = C(dv/dt)].
This flow of charges increases with an increase in the rate of rise of anode to cathode voltage of SCR. The higher the rate of rise of applied voltage, the higher will be the flow of charges. When the flow of charges due to the rate of change of voltage becomes equal to the flow of charges as injected when the gate is energized, then the SCR will turn ON.
The dv/dt rating of an SCR has an inverse relation with its junction temperature. The higher the junction temperature, the lower will be the dv/dt. In practice, dv/dt triggering is never employed because it leads to random and false triggering. Snubber circuits are arranged across each SCR to limit the maximum dv/dt applied to an SCR as shown in the above figure.