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Recently, omnidirectional wireless power transfer systems have been studied intensely due to their improved flexibility when compared with their planar counterparts. In this paper, a novel wireless charging bowl with multiple transmitter coils is proposed to power portable devices. The bowl-shaped transmitter coil is optimized to provide sufficiently strong and nearly uniform omnidirectional field distribution. Inside the bowl, a planar receiver coil can be charged with free-positioning and arbitrary orientation. This unique benefit is particularly attractive for low-power portable devices. In the experiment, the proposed transmitter coils are built and operated at 6.78 MHz. The magnetic field is measured to verify the uniform and omnidirectional magnetic field distribution. The coil to coil efficiency is 85% to 95%. A complete system including the inverter and rectifier stage is implemented. When charging a 5W smart phone receiver, the overall efficiency varies within a range from 68-80% for any possible position.
Feng et al. (Fri,) studied this question.