# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 473

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Mainly electrolytics are used as capacitors, but also film capacitors. The total DC-bus
capacitance depends on the requirements of the application and – by paralleling –
ranges from 100µF to 100mF. For higher DC-bus voltages, the capacitors need to be
connected in series so that thewholeDC-bus often consists of amultitude of series and
parallel connected capacitors.When connecting capacitors in series, it is necessary that
resistors are connected in parallel to force the voltage balanced (symmetry) in order to
avoid one of the capacitors in the chain inadvertently taking up an excessive part of the
voltage.
shows two examples of inter-connecting DC-bus capacitors. The
is that with increasing numbers of capacitors the
balancing of the voltage division and the leakage currents between the capacitors of the
top row and the bottom rowwill more andmore converge. At best, additional balancing
resistors may not be necessary at all here. The disadvantage is that in case of one
capacitor failingwith a short circuit then the opposing capacitor bankwill get the full DC-
bus voltage and will fail as well. Only when the capacitor that failed originally does not
show a short but an open circuit instead, will the application continue to operate, albeit
under reduced performance. The variant in
, however, exemplifies a parallel
connection of individually balanced series capacitors. In this case, the failure of one
series connected capacitor set does not lead to an instant failure of the application. The
disadvantage of this circuit is, of course, the extra cost for the balancing resistors and
the space and production expenditure required.
Fig. 13.3
Set-up of capacitors and balancing resistors inaDC-bus
The following rule of thumb can be used according to
to calculate the balancing
resistorsR
B
for two capacitors or capacitor sets:
C
max ,DC C
B
UC 0015 .0
U U2
R
⋅ ⋅
− ⋅
Eq. 13.5
U
C
:
Maximum designated voltageat each capacitor [V]
U
DC,max
: MaximumDC-bus voltage [V]
C:
Capacitance of each capacitor [F]
When selecting suitable DC-bus capacitors, which in most high power applications will
be electrolytic capacitors, then themaximum ambient temperature and the ripple current
aremost important next to the nominal voltage.With regard to the ambient temperature,
it is noteworthy that the forming voltage
determines the upper temperature limit for
2
The formation produces an oxide layer on the anode surface, which later builds the dielectric of the capacitor.
The magnitude of the formation voltage affects the thickness of the oxide layer: The higher the voltage, the
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