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In the Airbus, the power supply for the air-conditioning
controls is cooled by the very system it controls. The IGBT
module's is built without conventional base plate, so that
IBGT chips and power diodes are mounted directly to the
die cast housing of the turbine that circulates conditioned air
throughout the cabin. Heat is conducted through the housing to
stator blades that are immersed in the air drawn through the
turbine.
To adequately cool the power module, the turbine's die casting
must be thick enough to spread the heat to the blades, and the
number and pitch of the blades sized to dissipate the heat at 70
deg. C ambient and inlet temperatures. The air-conditioner
manufacturer had developed a proposed configuration, but
contracted with thermal consulting firm, AMS Technologies, AG of
Munich, Germany to analyze its design and make recommendations.
The approach velocities in the turbine are above 50 m/s and the
velocity between blades approaches the speed of sound. The heat
transfer coefficient to air is close to maximum and can not be
improved. Therefore, the junction-air thermal resistance is
governed by the heat conductance from the module to the blade
surface and by the effective cooling surface area.
Because a detailed simulation of the entire turbine would have
exceeded available computer capacity, a simplified Coolit model
consisting of a segment of the unrolled annulus was built. All the
details of the segment were modeled, including the curved stator
blades. The accuracy of the model, regardless of the die casting
thickness, was estimated to be within +/- 5%.
The analysis showed that optimum heat transfer occurs when the
components are positioned precisely above the blades and that even
a small offset is detrimental over the entire design temperature
range. Varying other design parameters, such as number of blades,
blade pitch, etc. produced a critical variation of temperature
drop of 10-17 K between junction for the hottest chip and the
sensor of the IGBT module. The original IGBT module manufacturer's
method of calculating the hottest IGBT chip junction temperature
as a function of the sensor reading needed to be modified for this
application. (IGBT modules with conventional base plates would not
have shown this sensitivity.)
AMS also investigated the heat dissipated by the electrical
windings in the core of the annulus. Initially, there was concern
was that this heat might raise the IGBT temperature significantly.
However, the Coolit analysis showed that even maximum temperature
conditions only slightly increased the junction temperature of the
IGBT.
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