Thermal Design of Airbus AC Power Supply
The Airbus 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 such 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.
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.
The analysis with a thermal design software product 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.)
Maximum cooling occurs when chips are mounted directly over stator blades.
Our Thermal Design experts 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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