IGBT Modules - Technologies, Driver and Application (Second Edition) - page 389

11B
377
Fig. 11.7
Rectifier diode failure in solder-bond technology caused by surge current
stress
The examples of rectifiers discussed so far are not able to feed energy from theDC-bus
back into themains. If, therefore, as amotor is decelerated, the energy that is fed from
themotor to the DC-bus will have to be removed in other ways. Otherwise, the DC-bus
voltage would rise significantly with the charging of the capacitor. The easiest way to
reduce the regenerative energy of the load in the DC-bus is to use a brake chopper
(dynamic brake).
shows the use of two rectifiers, each with a brake chopper
consisting of an IGBT T
7
and two freewheeling diodes D
7
and D
8
. Once the DC-bus
voltage exceeds a certain value, T
7
is pulsed and the energy is dissipated in the brake
resistor R. In order to avoid dangerous over-voltages due to parasitic inductances
during the turn-off of T
7
, the diodeD
7
is necessary and it needs to be able to handle the
full brake current (just like the IGBT T
7
). The freewheeling diode D
8
, however, can be
much smaller since the effective parasitic inductances are lower and thus will store less
energy. The switching frequency of the chopper IGBT is usually only a few 100Hz, so
that only low switching losses occur. IGBTs in chopper circuits are therefore smaller
than the inverter IGBTs.
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