A 6.78 MHz, 95.0% Peak Efficiency Monolithic Two-Dimensional Calibrated Active Rectifier for Wirelessly Powered Implantable Biomedical Devices

In this paper, a fully integrated active rectifier with triple feedback loops is proposed to enhance power conversion efficiency (PCE) over a wide loading range by calibrating both the gate transition timing and power switch size. The on- and off-transitions of the power switches are calibrated using a hybrid delay-based gate control circuit (HDGCC) with hybrid feedback loops. Conventional active rectifiers that only focused on calibrating the gate transition timing of a NMOS power switch with a fixed power switch size exhibit a low PCE when the loading condition deviates from the predetermined range. Thus, an automatic size selector based on a third feedback loop is proposed, which changes the power switch size based on the loading condition and ensures a stable operation of the hybrid loops by maintaining the voltage drop across the NMOS switches. An active rectifier was fabricated using the standard 0.18 μm CMOS process. The effectiveness and robustness of the two-dimensional calibration were verified through measurements under an AC input voltage ranging from 2.5 to 5.0 V and an output power ranging from 1.25 to 125 mW. The peak voltage conversion ratio and peak PCE were 97.6% and 95.0%, respectively, at R L = 500 Ω.