Systematic Design of a 100 W 6.78 MHz Wireless Charging Station Covering Multiple Devices and a Large Charging Area

Wireless power transfer (WPT) promises to offer safe and convenient charging in consumer electronics applications. The application ranges from sub-watt medical implant device charging to watt-level household charging, further to the kilowatt electric vehicle (EV) and railway charging. At present, two industry standards are widely used to guide the WPT product design. The Qi standard describes an application that requires the receiver to be placed close to the transmitter, called "tightly coupled" WPT. The receiver coil typically has a similar size compared to the transmitter. And the operation frequency is in the kHz range. The Airfuel standard, on the other hand, uses the 6.78 MHz ISM band as the operating frequency, which facilitates high induced voltage when the coupling between the two sides is "loosely coupled". Thus, the Airfuel standard can be adopted in applications where the transmitter size is larger than the receiver for multi-receiver applications. Other benefits for the Airfuel standard include high-Q coil design for high system efficiency, small component size, and less interference with neighboring metals.

Targeting the application described by Airfuel, this work presents the analysis and design of a 100W, 6.78MHz WPT station covering 0.5m x 0.5m and multiple receivers. The station provides three key features: 1) free-positioning: receivers have constant induced voltage anywhere on the transmitter; 2) free-loading: operation status of one receiver do not affect another receiver; 3) high overall efficiency.

To fulfill 1), an interleaved coil is proposed that has various current distributions in each turn. To fulfill 2), a passive impedance matching network is detailed. To achieve 3), soft-switching of MHz converters is analyzed, and a systematic design method is proposed. Using the resulting system parameters from the systematic design, a prototype is built and tested. The experimental results show that the magnetic field variation on the charging surface is 15.9%, validating the free-positioning function. The free-loading function is verified by the load-change test. The measured transmitter coil current and the induced voltage on receivers are constant. The measured dc-to-dc efficiency at full load is 92.8%, which is high compared to other systems in the literature.