A new digital locking MPPT control for ultra low power energy harvesting systems

Abstract

This paper presents a new control technique for maximum power delivery of solar energy harvesting systems. The new technique is based on studying the solar cell characteristics, then determining the trajectory of the maximum power across different lighting conditions. The study is based on connecting the solar cell to the power converter, and finding a relationship between the charge pump optimum frequency and the solar cell optimum voltage that delivers the maximum power of the solar cell. The goal of the control unit is to match this frequency-voltage relationship without sensing circuits and/or decision generation circuits. The solar model used maximally delivers 1.5mW, the open circuit voltage is below 550mV. The harvester should deliver a 1.2 V supply voltage. The control unit consists of an 8-bit low power SAR analog-to-digital converter, exponential decoder and a digitally-controlled oscillator. The control unit power consumption is less than 120μW. The power efficiency reaches 43.6% at 975μW available solar power. The technology used for simulations is Global Foundaries 65 nm CMOS.