The DESAT diode’s function is to conduct forward current, allowing sensing of the IGBT’s saturated collector-to-emitter voltage, V_(CESAT), (when the IGBT is on) and to block high voltages (when the IGBT is off). During the short transition time when the IGBT is switching, there is commonly a high dV_CE/dt voltage ramp rate across the IGBT. This results in a charging current I_(CHARGE) = C_(D-DESAT) × d_VCE/dt, charging the blanking capacitor. C_(D-DESAT) is the diode capacitance at DESAT.

To minimize this current and avoid false DESAT triggering, fast switching diodes with low capacitance are recommended. As the diode capacitance builds a voltage divider with the blanking capacitor, large collector voltage transients appear at DESAT attenuated by the ratio of 1+ C_(BLANK) / C_(D-DESAT).

Because the sum of the DESAT diode forward-voltage and the IGBT collector-emitter voltage make up the voltage at the DESAT-pin, V_F + V_CE = V_(DESAT), the V_CE level, which triggers a fault condition, can be modified by adding multiple DESAT diodes in series: V_CE-FAULT(TH) = 9 V – n × V_F (where n is the number of DESAT diodes).

When using two diodes instead of one, diodes with half the required maximum reverse-voltage rating may be chosen.