Light-harvesting photovoltaic charger–supply microsystems
Damodaran Prabha, Rajiv
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Wireless microsensors not only enable miniaturized applications, like biomedical implants and remote monitors, but can also add intelligence to expensive, previously inaccessible, and difficult-to-replace technologies of scale, like industrial power plants and farms, that improve performance, save energy, and extend operational life. The driving challenge with these devices is limited space because small batteries, for example, store little usable energy, and replacing or recharging a battery requires prohibitively expensive recurring personnel costs. Harvesters circumvent this basic space and cost challenge by continually harnessing energy from the surrounding environment. Of available sources (like thermal, mechanical, magnetic, chemical, and light), solar light produces the highest power density, and although indoor lighting is not as rich, thermal and magnetic sources produce even lower power densities and mechanical and chemical transducers are difficult to integrate. Nevertheless, for a millimeter scale microsensor the area of light exposure is small and therefore harvestable power is low even for solar light and much smaller for indoor conditions. As a result the charger–supply systems that transfer these low power levels have to be efficient, and this at low power levels is difficult. Switched-inductor power transfer circuits provide higher conversion efficiency in comparison with the switched-capacitor counter parts; however they generally use bulky inductors that occupy large volume. Therefore to achieve high efficiency and power density the charger–supply power stage can use no more than a single millimeter scale inductor, input and output capacitors as external passives. Nevertheless achieving high efficiency with millimeter scale inductors that have large series-loss incurring parasitic-resistances is extremely challenging. The problem here is that state-of-the-art light harvesting circuits require almost as much, if not more, power to transfer energy and charge a battery than a miniaturized PV cell can generate. The objective of the proposed research is to study, explore, develop, design, simulate, fabricate, build, test, and evaluate a low-loss CMOS charger-supply photovoltaic (PV) system that draws power from a millimeter CMOS PV cell and assistance from a tiny battery to regulate and supply a milliwatt microsystem and recharge the battery. This research identified the CMOS PV cell configuration that produces maximum output power and built a charger–supply system that is 95% efficient with a 7.2× smaller (lossier) inductor and 88% efficient with a 95× smaller inductor.