IGBT Modules - Technologies, Driver and Application (Second Edition) - page 356

case, the current flowing in the chokes in the opposite direction would be equal. There
would be nomagnetic resistance and the choke acts as a low-impedance resistor.
If the currents in the phases connected in parallel are different, the chokes exert
different inductances. In
the three legs are inter-connected, so the system
balances itself. It should be borne in mind that the power dissipation generated in the
chokes is dependent on the current balance. The more imbalanced a system, the
greater will be the dissipation in the chokes.
Fig. 8.17
Ring connectionof chokes
Powder core toroids have proved useful for output currents per switch of less than
400A. If these are wound as shown in
the electric fields compensate each
other for the same current or are added if the currents are imbalanced.
Fig. 8.18
Ring connectionof chokes using apowder core toroid
8.3 Series connection
The reason for wanting to connect IGBTmodules in series in the application is usually a
result of the following factors, either singly or in combination:
There is no IGBTmodule available in the required voltage rating. For example,
switching a DC-bus voltage with a nominal rating of 6kV is required. Turn-off
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