IGBT Modules - Technologies, Driver and Application (Second Edition) - page 324

7.9 IGBT reverseblocking voltage
Generally IGBTs are able to take up a certain blocking voltage in reverse mode. There
is a freewheeling diode connected anti-parallel to the IGBT, because in the majority of
all applications in which IGBTs are used inductive loads are common. Without this
freewheeling diode very high voltages would occur which would lead to the destruction
of the IGBT. This is even the case if the occurring voltage is below the IGBT breakdown
voltage. The reason is that the breakdown voltage is only specified in IGBT forward
direction. An occurring voltage in reverse direction may be blocked in principle,
however, is not specified. Therefore, voltages in IGBT reverse direction should be
avoided. The easiest way to achieve this is to implement a freewheeling diode, as
described above.
In the application and at turn-on of a diode connected anti-parallel to the IGBT, a
voltage U
higher than the diode forward voltage U
will appear during the
commutation, as detailed in chapter
This voltage may be anything from some
volts to several hundred volts depending on the diode and the current commutation
. This voltage will therefore appear across the IGBT in reverse direction.
Because the reverse blocking capability is not defined (as detailed above), the diode
commutation itself poses a critical operating point for the IGBT if the voltages get too
high in value. In practical terms, one has to make sure the voltage U
does not
exceed a value of 100V. In
for example, a voltage U
of approx. 32V
appears (40V/div) before the voltage at T
returns to its nominal valueU
Fig. 7.19
Reverse blocking voltage across the IGBT during turn-on of the anti-parallel
7.10 Si IGBTwithSiC freewheelingdiode
As discussed in chapter
SiC Schottky diodes do not show a reverse recovery
and, hence, no (relevant) recovery peak I
when turning off. Thismakes that
type of diode interesting as an IGBT freewheeling diode to reduce the IGBT turn-on
losses for one and, secondly, to increase the turn-on speed of the IGBT (which in turn
leads again to a reduction of the IGBT turn-on losses). When using Si freewheeling
diodes the turn-on speed of the IGBT is limited by the diodes. If the IGBT turns on with
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