Substitution of Solid Encapsulation Materials by Dielectric Fluid in Power Modules: Potentialities and Challenges

In conventional power modules, the solid insulating material (epoxy, silicone gel) that covers the top of the devices, does not efficiently contribute to the heat extraction. Moreover, thermal conducting paths are formed by heterogeneous layers for electric insulation, causing inevitably high thermal resistance. As an alternative, the direct cooling of the devices using dielectric liquids in forced convection can pave the road toward a new class of high performance power modules. The aim of this paper is to evaluate the potentialities of using flowing dielectric liquids as encapsulating material in order to ensure the direct cooling of the devices as well as the electric insulation. The thermal interest of the solution is validated using thermal-hydraulic simulations while the dielectric properties of various liquids are evaluated experimentally over a wide temperature range. Results show that by selecting package structure allowing a high exchange area between the top device interconnection and the dielectric fluid, a reduction by more than 40% of the junction to fluid thermal resistance can be achieved. Moreover, based on the combination of the variation of the dielectric properties of liquids in motion issued from the literature and the measured dielectric properties (relative permittivity, electrical conductivity, breakdown voltage) of the dielectric fluids, the three evaluated candidates show their ability to insure a good insulation at temperature up to 175 °C for high voltage applications.