Optimization of Sine-Wave Clocking for High-Frequency AC–DC Conversion

Wireless energy harvesting devices convert received ac energy into dc voltages suitable to power the back-end functionality of the devices. The low energy available to the devices require high ac-dc conversion efficiency in order for enough power to be delivered to the load. This paper presents a model to characterize loss through charge pump cells in wireless energy harvesting devices. The proposed model includes the time-domain effects of the input radio-frequency energy wave and provides additional insight into how clock and switch parameters along with architecture considerations can be used to improve the efficiency of ac-dc conversion. Results are verified using simulation in a 0.18-μm CMOS technology. We show the impact of threshold voltage on reverse conduction and the limitations on increasing transistor switch sizes to support high current loads. Design examples use the presented model to optimize design parameters to decrease loss in the charge pump. We compare the performance between sine-wave and square-wave clocked charge pumps to show the tradeoff between charge pump loss and clock generation power consumption. Furthermore, the benefits of easily computing architectural changes is demonstrated using the proposed model showing how the calculated equivalent resistance can be used to determine the benefits of mixed-mode clocking.