Wireless Charging of a Metal-Encased Device

In this article, a metal resonator design of enabling wireless charging of a metal-encased device was demonstrated using a two-coil magnetic resonant system operating at 6.78 MHz. Despite high metal population on metal housing, the transmitting magnetic field was efficiently linked to the receiving resonator through segmenting the metal housing with a small gap. In addition to the initially transmitting magnetic fields passing through the gap, extra magnetic fields were generated by the eddy currents induced on the segmented metal housing. These two magnetic fields were added up in phase and used to couple to the power-receiving coil. The principle of operation was demonstrated with electromagnetic simulation and measurement. Furthermore, a prototype of metal housing, including the power-receiving coil and ferrite, was fabricated and integrated with a commercial smartphone. The measurement results showed 86% of resonator coupling efficiency (RCE) with an aluminum metal casing of the device when tested with a regular Class 3 AirFuel Alliance (AFA)-compatible transmitter. Actual measured power delivered to a smartphone battery was 8.7 W at maximum coupling position with 89% of the RCE.