The DESAT protection has been widely used for short circuit protection of IGBTs. This circuit indirectly measures the drain-source voltage of the MOSFET during the on-state to detect short circuits. Figure 3-3 illustrates the simplified circuit.

The DESAT protection circuit consists of a resistor blanking capacitor, and a diode. When the device turns on, a current source charges the blanking capacitor and the diode is conducted. During normal operation, the capacitor voltage is clamped at the forward voltage of the device. When short circuit happens, the capacitor voltage is quickly charged to the threshold voltage which triggers the device shutdown.

As shown in Figure 3-4, to make sure that the switching transients do not interfere with the desaturation detection, the current i_desat and the capacitor C_desat need to be carefully chosen to define a proper blanking time t_blk. To increase the current i_desat and decrease the reaction time, the diode D₂ with the forward voltage u_D2 and the resistor R₂ are implemented. Neglecting the time that is needed to block the diode D₁, the blanking time t_blk can be estimated with

where U_dd is the supply voltage of the gate driver.

In case of a fault detection, the gate driver initiates a soft turn-off pulling a constant current of 400mA out of the MOSFET gate to verify low overshoots in the drain-source voltage.