Experimental investigation of discrete air cooled device thermal resistance dependence on cooling conditions

This problem is discussed here based on the practical example of an air cooled power device operating at different power dissipation levels and in variable cooling conditions. The experiments presented in this paper demonstrate that the notion of junction-to-case thermal resistance is ambiguous since the cooling conditions affect the heat diffusion processes inside the package and consequently influence the thermal resistance value. For the analysed device, a simple four-stage RC Foster ladder model is proposed here to simulate the device dynamic thermal behaviour. Owing to the fact that this model is generated in a structure preserving way, its element values can be assigned some physical meaning.