MT5F35778a © Fuji Electric Co., Ltd. All rights reserved.
3. Overvoltage Protection and Safe OperatingArea
3.1 Overvoltage protection
Due to the fast switching speed of IGBT, high di/dtis generated during the IGBT turn-off and the
IGBT turn-on / FWD reverse recovery. This high di/dtcauses a high surge voltage due to the external
wiring stray inductance. If the surge voltage exceeds the module’s maximum rated voltage (VCES), it
can lead to the destruction of the module. There are several methods to suppress this overvoltage
such as adding a snubber circuit, adjusting the gate resistance RG,or reducing the inductance of the
main circuit.
To show the correlation between the surge voltage and the related parameters, an example of surge
voltage characteristics for the SiC hybrid module 2MSI400VAE-170-53 is shown.
Fig.2-2 shows an example of the dependency between the stray inductance and the surge voltage at
turn-off.As shown in the graph, the surge voltage increases as the stray inductance increases. It can
be seen that the effect on the turn-off surge voltage is particularly large.
Fig.2-3 shows an example of the dependency between the collector voltage and the surge voltage at
IGBT turn off. The surge voltage becomes higher when the collector voltage increases.
Fig.2-4 shows an example of the dependency between the collector current and the surge voltage at
IGBT turn off. The surge voltage at IGBT turn off will be higher when the collector current is larger.
As described above, the value of the surge voltage generated in IGBT modules varies greatly
depending on the main circuit stray inductance. Besides this, external circuit conditions such as
snubber circuits, capacitor values and gate drive conditions also have an influence on the surge
voltage.
When using IGBT modules, please make sure that the surge voltage is within the Reverse Bias
Safety Operating Area (RBSOA) under all operating conditions. If the surge voltage exceeds the
RBSOA, please take countermeasures such as changing the gate resistance, reducing the stray
inductance or adding a snubber circuit.
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