Passive Cooled Charging Pad for Interoperable Wireless Power Transfer

Wireless Power Transfer for Electric Vehicles has been developed for many years. After a decade, several international standards have been published. Inductance values, coupling factors, currents, voltages and efficiency limits have been defined for an interoperable wireless charging system. This paper presents a concept and a build-up for a passively cooled charging pad. The coil design according to the limits out of the standard is adjusted and optimized to the new charging pad. In detail the compensation of the bifilar litz wire is changed to achieve a uniform current sharing. Furthermore, the ferrite layer design described in the standard is adopted for enhanced thermal performance and better manufacturability. In the new design, the standardized aluminum plate is not only used for shielding purposes, but also acts as a heatsink. Litz wire and ferrite tiles are potted for an optimized thermal path to top and bottom surface of the charging pad. Additionally, the potting enhances the mechanical stability for drive over capability. For verification a 11 kW charging system is built. Inductance values and coupling factors are evaluated by coupling measurements. The minimum efficiency, voltages and currents are measured and compared to the defined limits out of the standard, at rated full power transfer. Due to all improvements of the charging pad, full power transfer even at worst ambient conditions and after drive over tests is possible.