IGBT Modules - Technologies, Driver and Application (Second Edition) - page 357

overvoltage requirements mean that the module must have an appropriate
higher blocking voltage. However, the largest module currently being offered
bymanufacturers only has a nominal rating of 6.5kV. Hence twomodules have
to be connected in series.
There is no IGBTmodule available in the desired package configuration in the
correct voltage range. For example, only modules of a specific package
configuration may be used. As each package configuration has a maximum
voltage limit within the package family, e.g. currently 1.7kV for 62mmmodules.
If, for any reason, it is not possible to use a different configuration instead, this
will also be a case of having to use series connection, if the expected switching
voltageexceeds the properties of the individual module.
For cost reasons, it has been decided not to use themodule with the greatest
insulating capacity. Cheaper modules with lower insulating capacity can be
used instead. For example, instead of using a 6.5kV module, two 3.3kV
modules can be connected in series.
For reasons of efficiency, it has been decided not to use the module with the
greatest blocking capacity, but to use one with the closest lower voltage rating
instead. This reduces the switching losses. The switching frequency can
possibly be increased, which reduces the extent of the required filter.
Fig. 8.19
Asymmetry in series connectionof IGBTs
When IGBT modules are to be connected in series, several specific points must be
taken into account. If the series connection is not set up properly – including selection of
appropriatemodules, the design of the driver stage and passive or active voltage divider
– slight or severe voltage mismatching can occur across the modules. This can
eventually cause overloading or exceed the maximum blocking voltage of a module,
causing an application to fail. When connecting in series, it is therefore important to
ensure that, in both static and dynamic operation, the voltage is always properly
distributed across all modules connected in series. Furthermore, cooling must also be
taken into account as an additional parameter.
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