After detecting the short-circuit or overcurrent event with either voltage-based or current-based detection methods, the power switch needs to be shut down safely and efficiently to prevent power switch damage and system failure.

If the switch is turned off quickly with a high sink current (several or tens of amps), there will be a very large, negative di/dt through the switch. This di/dt, coupling with the parasitic inductances in the power loop, can cause the voltage across the switch to rise up. This V_CE/V_DS overshoot can be hundreds of volts, and has the potential to overvoltage the power switch and cause power switch failure.

Thus, after detecting short circuit or overcurrent, it is preferable to use either soft turn-off (STO) or two-level turn-off (2LTO) to turn off the power switch. During soft turn-off, shown in Figure 6-1, a smaller, constant sink current is used to switch off the device. The turn-off time is increased, turn-off speed is reduced, and the gate is slowly discharged. During two-level turn-off, shown in Figure 6-2, the gate is first pulled down to a mid-voltage level and stays for a fixed amount of time. After the fixed time expires, the driver continues to pull down the gate voltage with a smaller, constant sink current. Both of these methods can reduce the turn-off speed, reduce the di/dt, and reduce V_CE/V_DS overshoot to protect the device.