Analysis, modelling and measurement of the effects of aluminium and polymer heatsinks on conducted electromagnetic compatibility in DC-DC converters

Effects of polymer heatsink materials on electromagnetic interference (EMI) noise in converters have not been adequately investigated. Heatsinks provide a path to ground for the common-mode (CM) noise. Selecting an appropriate heatsink material can therefore help reduce CM noise by increasing noise path impedance. The critical parameter is the heatsink-to-device capacitance and accurate models of the heatsink impedance need to be developed. This is necessary for results that are accurate enough to allow prequalification of a converter as would happen in an accredited environment. However, simplified CM and differential-mode models of the step-down DC-DC converter are adequate to predict the effects of the heatsink on the conducted noise. This study will demonstrate that compared to a solid aluminium material, using polymer material incorporating conductive fillers can greatly lower the device-to-heatsink capacitance, and still be adequate for heat dissipation in low power converters. This in turn reduces CM noise in the frequency band below 30MHz. The measurement test setup is configured according to MIL-STD-461F standard. It consists of a wideband two-port line impedance stabilising network (LISN), a properly grounded copper sheet, a suspended feed-line from the LISN to the device under test and an oscilloscope recording the data.