volume | PIER Journals

Abstract

Cylindrical ferrite cores are widely used in wireless implantable medical devices (WIMDs) to enhance magnetic coupling performance. Conventional uniform-permeability models feature simple formulations and low computational costs, making them suitable for rapid engineering analysis. However, they cannot describe the axially varying magnetic response inside finite-length ferrite cores. Due to the demagnetizing effect caused by finite core dimensions, the effective permeability shows a non-uniform axial distribution. To address this issue, an analytical model for the axial effective permeability distribution of finite-length cylindrical cores is established based on demagnetizing field theory and the magnetic charge model. Furthermore, a unified-parameter correction model and a piecewise correction model are developed based on finite element simulation results to improve the prediction accuracy. Over the aspect-ratio range of 1-10, the piecewise correction model reduces the maximum axial relative error from 15.64% to 2.67% compared with the unified-parameter correction model. An effective permeability measurement platform is further established, where the average effective permeability at different axial positions is obtained by varying the relative axial position between the test coil and the magnetic core, enabling experimental validation of the correction models. The results show that the axial effective permeability of cylindrical cores gradually decreases from the center toward both ends and exhibits a symmetric distribution with respect to the center. Experimental results are generally consistent with predictions of the piecewise correction model in terms of the axial distribution characteristics, while noticeable deviations remain for cores with small aspect ratios. The proposed model enables direct calculation of the axial effective permeability distribution of cylindrical ferrite cores from their geometric parameters, providing a theoretical basis for the analysis and optimization of WIMD receivers and other electromagnetic devices employing cylindrical ferrite cores.

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Dr. Xinyu Li

Dr. Kewei Zhu

Dr. Yitong Wu

Dr. Jinyang Gao