A Highly Efficient Ultralow Photovoltaic Power Harvesting System With MPPT for Internet of Things Smart Nodes

Implementing a monolithic highly efficient ultralow photovoltaic (PV) power harvesting system is pivotal for smart nodes of Internet of things (IOT) networks. This paper proposes a fully integrated harvesting system in 0.18-μm CMOS technology. Utilizing a small commercial solar cell of 2.5 cm², the proposed system can provide 0-29 μW of power, which is much higher than the commonly used passive radio-frequency identification devices in IOT application. The hill-climbing maximum power point tracking algorithm is developed in an energy-efficient manner to tune the input impedance of the system and guarantee adaptive maximum power transfer under wide illumination conditions. The detailed impedance tuning approach is implemented with a capacitor value modulation to eliminate the quiescent power consumption as well as to achieve a higher efficiency than the traditional pulse-frequency modulation scheme. A supercapacitor is utilized for buffering, energy storing, and filtering purposes, which enables more functions of the IOT smart nodes such as active sensing and system-on-chip (SOC) signal processing. The output voltage ranges between 3.0 and 3.5 V for different device loads, such as sensors, SOC, or wireless transceivers. The measured results confirm that this PV harvesting system achieves both ultralow operation capability under 20 μW and a selfsustaining efficiency of 89%.