Design of Sub-Gigahertz Reconfigurable RF Energy Harvester From −22 to 4 dBm With 99.8% Peak MPPT Power Efficiency

To overcome the low-efficiency and limited working range of the existing RF energy harvesting (EH) systems for the wireless Internet-of-Things (IoT) sensors, a novel reconfigurable system is proposed with integrated hill-climbing, maximum power point tracking (MPPT) function for wide input power from −22 to 4 dBm. A conceptual linear model with high accuracy is also proposed to analyze the rectifier efficiency for MPPT operations. The rectifier with off-chip matching is designed with a patch antenna at 915-MHz the industrial, scientific and medical (ISM) band. To further improve the end-to-end efficiency, the harvested power is used to power up the circuit block in system on a chip (SoC) directly, avoiding additional conversion loss. Our proposed reconfigurable 12-stage rectifier with matching network achieves −18.1-dBm sensitivity for 1- \text{M}\Omega loading and 36% peak efficiency at 1 dBm. The proposed MPPT function can detect and determine the optimal rectifier stage for loading from 10 \text{K}\Omega to 1 \text{M}\Omega . The measured MPPT accuracy is over 87% from −22 to 4 dBm compared to external tuning conditions. The minimum stand-by power is 20 nW at 0.5 V and the overall MPPT power efficiency is over 72% with a peak value of 99.8% including dissipated power. Measurements also show the system can achieve self-startup and self-sustained functions with a 10- \mu \text{F} external capacitor buffer.