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A 13.56 MHz wireless power transfer system with a 1X/2X reconfigurable resonant regulating (R 3 ) rectifier and wireless power control for biomedical implants is presented. Output voltage regulation is achieved through two mechanisms: (1) a local PWM loop at the secondary side controls the duty cycle of mode-switching of the rectifier between the 1X and 2X modes; and (2) a global control loop obtains the mode-switching information from the secondary side and send it back to the primary side through the wireless channel and adjusts the transmitter power of the primary coil to adapt to load and coupling variations. Two novel backscattering uplink techniques are proposed for fast and energy-efficient data feedback. The first is for general data transmission using Manchester code; and the second is for fast duty cycle feedback to cater for fast load-transient responses. Stability analysis of the entire system with the two control loops is also presented. The primary transmitter and the secondary R 3 rectifier are fabricated in 0.35 μm CMOS process with the digital control circuits implemented using FPGA. The measured maximum received power and receiver efficiency are 102 mW and 92.6%, respectively. For load transients, the overshoot and the undershoot are approximately 110 mV and the settling times are less than 130 μs.
Li et al. (Fri,) studied this question.