Automotive Engineering - May 2021 - 9

TECHNOLOGY REPORT

Additive manufacturing " lets us design in 3D without these constraints and
we see great benefit in applying the
technique to EV inverters, " he continued. " These have a significant impact
on how hard you can drive other components of the EV powertrain, so even a
modest advance can create a virtuous
circle of improvements in efficiency,
packaging and energy density. "

Overcoming constraints

BOTH IMAGES: UNIVERSITY OF BATH

To prove this, Wilson is leading a multidisciplinary research team that is completing the first phase of a project that
looks at the potential for 3D printing select inverter components. This could
allow key constraints to be overcome
- including thermal management, electrical noise and packaging volume.
The aim is to achieve significant design
improvements that can be manufactured in volume. Although it is early
days, the inverter project is looking
promising, he said.
At present, inverters are typically designed via 2D processes, stacking flat
component boards. At the bottom of the
stack is a thick aluminum cold plate
heat-sink that is typically liquid-cooled.
" The efficiency, reliability and performance of SiC switches falls significantly
as temperature rises, so the ability of this
plate to extract heat from the inverter is
critical, " Wilson said. " A conventional
cold plate has machined cooling channels. But these inhibit the creation of

Oliver Holt's design points the way to making
the inverter significantly more power dense.
AUTOMOTIVE ENGINEERING

Bus bar connections (top surface) link directly to the new heat transfer lattice within the cold plate.

intricate shapes and you can't specify
very thin walls between the channels. As
well as making it less effective, that
means the plate is heavier and bulkier
than you need and introduces additional
thermal resistance. "
This is where 3D printing comes in.
Oliver Holt, a member of Wilson's specialist team, has designed a plate with a
complex lattice internal structure constructed of walls less than 1-mm thick.
The more efficient design will allow the
power electronics to carry up to 10%
higher current, making the inverter significantly more power-dense. " The
manufacturing solution would be to
3D-print just the complex shapes and
drop them into a conventionally-manufactured plate, " Holt said. " We only employ the more expensive technique
where it allows us to do something that
adds a lot of value. "
The next challenge is packaging,
which is also linked to how quickly
the SiC switches can be instructed to
turn on and off. Electro-Magnetic
Interference (EMI) is created not only
by a high switching frequency, but also
by the rate of change of voltage (dV/dt)
in the transition from conducting to
not-conducting. Detailing the finer
points, Holt explained that EMI " is made
significantly worse when the distance
between switches and the gate drivers,
together with the area of the connection, is large. "

This is because even such a small
inductance leads to damped-resonant
ringing of the switches as they transition. The traditional solution is to operate the switches at a lower speed,
meaning the designer is unable to take
full advantage of the SiC technology.
Holt added that 3D printing " facilitates manufacture of the connections
with highly optimized 3D geometries,
which allows the gate drivers to be
moved closer to the switches, improving packaging as well as reducing the
loop inductance that can limit the
achievable switch performance. "

Simplified cooling

What all this delivers, Wilson said, is a
reduction in switching losses that translates to both an increase in converter
efficiency - especially at low loads.
There is also an increase in inverter
power without upgrading the cooling
system, or a simplified cooling system
for the same power. " It's a virtuous circle enabled by additive manufacturing's
ability to let us design in 3D, " he said.
Because the new approach eclipses
conventional manufacturing constraints,
it delivers " the ability to fully exploit
proven wideband gap semiconductor
devices that will also allow so many
other systems in the EV powertrain to
work more efficiently - permitting designers to apply it to production. "
Stuart Birch

May 2021 9

Automotive Engineering - May 2021

Table of Contents for the Digital Edition of Automotive Engineering - May 2021