Analysis of Series-Connected Double-Layer Coils for MHz Inductive Power Transfer

Abstract

Double-layer coils (DLCs) have been extensively investigated for compact inductive power transfer (IPT) systems operating in the MHz frequency range. Different from previous studies which focus mainly on realizing self-resonance and enhancing the Q factor, this paper enhances both the Q factor and the self inductance to achieve high efficiency and guarantee target output voltage when deployed in IPT applications. By using a lumped-element model derived from transmission line concept, this paper shows that the self inductance when the two layers are serially connected is approximately more than 3 times of that when the two layers are open-ended. Motivated by this feature, we focus on the series-connected DLC and investigate a resonance scheme using two external capacitors: one inserted between the two layers and the other inserted outside the coil. In this resonance scheme, parameters of the capacitors are chosen not only to enhance the Q factor but also to maintain the self inductance. Our air-cored sample coils of 100 mm outer diameter exhibit self inductance of 7.69 μH, Q of 308 at 6.78 MHz, and 94% power transfer efficiency at 50 mm transmission distance. These results are attractive when compared to recent self-resonant open-ended DLCs having similar dimensions.