A frequency-tuned magnetic resonance-based wireless power transfer system with near-constant efficiency up to 24 cm distance
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In magnetic resonance-based wireless power transfer systems, the main challenge arises from varying distances between coils during power transfer because distance variations ultimately reduce the power transfer efficiency. Frequency-tuned wireless power transfer systems provide an almost-constant output power to the load up to a critical coupling distance. However, the critical coupling distance and power transfer efficiency are dependent on coil size, source, and load resistances. The purpose of this paper is to discuss this dependency with an equivalent circuit model, determine a suitable coil size for given design specifications, and set up a frequency-tuned system that is based on tracking the resonance frequency at the transmitter side. The coil's lateral size is usually limited by the size of the device to be charged in wireless power transfer applications; therefore, radii of coils are fixed to 25 cm. Coil size is varied by changing the number of turns during simulations. Two identical coils are fabricated based on the simulations using an equivalent circuit model and characterized by S-parameter measurements. A frequency-tuned system is then realized using the fabricated coils, a radiofrequency bidirectional coupler, and a suitable load resistance. Measured power transfer efficiency exhibits good agreement with that predicted by the circuit model. An almost-constant power transfer efficiency of more than 75 % is obtained for up to 24 cm coil separations.