A 6.78-MHz Wireless Single-Stage Resonant CC-CV Charger with Automatic Coupling Range Extension Scheme for Implantable Medical Devices

A $6.78-\mathrm{MHz}$ wireless, single-stage resonant charger (SSRC) that performs constant current-constant voltage (CC-CV) and automatic coupling range extension for implantable biomedical devices is presented. The proposed SSRC alternates between normal and resonant modes of operations according to environment variations, which improves the available charging range by $2.18 \times$. Also, the proposed charger limits the maximum current in resonant mode, thus guaranteeing safe charging of a given battery within its maximum current value. The proposed SSRC has been fabricated in a $0.18-\mu \mathrm{m}$ BCD process, occupying a chip area of $0.575 \mathrm{~mm}^{2}$. The measurement results verified energy delivery to batteries, while extending the charging range from 22 mm to 48 mm and preventing over-current charging. The measured peak power conversion efficiencies (PCE) were 89.42% and 75% in the normal and the resonant modes, respectively. These results demonstrate that the proposed SSRC can perform high energy-efficient charging even with environment variations.