Enhanced Charging Energy Efficiency via Optimised Phase of Directly Charging an Energy Storage Capacitor by an Energy Harvester

Abstract

This paper presents a technique to enhance the charging time and efficiency of an energy storage capacitor that is directly charged by an energy harvester from cold start-up based on the open-circuit voltage (VOC) of the energy harvester. The proposed method charges the capacitor from the energy harvester directly until the capacitor voltage reaches 0.75VOC of the energy harvester, which corresponds to an equivalent charging efficiency of over 81% based on theoretical analysis. This charging efficiency is much higher than relying on a boost converter to charge up the capacitor when the energy harvester is low as most of the functioning blocks cannot operate at low voltage. A control circuit based on RC high-pass and low-pass filters, and a comparator was introduced to detect the instance when the capacitor is charged up to 0.75VOC of the energy harvester. A circuit was built to verify the proposed method and tested with a piezoelectric energy harvester. The proposed circuit achieved an enhanced charging time of over 91% faster and a final capacitor voltage of 317% higher than a conventional boosting scheme.