Which ever measure is chosen in the end, internal gate resistors of the IGBT module
will always allow higher gate voltage which in turn will lead to a higher short circuit
current. The external measures described above will also work to limit the short circuit
current with those IGBT modules. However, they cannot compensate the effect of the
internal gate resistor.
Another important point in connection with a real short circuit is its duration from the
beginning to its turn-off. Most of the IGBTs that are characterised for a short circuit
require that the short must not last beyond 10µs. The duration is derived from the
energy of the losses E
, which are generated inside the IGBT during the short circuit.
Thismay not exceed a specific value defined by themanufacturer. Because it is usually
very difficult or impossible to measure the short circuit energy in the application, for
practical reasons one uses themaximum short circuit duration quoted in the datasheet.
Within that time, the entire sequence has to happen, from "detection" and "processing"
to "turn-off". The dependency of the maximum short circuit duration t
and DC-bus voltage U
is important in this context and is as follows:
The higher T
the shorter will be the maximum permissible short circuit
duration. In that way the timeavailablewill be reduced for the progression above.
Modern driver units with their integrated protection functions such as desaturation
usually provide sufficient performance in detection and turn-
off capability. Only with IGBTs specified for t
= 6µs (at 150°C) special care has to be
taken when selecting the driver stage or implementing short circuit detection. Not all
drivers or circuits are able to guarantee the required 6µs under all operating conditions.
The difficulties in applications arise less with short circuits of the SC1 category rather
thanwith the category SC2. The following looks at various aspects, whichmay illustrate
this. A critical case is when, during a short circuit SC2, the IGBT is turned off precisely
at that moment, when it goes from saturation to desaturation. In this instance, two things
may happen together or separately. For one, this case usually produces the highest
collector-emitter over-voltages due to the high
values in connectionwith the stray
inductances in the commutation path. If the resulting collector-emitter voltage exceeds
the IGBT voltage rating U
then it may suffer destruction. Here too, appropriate
protectionmeasures need tobe taken in the application:
Active clamping (chapte
Low inductance design of theDC-bus (chapter
Use of snubber capacitors (chapter
Apart from that, power losses generated in the IGBT (U
rise to apoint where they destruct the IGBT thermally.
Another problem is the effective inductance in the short circuit, which limits the short
circuit current according to its size.
is an example.
shows a low
short circuit inductance, which pushes the IGBT into desaturation within a pulse time of
6µs and thereby forces a turn-off via the existing U
monitoring (the measurements
here were done using Active Clamping to limit the collector-emitter over-voltage at the
time of turn-off). I
, however, the short circuit inductance has been increased.
In this case, the IGBT does not reach desaturation within the pulse time of 6µs,