Design of a Battery Charger Boost Converter for Extremely Low Supply Voltages Below 10 mV in Energy Harvesting: A Return-on-Investment Approach

This paper proposes a battery charger boost converter for extremely low supply voltage targeting values below 10 mV in energy harvesting applications, based on a return-on-investment (ROI) approach. The gate control circuit is powered by a battery $V_{B}$, providing a sufficient voltage for operation (investment), while the power from an energy transducer with an extremely low supply voltage $V_{I N}$ is supplied to the battery (return). The boost converter is designed to minimize $V_{I N}$ under the condition that the return exceeds the investment, i.e., the battery receives a net positive charging current. A circuit model is proposed for designing this ROI-based battery charger boost converter, demonstrating operation with $V_{I N}=10 \text{mV}$ and $V_{B}=1.5 \mathrm{V}$. The boost converter was fabricated in 180 nm CMOS process with 1.8 V transistors to evaluate its characteristics as a function of the operation frequency and the channel width of the pull-down NMOSFETs. Additionally, the boost converter was evaluated as a battery charger for a 2.3 V all-solid-state battery.