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Designing the Airbus a/c power supply

Courtesy of AMS Technologies, AG

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.

Coolit-based power supply design flies high with Airbus.
Maximum cooling occurs when chips are mounted directly over stator blades.

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