A 40.68-MHz Dual-Output Wireless Power Transfer System for Millimeter-Scale Biomedical Implants

This article presents a single-link, dual-output wireless power transfer (WPT) system operating at 40.68 MHz for miniature, high-power biomedical implants. The elevated carrier frequency enables a millimeter-scale receiver (RX) coil while maintaining link efficiency and output power comparable to low-frequency WPT designs. End-to-end (E2E) efficiency is optimized through global power modulation using a dynamic <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small>-time (DOT) algorithm in conjunction with a fully integrated load-shift-keying (LSK) uplink. At the RX, a single-stage dual-output resonant-current-mode (DORCM) rectifier with single-mode zero-crossing-based control achieves zero-voltage switching (ZVS) and robust adaptability over varying coupling conditions. At the transmitter (TX), an adaptive-ZVS (AZVS) class-D power amplifier (PA) minimizes switching losses and electromagnetic interference (EMI). Both the TX and RX chips are fabricated in a 180-nm BCD process. Measurement results demonstrate dual-output voltage regulation at 1.2 and 2 V under DOT control (DOT Ctrl), with smooth switching behaviors observed at both the TX and RX. The DORCM RX supports seamless cold-start-up operation and achieves a peak power conversion efficiency (PCE) of 90.3% at 90.4-mW output power. The peak E2E efficiency is 51.2% at a coupling factor of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k$</tex-math> </inline-formula> = 0.2. Enabling DOT modulation improves E2E efficiency by up to 18.2%, corresponding to a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.2{\times }$</tex-math> </inline-formula> reduction in TX input power. The system delivers up to 149.7-mW output power with a TX input voltage of 5 V.