IGBT Modules - Technologies, Driver and Application (Second Edition) - page 338

static and dynamic operation, the current is always properly distributed through all
modules connected in parallel. Static symmetry used to be considered very important
but, with the use of modern chip generations, this is no longer true, as dynamic balance
in parallel connection is much more significant. This will be dealt with in more detail
below. Furthermore, cooling must also be taken into account as an additional
8.2.1 Noteson staticoperation
The classicway of determining current distribution inmodules connected in parallel is to
use the static approach, where "static" describes only the behaviour of the power
electronics component in its environment in a state of being permanently switched on.
This section discusses the saturation voltage and the forward voltage of the
components and their behaviour at different temperature levels in detail. While the first
generations of IGBT chips, which were based on the punch through concept, had a
negative temperature coefficient, modern IGBT chip generations – as described in
have a positive temperature coefficient. Increasingly, technologies are
being used for freewheeling diodes that leave them with only a very slight negative
temperature coefficient or even allow them to change to a positive temperature
coefficient at nominal current and above. This development provides extensive
assistancewhen IGBT components are connected inparallel.
Parameter variance for power semiconductor components has also improved a great
deal. Continually rising production runs have optimised the manufacturing process to
such an extent that inmost cases, it is sufficient to use IGBTmoduleswith identical date
connected in parallel. It is no longer common practice to select according to
or U
The following example shows the relationship between the output characteristics of an
IGBT and the way the current is distributed at typical and maximum values
This calculation becomes necessary when components are connected in parallel
without being selected, i.e. without knowledge of their U
values or of the date code.
In this case, themaximum parameter variance is taken into account; this is specified in
the manufacturer's datasheet. The first step is to linearise the characteristics for the
temperature under consideration. In the example below, the temperature of 125°C has
been selected. Assuming that the U
value is identical for both the typical and the
maximum values, only the linearised characteristics need be shown, by starting from
in each case, and drawing straight lines to the values stated in the datasheet for
. In parallel connection, the lowest U
voltagewill always appear
across the IGBTs, so that the IGBT with the lowest value (here U
) carries the
highest current and the IGBT with the highest value (here U
) carries the lowest
current. In the example below, therefore, the IGBT with U
has a static current of
1000A and the other IGBT has a static current of 756A. In order not to overstrain the
power electronics component statically, this means that the total nominal current
(1756A) of the two IGBTs in parallel connection is less than the total if they are switched
individually (2000A).
Thedate code indicates the date onwhich the components weremanufactured, printed on the components as a
number or in a barcode. It cannot automatically be assumed, however, that IGBT modules with the same date
codealso contain IGBT chips from the samebatchof wafers.
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