Efficiency analysis and optimal design of a circular loop resonant coil for wireless power transfer

In this paper, wireless power transfer efficiency between two magnetic resonant coils is analyzed for optimal design of a magnetic resonant coil. A simple power efficiency formula is obtained, by assuming that source and device resonant coils are a single circular loop with parallel plate capacitor and identical. It is found that the efficiency η is determined by a coupling coefficient κ of an intrinsic decay rate Γ and κ/Γ is determined by a wire radius (a), a coil loop radius (r) and a distance (D) between two resonant coils. From the relations, the maximum power transfer efficiency can be calculated by fixing dimensions (a, r, D) of the resonant coil. And then, the optimal resonant coil can be designed. By simulation, the efficiency of designed coil is verified. The optimal design procedure can be also applied to other resonant coils.