33.6 A Wireless Power Transfer System with Up-to-20% Light- Load Efficiency Enhancement and Instant Dynamic Response by Fully Integrated Wireless Hysteretic Control for Bioimplants

Wireless power transfer (WPT) systems are becoming increasingly popular for sub100mW biomedical applications [1] -[5]. Because the received power is sensitive to coupling and loading conditions, power/voltage regulations are essential to achieve stable and accurate power delivery, fast transient response, and high end-to-end (E2E) efficiency, which includes all the power losses in the transmitter (TX), wireless power link, and the receiver (RX). Many existing WPT designs operated in open-loop [3] -[5]; or achieved voltage regulation but only in the RX [6], with the TX remained unregulated and designed to operate at full capacity, thus degraded E2E efficiency at light-load conditions. Because lower-power or standby mode typically contributes to the majority of the operation time, light-load efficiency is always an important specification of power management circuits, especially to extend the run time for battery-powered devices, e.g., a wearable/portable WPT transmitter supporting bioimplants. [1], [2], [7] -[9] have reported different approaches to achieve TX regulation; however, all required extra discrete components, which increased the form-factor and cost. [7], [8] required a wire to close the loop. [1], [2], [9] utilized load-shift-keying (LSK) backscattering for TX regulation, which was proved an effective solution. However, [2], [9] relied on lots of off-chip components, including power inductors, diodes, DACs, FPGAs, etc., due to the analog control methodologies. The linear control also introduced small-signal bandwidth limitations, which required careful design to ensure stability at different loading/coupling conditions with PVT/component variations, and resulted in significant compromise in dynamic performance. [1] introduced a nonlinear constant-idle-time control to eliminate the bandwidth limitations and most of the off-chip components; however, the light-load efficiency still suffered. In addition, [1] still required an extra sensing coil to extract LSK signals that increased the TX coil area by 86%.