Fast and accurate simulation of time-variant air-cooling systems

A number of challenges in the thermal simulation of air-cooling systems for power-electronic devices, in particular IGBT switches, are addressed, including (i) strongly dynamic power losses; (ii) thermal cross-talk between IGBTs and diodes; (iii) variable fan speed, making the cooling system time-variant; (iv) fast simulation of transient operation over long periods of time; (v) fins with surface enhancement in the form of ripples. A simulation methodology is presented employing a 3-d model to generate thermal impedances, which are then fitted with partial-fraction thermal networks of Y-topology. The behavior of the network models is studied for the time-variant case and a method of resistance adaption proposed to reduce errors. The methodology is verified by comparing the 3-d model with measurements, and the network model with the 3-d model. The methodology proves to be fast, robust and of high accuracy. It is well suited for the industrial product development process, where time histories of semiconductor junction temperature are needed as an input for reliability and life time analysis.