A High Frequency Wireless Power Transfer System for Electric Vehicle Charging Using Multi-layer Non-uniform Self-resonant Coil at MHz


With the increasing public awareness of the environmental impact of greenhouse gas emissions, electric cars have drawn worldwide attention as the potential successor of gas cars. Wireless power transfer (WPT) allows EV battery charging without a physical, cable connection in a safe, robust, and unobtrusive way in contrast to the conventional wired EV chargers. The wireless charging coils are buried underground and charging station space is reduced. In addition, no plug or cable is needed, which could be easily damaged or cause electric safety concern without supervision. IPT is a relatively mature technology for WPT systems for EV chargers with air gap up to dozens of centimeters. However, the efficiency and power rating of IPT systems are often limited by high leakage flux due to the large air gap between the transmitter coil and receiver coil. This issue can be addressed by using more litz wire windings or heavier magnetic cores and having larger coils up to 400 mm-500 mm in radius to gain larger inductance value and higher coupling coefficient. However, the efficiency and power density are often sacrificed, and overall manufacturing cost is inevitably increased.


Three-layer non-uniform SR coil structure

Coil design plays a critical role for any high efficiency, high power density, low cost and robust WPT system. Among many WPT coil structures, the SR coil has many advantages for MHz range applications. Based on the multi-turn spiral SR coil structure, a multi-layer non-uniform SR coil with series resonance is proposed, improving the quality factor by distributing the source current among multiple layers while maintaining a multi-turn structure with large inductance. The proposed structure can be implemented with varying size and number of turns, making it applicable in WPT systems for consumer electronics and electric vehicles. Compared to the conventional SR coil designs, it has the advantages of high inductance, high quality factor, and exhibits a series L-C resonant characteristic.
The structure of a three-layer case is shown in the attached picture. Three layers of copper spiral traces are stacked vertically, with two layers of dielectric material sandwiched between. Similar to the two-layer case, the current will flow from terminal a on the top layer, to the terminal b on the bottom layer. In the multi-layer structure, the current will distribute between the three layers, but at any point along the length of the spiral, the sum of the currents in each of the three layers is equal to the source current. As the current gradually transitions from terminal a to terminal b, it must flow through both dielectric layers in series. Together with the inductance from spiral coils, the three-layer coil still works as a series L-C resonant network.

How WBG Can Help

High switching WBG devices enable the application of SR for wireless EV charger. By pushing for higher frequency, smaller air-core coils with less inductance value can be applied for the same power transfer. Litz wire is excessively expensive for strand diameters below 50 µm and may result in higher loss than solid-core copper for MHz frequencies with larger strand diameter. The multi-layer self-resonant coil structure used in this work allows high quality factor coils to be fabricated from layers of inexpensive copper foil and dielectric film. Additionally, the self-resonant coil utilizes its interlayer capacitance for resonance, eliminating the external compensation capacitor and shrinking the overall volume of passive component to increase power density.

Personnel Involved

  • Ruiyang Qin
  • Jingjing Sun