Tech Briefs Magazine - June 2022 - 18

Automotive Design
Figure 3. 3D model image showing simulation of EM effects inside a DC link capacitor design.
needs in order to operate. If connected
to DC current, the motor simply will not
rotate. Instead, it must be provided with
alternating current with three distinct
but complementary waveforms, enabling
the motor's three-part field coil to magnetically
attract the segments of its rotor
in a sequential pattern. " To control the
activity of the motor, we must control the
amplitude and frequency of the inverter's
current output, " explained Kessler.
" The speed of the motor is proportional
to frequency, while amplitude helps determine
its torque. "
" The desired current waveform through
the transistors has a relatively steep gradient.
The only way to achieve switch-mode
current with this high gradient is to have
very low inductance in the source path, "
he said. Inductance is the force opposing
changes in current flow. Every slight
change in current will be limited by an
induced counteracting voltage, which will
disrupt the desired waveform - and the
smooth rotation of the motor.
To reduce the inductance in the
source path of the transistors, a capacitor
is placed in parallel across the input
lead from the battery, which is called the
DC link. The DC link capacitor (Figure
2) is placed in direct proximity to the
transistors and provides the desired current
waveforms through the transistors.
The low impedance of the capacitor
minimizes any remaining ripple voltage
on the battery side.
A typical capacitor consists of two electrodes
separated by an insulating gap,
which may simply be airspace or some
kind of material. In this application,
Bosch uses capacitors made with metallized
polypropylene film. A thin coating
of metal (forming the electrodes) is
sprayed on each side of the film, which
provides the necessary dielectric gap.
The metallized film is then wound tightly
into a canister shape. As with the inverter
itself, the capacitor's conceptual
simplicity conceals a multifaceted engineering
design problem.
Capacitors are widely available components
that are installed in countless electronic
devices, but they can't be just
picked up from the marketplace. " There
are multiple interdependent factors at
work. First, we have high demands for
performance and reliability. Second,
there are very tight spatial requirements.
Third, we face difficult thermal constraints,
as the polypropylene film in a
capacitor can only withstand temperatures
up to around 105 ┬░C. This issue is
compounded by the interaction of electromagnetic
and thermal activity throughFigure
4. A model of the electromagnetic field
generated by the capacitor, which aids the calculation
of loss distribution in the unit.
out the inverter. And finally, the capacitor
is relatively expensive, " Kessler said.
Simulation Solves the Black
Box Problem
To meet the design challenges of a DC
link capacitor, Kessler developed a process
that combines experimental testing
with multiphysics simulation. As an example
of why simulation-based analysis is
a necessary part of his work, he cites the
difficulty of finding and measuring potential
hot spots, where high heat and
coupled effects can cause failures. " We
try to locate hot spots by placing a lot of
thermocouples inside prototypes and
measuring temperatures at various load
points, " Kessler said.
" A simple 2D model of a capacitor is
also insufficient, " Kessler said. " The inverter
is a distributed system with internal
resonances and a complex loss distribution.
Our coupled EM and thermal
analysis must account for skin effects
and proximity effects. We cannot calculate
an absolute value for peak temperatures
without a 3D finite element approach,
which also enables us to model
the spatial distribution of coupled EM
and thermal effects. This is an ideal task
for the COMSOL Multiphysics®
ware, " he added. (Figures 3-5)
softFigure
5. A 3D model showing simulation of thermal effects inside a DC link capacitor design, and a cutaway view showing the hotspot location in
the capacitor.
18
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