A Discontinuous Charging Technique with Programmable Duty-Cycle for Switched-Capacitor Based Energy Harvesting Circuits in IoT Applications

Abstract

This paper presents a discontinuous charging technique for switched-capacitor (SC) converters that delivers current in bursts to the load. The purpose of the proposed approach is to improve power conversion efficiency(PCE) at low ambient input power in energy harvesting circuits for Internet of Things applications. Current is delivered to the load only during non-overlap time $\pmb{(\text{t}_{\text{NOL}})}$ . The $\pmb{\text{t}_{\text{NOL}}}$ time, and consequently the clock duty cycle, are programmed to control the average power delivered to the load based on ambient power levels, as opposed to battery voltage level in existing chargers. This allows the system to tune the maximum PCE to the expected, application-specific, ambient power levels. The circuit is modelled and designed in 90nm CMOS process. For a given 40k!Ω load, the proposed discontinuous system achieves 71.5% peak power conversion efficiency at 6.87uW input power when operating at 258KHz with 10% $\mathbf{t}_{\pmb{\text{NOL}}}$ . Compared to continuous SC charging, the proposed design achieves the same peak power conversion efficiency at 78% less input power.