To gain a better understanding of the thermal requirements, basic calculations and FEA modelling of a simplified system was performed. The graph on the left shows the predicted losses based on the DC-DC convertor losses per each component given the required voltage change, the battery state of charge, and the discharge cycle. 

Several different thermal configurations were simulated. The configurations altered the heat generating components, their location, size, performance, as well as the heat sinking materials and geometry. The design that also met cost and manufacturability requirements was moved forward.

The MOSFETs and inductors generated the most heat and therefore were placed as close as possible to the primary heatsink, the top surface. To add additional heatsinking a heat pipe was embedded in the inductor potting material and routed to the bottom surface which acts a secondary heatsink. 

MOSFET Contact Pressure

To ensure good thermal contact between the MOSFET and heatsink a number of design considerations had to be made:

• Thickness and material of a thermal pad that meets compliance requirements, thermal conductivity, and desired compression ratio.

• Tolerance analysis of the stack up  of the  MOSFET PCB  to heat sink.

• Design of clamping mechanism that was easy to assemble, met creepage requirements, and provided target pressure  based on MOSFET datasheet.

• Collaboration with PCB vendor to design a jig to bend and locate MOSFETs.