Power semiconductors like MOSFETs and IGBTs ideally operate in a fully saturated state: ohmic region for MOSFETs and saturation region for IGBTs. The voltage across, V_DS for MOSFETs or V_CE for IGBTs, remains low to minimize power dissipation. However, when these devices enter an oversaturated state as shown in Figure 1-1, small increases in current cause large voltage increases, leading to excessive power dissipation and potential damage.

Figure 1-1 Example IGBT I-V Curve

Some isolated switch drivers include a desaturation (DESAT) protection feature that monitors V_DS or V_CE and quickly shuts down the semiconductor during an overcurrent event. The TPSI31xx is a family of fully integrated isolated switch drivers, which when combined with an external power switch, forms a complete isolated solid-state relay solution.

The TPSI3133, as shown in Figure 1-2, is a variant specifically intended for DESAT due to its internal pulldown MOSFET on the fault comparator input. This internal MOSFET prevents false positives while the IGBT/SiC MOSFET is off by pulling down the fault comparator input and keeping it pulled down for an additional 100ns after EN goes high, allowing the IGBT/SiC MOSFET to fully turn on.

Figure 1-2 DESAT With TPSI3133

DESAT protection circuits are typically configured with IGBTs because IGBTs show distinct voltages (V_CE) across current (I_C) in the saturation region which makes detection easier, have sharper transitions into the active region, and generally able to handle more power due to larger die size (many have a short circuit withstand rating).

DESAT can also work with SiC MOSFETs. The problem with low voltage MOSFETs is since the overcurrent threshold is typically set well above normal operation to avoid false positives, a low voltage MOSFET may be damaged by overheating well before the circuit even detects an overcurrent event.

By implementing DESAT protection, engineers can ensure power semiconductors stay within safe operating area.