A 13.56 MHz CMOS Active Rectifier With Switched-Offset and Compensated Biasing for Biomedical Wireless Power Transfer Systems

A full-wave active rectifier switching at 13.56 MHz with compensated bias current for a wide input range for wirelessly powered high-current biomedical implants is presented. The four diodes of a conventional passive rectifier are replaced by two cross-coupled PMOS transistors and two comparator- controlled NMOS switches to eliminate diode voltage drops such that high voltage conversion ratio and power conversion efficiency could be achieved even at low AC input amplitude |V_AC|. The comparators are implemented with switched-offset biasing to compensate for the delays of active diodes and to eliminate multiple pulsing and reverse current. The proposed rectifier uses a modified CMOS peaking current source with bias current that is quasi-inversely proportional to the supply voltage to better control the reverse current over a wide AC input range (1.5 to 4 V). The rectifier was fabricated in a standard 0.35 μm CMOS N-well process with active area of 0.0651 mm². For the proposed rectifier measured at |V_AC| = 3.0 V, the voltage conversion ratios are 0.89 and 0.93 for R_L=500 Ω and 5 kΩ, respectively, and the measured power conversion efficiencies are 82.2% to 90.1% with |V_AC| ranges from 1.5 to 4 V for R_L=500 Ω.