Tutorial: Tiny light-harvesting photovoltaic charger-supplies

A fundamental challenge wireless microsystems face is size, and in consequence, lifetime because tiny batteries exhaust quickly. Although small fuel cells and atomic sources store more energy than lithium-ion batteries and super capacitors, they source less power, so they cannot power as many functions. Small batteries and capacitors, however, cannot sustain life for long. Thankfully, the environment holds vast amounts of energy. And of typical sources like light, motion, temperature, and radiation, sunlight produces the highest power density, but only when available. Combining photovoltaic (PV) cells with tiny batteries or capacitors can therefore be more compact, reliable, and longer lasting than any one of these technologies alone. Managing a hybrid system of this sort to supply a milliwatt application, however, requires an intelligent, low-loss charger-supply system. This talk surveys and describes how smart PV-sourced microsystems can draw power from tiny PV cells and supplementary power from small batteries to supply a load and replenish the battery with excess PV power. To that end, the material reviews and discusses miniaturized PV cells, power-efficient charger-supply circuits, and reliable feedback controllers. The presentation ends with measurement results from a prototyped example.