# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 355

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power dissipation in the windings. This can damage the windings, so the
dt
du values
need to be reduced, especially for large machines. As this usually involves inverters
with IGBT modules connected in parallel, the principle of
dt
du
filters will be briefly
described in the following.
Fig. 8.16
dt
du
Filters in parallel connection
As described in chapters
and
modern IGBTs are essentially not controllable when
turning off. Reducing the
dt
du
of modern IGBTs when turning off causes a massive
increase in turn-off losses. To protect the electric machinery from excessive
dt
du
when
the IGBT turns on and off,
dt
du
filters are used in the output of the inverter. A capacitor,
C
du/dt
is connected parallel to the load inductance L
. If, for example, the IGBT T
1
is
now turned on, the load flows into the capacitor the instant the device is turned on, and
all the voltage is present across the inductance L
11
. Once the capacitor is charged the
voltage can increase across C
du/dt
, and therefore across the L
located parallel to the
capacitor. This means that the
dt
du
on the load is determined not by the connected
IGBT, but by the relationship between L
11
, R andC
du/dt
. R limits themaximum current of
the capacityC
du/dt
. If the switching voltage of the IGBT is considered to be ideal, the
dt
du
across the load can be calculated according to
dt /du
11
DC
C L
U
dt
du
=
Eq. 8.4
As well as using output inductors, there is a second method of current balancing in
parallel connection: Ring connection of chokes. This makes use of the well-known
common mode effect. The idea comes from the era when thyristors were connected in
parallel and can be applied in the same way to modern IGBT connections. Ideally, all
currents in the parallel connection should be of the same value and direction. In this
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