Maximum Achievable Efficiency in Near-Field Coupled Power-Transfer Systems

TLDR

Wireless power transfer is commonly realized by near‑field inductive coupling and is critical to many existing and emerging biomedical engineering applications. This paper presents a closed‑form analytical solution for the optimum load that maximizes power efficiency under arbitrary input impedance conditions using two‑port parameters, enabling the design of optimized wireless power transfer links through biological media with standard EM simulation tools. Using a two‑port network model, the authors predict power‑transfer efficiency across any frequency, coil geometry, and surrounding media, applicable to inductive or capacitive coupling, and illustrate the approach with case studies for typical applications. The method generalizes known special cases and decouples inductive‑coupling two‑port design from load and power‑amplifier considerations.

Abstract

Wireless power transfer is commonly realized by means of near-field inductive coupling and is critical to many existing and emerging applications in biomedical engineering. This paper presents a closed form analytical solution for the optimum load that achieves the maximum possible power efficiency under arbitrary input impedance conditions based on the general two-port parameters of the network. The two-port approach allows one to predict the power transfer efficiency at any frequency, any type of coil geometry and through any type of media surrounding the coils. Moreover, the results are applicable to any form of passive power transfer such as provided by inductive or capacitive coupling. Our results generalize several well-known special cases. The formulation allows the design of an optimized wireless power transfer link through biological media using readily available EM simulation software. The proposed method effectively decouples the design of the inductive coupling two-port from the problem of loading and power amplifier design. Several case studies are provided for typical applications.