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The power rating of todays IGBT frequency converters is not only limited by the silicon, but by the heat dissipating capability of the employed heatsink. There are many applicatiom where water cooling is strictly unwanted, like in wind-turbine converters especially for oCshore applications.Using 3rd-generation IGBTs makes the problem more ditliculk The footprint of the modules is smaller compared to footprints of older modules. Hence, the heatsink must not only dissipate but it must spread (heatspreader) the power losses.This paper describes a way to develop a suitable lumped R-C-model based on the geometrical and material data of a given application The model is implemented as a kind of development-tool in Matlab. With this tool the developing engineer is able to optimize the thermal static and dynamic behavior. As input a time constant or time depending thermal power dissipation can be selected. Then the model is capable to evaluate the iduence of e.g. the thickness of the baseplate to the fluctuations of the die-temperature. I.07803-8399-0/04/$20.M) 02004 IEEE.