Maximizing the Power Conversion Efficiency of Ultra-Low-Voltage CMOS Multi-Stage Rectifiers

This paper describes an efficient method to explore the design space of AC/DC converters for energy harvesting circuits. A simple analytical model of the rectifier circuit valid down to ultra-low-voltage operation (input voltage below the thermal voltage) is proposed. Based on the Shockley diode equation, the use of an optimization equation that relates the number of stages of the rectifier to the saturation current of the diodes is demonstrated. A set of universal normalized curves to guide the designer regarding the minimum available power from the AC power source is presented. Three rectifiers with different numbers of stages and saturation currents along with matching networks integrated in a 130 nm CMOS technology demonstrated a conversion efficiency of around 10% for an available power of around -20 dBm and DC load voltage and current of 1 V and 1 μA, respectively.