IGBT Modules - Technologies, Driver and Application (Second Edition) - page 162

and short circuit voltage over the time. Note however, that the higher the short
circuit current will be the shorter the permitted short circuit time. If this time is
exceeded it may destroy the component. Within the time t
the short circuit
has to be recognised and turned off which places certain requirements on the
IGBT driver stage. Usual values for t
are in the range of 6µs to 10µs for short
circuit currents of the 4 to 8 fold of the nominal current. Note, however, that t
only applies for the conditions as specified in the datasheet. If the IGBT is
operated at a much lower DC-bus voltage for example, then t
may quite
extend to longer values. However, these dependencies are rarely published by
themanufacturers. Also the short circuit time t
is not the relevant criterion, if
looked at closely, but instead the energyE
transformed in the chip during the
short circuit. An influence on the energy have the effective collector-emitter
voltage which, amongst others, depends on the stray inductance in the
commutation path, on the short circuit current which, amongst others, depends
on the gate voltage, which itself is influenced by factors such as the voltage
as well as the junction temperature of the chip. Because all
these dependencies are difficult to specify, as they are derived from the actual
frame conditions of the application, to simplify, the maximum short circuit time
is given by themanufacturers as a reference value.
Junction temperatureT
This temperaturemay not have exceeded themaximum specified temperature
in the datasheet before the short circuit. As the IGBT heats up extremely
(>200°C) during the short circuit due to the generated losses, the component
would otherwise be destroyed. The heating of the IGBT is also the reasonwhy
themeasured collector current i
decreases. The short
circuit time t
in the datasheet is generally referenced to T
. For lower
junction temperatures this timemay be exceeded. Here too, the dependency is
usually not specified by the manufacturer. The junction temperature also
determines the absolute value of the short circuit current. The higher the
junction temperature is before the short circuit occurs the lower the resulting
short circuit current
IGBT technology
The IGBT technology determines, amongst others, the width of the MOS-
channel inside the IGBT (chapter
. Depending on the design, different
interactions as well as compromises result with regard to the switching
Turn-off losses/energyE
Thewider theMOS-channel, the higher the turn-off losses.
Saturation voltageU
The wider the MOS-channel, the lower the saturation voltage. These first
two dependencies arise from the IGBT trade-off diagram.
Short circuit time t
The wider the MOS-channel, the shorter the short circuit time. Because a
wider MOS-channel allows a higher short circuit current to flow due to the
lesser resistance, the IGBT heats up more. Hence for a wider MOS-
channel the time t
has to be limited in order to avoid the destruction of
the component.
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