Automatic load control for highly efficient microwave rectifiers

This paper presents an adaptive microwave rectifier at 2.45 GHz with automatic load control to improve the conversion efficiency. The conversion efficiency of a rectifier usually changes as the input power level changes, mainly due to the diode efficiency loss associated with built-in voltage or breakdown effect. Therefore, the conversion efficiency can achieve an optimal value only at a specific input power level, and decreases when the input power deviates from this value. The proposed microwave rectifier can automatically tune the load to the optimal value at each input power to maintain a high conversion efficiency within a wide range of input power. An automatic control circuit uses a micro controller unit (MCU) to select various load resistors. By sensing the rectified DC voltage and the current load resistance, the MCU can automatically tune the load to the desired value based on a pre-loaded lookup table. When the input power changes from 5 to 70 mW, the rectifier can achieve optimal conversion efficiencies over 62.5% through automatic tuning the load from 800 to 250 Ω.