SLLSEQ0C august 2015 – may 2023 ISO5852S
PRODUCTION DATA
The function of the DESAT diode is to conduct forward current, allowing sensing of the saturated collector-to-emitter voltage of the IGBT, V(DESAT), (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, a commonly high dVCE/dt voltage ramp rate occurs across the IGBT. This ramp rate results in a charging current I(CHARGE) = C(D-DESAT) × dVCE/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, VF + VCE = V(DESAT), the VCE level, which triggers a fault condition, can be modified by adding multiple DESAT diodes in series: VCE-FAULT(TH) = 9 V – n × VF (where n is the number of DESAT diodes).
When using two diodes instead of one, diodes with half the required maximum reverse-voltage rating can be selected.