A 6.78-MHz Wireless Power Transfer System With Dual-Output Resonant Current-Mode Regulating Rectifier and Transmission Power Regulation

This paper introduces a 6.78-MHz wireless power transfer (WPT) system for implantable medical devices (IMDs). The proposed dual-output resonant current-mode (RCM) rectifier cumulates energy from loosely-coupled coils and generates two output voltages (VH = 3-V, VL = 1.8-V) regulated by adaptive power control (APC) and local-loop power control. Due to transmission power regulation (TPR), the transmitter ( $T_{X}$ ) delivers appropriate power to the receiver ( $R_{X}$ ) to realize global-loop power control. Thus, the power transfer efficiency (PTE) is improved, especially under light load. Furthermore, zero-voltage switching and zero-current switching techniques enhance ${R} _{X}$ power conversion efficiency (PCE). $T_{X}$ and $R_{X}$ chips were fabricated in a 0.18- $\mu \text{m}$ CMOS process. The measurement results show that the proposed WPT system successfully regulates outputs at VH = 3-V and VL = 1.8-V at a 1.5-cm coupling distance. With the proposed TPR, PTE is improved by 28.2% at PTOTAL = 1.6-mW, and input power is reduced by 94.8% at PTOTAL = 1-mW. The measured peak PCE and peak PTE are 85.1% and 31.3% at a coil distance of 10-mm, respectively.

I. Introduction

Owing to the miniaturization of electronic circuits, implantable medical devices (IMDs) have been widely adopted in various treatments [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Due to their limited energy capacity, conventional IMDs are powered by embedded batteries, which are replaced periodically. A noninvasive wireless power transfer (WPT) that can power or recharge IMDs is employed to reduce the risk of infection due to battery replacement [12]. Nevertheless, the long coupling distance, misalignment between coils, and the small receiver () coil in medical applications significantly reduce the power conversion efficiency (PCE) and available output power.