Enabling wireless power transfer though a metal encased handheld device

Wireless power transfer through a metal object is a major obstacle for the conventionally available tightly coupled inductive system. In this paper, we demonstrate that a metal encased mobile device is enabled for efficient wireless charging using a loosely coupled resonant system. To facilitate this approach, three coils are designed and distributed on a metal phone case and they are tuned for resonance at 6.78MHz. With electromagnetic analysis of the coil geometry, a prototype is fabricated by machining an aluminum. Measurement results show that 8W can be delivered to the load on a regular AFA (AirFuel Alliance)-resonant compatible charger. In practice, using all three coils is optional and a designer may use only one coil for the metal cover design.