Instrumentation & Measurement Magazine 26-3 - 46

Design and Operation of a CostEffective
Cooling Chamber for
Testing Power Electronics at
Cryogenic Temperatures
Stefanie Büttner, Julian Windisch, and Martin März
I
n recent decades, research in the field of cryogenic power
electronics has gained increasing interest, as it promises
advantages such as higher power density and higher efficiency.
Particularly in the mobility sector, lower weight and
smaller size are essential to advance electrification [1]. Another
incentive and benefit of lower power losses is the reduction in
operating costs. However, the study in [2] has shown that energetic
profitability of low-temperature cooling is achieved, in
particular, in applications where the necessary cooling for the
power electronics is available for free and synergy effects can
be realized within the overall system. Interesting areas of application
are, therefore, in the field of aviation, where the cold
ambient temperature of -55 °C is available, and in the mobility
sector. Cryogenically stored fuels such as liquid hydrogen
(LH2) or liquid natural gas (LNG) must be heated before they
can be used, for example LH2 for application in a fuel cell, for
which the power losses generated in a power electronic converter
can be used perfectly. This saves energy for extra heaters
and increases the efficiency of the power electronics [2]. One
challenge when operating power electronics at temperatures
below -40 °C is that most electronic components are not specified
from the manufacturer for these temperatures. Therefore,
a comprehensive characterization of all required electronic
components for a deep temperature operation is essential, for
which a suitable environment-a cryogenic cooling system-
with variably adjustable ambient temperature is required.
In the literature, various approaches to cryogenic cooling
of power electronics can be found, in which the cooling
capacity is provided via the heat transport mechanisms conduction
and convection. Heat conduction takes place within
a body or between two bodies in physical contact and depends
on the driving force of the temperature difference,
the heat-transferring surface and the thermal conductivity
of the body. The process of heat transfer between a surface
and a gas or liquid in contact with it is called convective
heat transfer, a distinction being made between natural
and forced convection. With natural convection, heating of
the cooling medium leads to a change in density and thus
a rising of the medium. If an external force, e.g., from a
46
pump or fan, is acting on the medium, this is called forced
convection.
There is a wide variety of approaches to implementing a
low-temperature cooling system. The fastest and most costeffective
method of testing power electronics at cryogenic
temperatures is convection cooling in a cryogenic liquid fluid,
as done in [3]. An advantage of this all-sided cooling method
is the low nitrogen consumption, especially when the power
dissipation of the test component is low. As long as the device
under test (DUT) is surrounded by liquid coolant, ice
formation due to humidity is avoided. This cooling method,
however, has some severe disadvantages. Only a constant
cooling temperature is feasible, corresponding to the boiling
temperature of the cryogenic fluid, e.g., 77 K for liquid nitrogen
or 20 K for liquid hydrogen. A continuous temperature
adjustment is not possible. The main problem, however, is the
extreme temperature shock during immersion, which causes
very high thermo-mechanical stresses and thus early failures.
A similar cooling method uses the vapor phase above the
boiling cooling medium, whereby a reduction of the temperature
gradients and a temperature adjustment is possible within
certain limits. The same can be accomplished by placing the
DUT over the liquid phase of a cryogenic fluid, with the DUT
being installed at one end of a copper strip that is immersed in
the cryogenic fluid at the opposite end. The cooling temperature
setting is made by the distance of the test object from the
liquid surface, with the test object to be immersed in the liquid
phase for the lowest temperature. Depending on the distance
to the liquid, a cooling temperature between the boiling and
ambient temperature is achieved [4]. However, without an inert
environment, there is the problem of ice forming on the
DUT. This conduction cooling method is more suitable for examining
individual electronic components or materials than
for examining complete power electronic converters.
As used in [5] or [6], heat can also be extracted locally from
the DUT by using a cold plate cooled with a cryogenic medium.
In this case, heat transfer is achieved by heat conduction
into the cold plate. Depending on the state of aggregation of the
fluid entering the cold plate, it can be either cryogenic liquid or
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
May 2023
Instrumentation & Measurement Magazine 26-3

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