Wireless Power Transfer Loss Simulation and Demonstrator for Electric Vehicle Fast Charging
Wireless power transfer (WPT) for Electric Vehicles (EVs) has several benefits over conductive charging including improvements in convenience, safety, automation, and vandalism resilience. WPT system power levels continue to rise and power levels up to 400kW may be needed to enable short charging times. With higher power levels, the amp-turns passing through the coils increase and stronger electromagnetic fields are created. However, for EV applications, these fields must be contained to certain extents without exceeding magnetic field limits. It is possible to do this. At Oak Ridge National Laboratory, 120kW wireless charging has recently been demonstrated at 97% DC-DC efficiency without exceeding stray-field limits .
This project is sponsored through the recent partnership between Volkswagen Group and the University of Tennessee, Knoxville  and seeks to apply WPT technology to Volkswagen Group electric vehicles. Some of these electric vehicles will be produced by Volkswagen at the Chattanooga, TN plant .
There are tradeoffs between varying objectives and constraints in the design of wireless chargers. These often lead to more complex coil designs. In this project, coils are directly designed to meet performance objectives with Fourier basis function weigh
As the power levels of wireless power transfer for electric vehicles rise, it becomes increasingly important to maximize the efficiency of the system while meeting design constraints such as stray-field inside or outside the vehicle extents, misalignment tolerances, and vehicle ground clearances. This project seeks to design wireless charging system with all of these in mind. Recently, is has been shown that more complex coil geometries, such as three-phase bipolar coils , can enable higher power levels under stray field limits vs. traditional circular or rectangular coils. Coil geometry design has been accomplished with finite element analysis (FEA) approaches and analytical methods. FEA-based methods often rely on brute-force iteration with full or partial 3D-modeling of parameterized coils in a specified geometry and many analytical methods are pertinent only to circular or rectangular coils.
This research project is focused on developing a new analytical method to rapidly design WPT systems with complex coil shapes for various objectives and constraints. The method directly designs WPT coil magnetic fields and currents to meet performance objectives and constraints through the optimization of Fourier basis function weights of varying spatial frequencies. Similar approaches have been applied in the design of MRI gradient coils, and electric machines. In the field of WPT, this method has many benefits and does not assume a specific coil geometry, number of turns, or rely on iterative finite-element analysis (FEA) simulations. After the continuous coil shape is optimized, the method can determine coil conductor paths and accurately predict self and mutual inductance for a desired number of turns and calculate the DC-DC losses for various gauges of wire and switching devices. This allows for convenient multi-objective optimization of a broad variety of coil shapes, operating frequencies, and DC-link voltages. Early stages of this project will validate the accuracy of this model with a 6.6kW prototype system.
How WBG Can Help
Wide-bandgap devices have aided the field of wireless power transfer by enabling higher switching frequencies and DC-link voltages with lower on-state resistance and parasitics than conventional Silicon devices. For electric vehicle, this can translate to smaller and lighter coils and ultimately more efficient, higher-power wireless chargers. As the cost of wide-bandgap devices decreases and the technology matures, these benefits have proven themselves in the field and many wireless fast-charging systems are being developed with Silicon-Carbide (SiC) or Gallium-Nitride (GaN) devices. In a similar way, early stages of this project will develop an inverter using 1.2kV SiC MOSFETs for a 6.6kW prototype.
- Andrew Foote
 J. Pries, V. P. N. Galigekere, O. C. Onar, and G.-J. Su, "A 50-kW Three-Phase Wireless Power Transfer System Using Bipolar Windings and Series Resonant Networks for Rotating Magnetic Fields," IEEE Transactions on Power Electronics, vol. 35, no. 5, pp. 4500-4517, 2020.