Thermal simulation of hybrid circuits with variable heat transfer coefficient

This paper demonstrates, based on a practical example of a test hybrid circuit, the importance of proper modeling of the heat transfer coefficient dependence on the surface temperature rise and fluid velocity in air cooled electronic systems. Hybrid circuits usually have large surface area and consequently important temperature gradients could occur in them, hence the local values of the heat transfer coefficient might differ considerably. In order to show the significance of the problem, dynamic thermal responses of the test circuit were measured for various dissipated power levels and air velocities. The measurement results allowed then the generation of compact thermal models. Owing to the fact that these models take into account the variation of heat transfer coefficient value with cooling conditions, it was possible to increase significantly the accuracy of thermal simulations.