It is sometimes unavoidable to use loosely coupled coils in applications, like biomedical devices, for transferring electric energy wirelessly. However, coil misalignment causes degradation of the power transfer efficiency. It is well known that the power transfer efficiency of classical parallel coils is primarily determined by the quality factors of the coils and the coupling coefficient between the coils, and is maximized by choosing an optimal turns-ratio between the coils. Changing the number of turns of the coils cannot effectively overcome such misalignment effect. This paper presents a structure that comprises two orthogonally placed windings for lessening the variation of the coupling coefficient due to the coil misalignment.

An output current summing technique that keeps the windings concurrently energized and combines the output currents of the windings will be studied. A canonical model will also be derived to describe the interactions between the coils. An experimental prototype has been built and evaluated on a test bed, which allows different degrees of lateral and angular misalignments. Results reveal that the proposed structure can effectively increase the minimum efficiency zone, allowing more lateral and angular misalignments. These investigations lay the foundation for future understanding more complex loosely coupled winding structures.

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