IGBT Modules - Technologies, Driver and Application (Second Edition) - page 342

Electric conductors are always surrounded by amagnetic field. In systems connected in
parallel themagnetic fields of the individual conductorsmay overlap. Depending on how
close the modules or module segments are placed to each other. This overlap is not
necessarily the same for all conductors in a system, however. For example, modules on
the edge of the system, which therefore have only one neighbour, experience a different
fielddistribution from those that have neighbours onboth sides.
Fig. 8.4
Dynamic current distribution in IGBTmodules connected in parallel
With a change in current, as occurswhen an IGBT is turned on or off, themagnetic field
will attempt to counteract the change, thereby influencing the switching speed. Hence
an imbalance in current rise or current fall occurs during switching, due to differences in
the magnetic coupling of the phase-legs connected in parallel. It is possible to
compensate for this imbalanceby tuning the stray inductances in the commutation path.
hows how to tune the stray inductances in the DC-bus. The result of this
adjustment can be seenmore clearly i
in themeasurement results. If the DC-
bus has been adjusted experimentally once, the results of this construction can be
transferred to the production run, taking all possible tolerances into account. As shown
in the example used in
there are now six "strings to pull". When adjusting the
stray inductance, the behaviour of the freewheelingdiodemust always be kept inmind.
Another aspect of dynamic current distribution is the variance of the threshold voltage
. The lower this voltage, the sooner theMOS channel of the IGBT opens and the
collector current begins to rise. Tolerances in the U
voltage cause the IGBTs to
turn on at different times, which is reflected in a dynamic mismatching of the turn-on
current. Generally, however, it can be said that the variance of U
inmodern IGBT
chips is very low. It may be advisable to use IGBTmodules with identical date codes in
order to narrowdown the variance.
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