UCC21759-Q1 initiates a soft turn-off when the overcurrent and short circuit protection is triggered. When the overcurrent and short circuit fault happens, the IGBT transits from the active region to the desaturation region very fast. The channel current is controlled by the gate voltage and decreasing in a soft manner, thus the overshoot of the IGBT is limited and prevents the overvoltage breakdown. There is a tradeoff between the overshoot voltage and short circuit energy. The turn off speed needs to be slow to limit the overshoot voltage, but the shutdown time should not be too long that the large energy dissipation can breakdown the device. The 400mA soft turn off current of UCC21759-Q1 makes sure the power switches is safely turned off during short circuit events. The timing diagram of soft turn-off shows in Figure 6-10.

Figure 7-6 Soft Turn-off