Coil Design for Wireless EV Charging
Wireless Power Transfer (WPT) is deemed as the emerging mainstay power conversion technology for Electric Vehicle (EV) charging application for now and in the future. Compared with traditional wired EV charger, WPT technique provides the opportunity of charging the automobile without a physical, cable connection in an unobtrusive way. The wireless charging coils are buried underground and charging station space can be reduced. In addition, no plug or cable is needed, which could be easily damaged or cause electric safety concern without supervision. By the application requirement of EV charging, the airgap between transmitter coil and receiver coil in the WPT system for EV application is typically at centimeter range, leading to the low coupling coefficient lower than 0.3 in most cases. To compensate that, current wireless EV charger uses with coil size ranging from 200mm up to 900mm to boost the coupling, inevitably sacrificing the system power density and efficiency. In this project, a self-resonant (SR) coil has been studied for high frequency WPT system at MHz range instead of conventional kHz operation of wireless EV. By pushing the switching frequency higher, the quality factor of coil could be improve. Since no external capacitor is needed for SR coil, the overall volume of passive component is smaller and system power density can be increased.
(a) 6.78MHz full bridge inverter with 3-layer non-uniform SR coil (b) Test result waveforms
Among many WPT coil structures, the series SR coil has been studied carefully for its many superiority over others. Instead of using Litz wire for inductor winding and lumped capacitor as compensated resonant component, SR coil consists of planar inductors made of copper foil. No Litz wire is needed and SR coil can utilize its parasitic capacitance as the compensation network. The total volume of passive component could shrink largely comparing with conventional coil at a given power rating.
Nevertheless, AC copper loss dominates the total ESR, diminishing the performance of SR coil especially at high operating frequency. Moreover, the specific designed capacitance value requires a certain amount of copper area provided by the planar inductor trace, limiting any method to cut down the copper loss. Aiming at increasing the copper area with the given capacitance value constraint, a multi-layer non-uniform SR coil structure has been proposed. Multiple layers of planar inductor are stacked up, with dielectric material in between any two adjacent layers. A 100mm radius multi-layer prototype has been fabricated using ceramic-filled PTFE laminate to prove the validity of this concept. The fabricated coil can reach quality factor over 200, which is almost doubled comparing with 2-layer uniform width SR coil. It has been tested with a 6.78MHz high frequency full bridge inverter in the lab and shows constant characteristics as expected.
How WBG Can Help
High switching WBG devices enable the application of SR for wireless EV charger. At MHz frequency, self-resonant coil will be resonating with its parasitic capacitance, with no external capacitor needed anymore. The overall volume of passive component is smaller and system power density can be increased. Accordingly, the fringing magnetic field at a given distance near the coil will decrease compared with large coil at kHz range, which could potentially relieve the burden of shielding component. Also by pushing the switching frequency higher, the quality factor of coil could be improved. For the same power transferred from transmitter coil to receiver coil, higher induced voltage is generated at higher frequency. In other words, less current is required at secondary side and power loss can be suppressed.