Electrostatic energy harvester and Li-ion charger circuit for micro-scale applications

Modern portable micro-systems like biomedical implants and ad-hoc wireless transceiver micro-sensors continue to integrate more functions into smaller devices, which result in low energy levels and short operational lives. Researchers and industry alike are consequently considering harvesting energy from the surrounding environment as a means of offsetting this energy deficit. Even with power efficient designs, low duty-cycle operation, smart power-aware network architectures, and batteries with improved energy density, the stored energy in micro-scale systems is simply not sufficient to sustain extended lifetimes. Fortunately, the surrounding environment is a rich source of energy, from solar and thermal to kinetic, but harnessing it without dissipating much power in the process is challenging. In this paper, an electrostatic vibrational energy harvester circuit is proposed and evaluated. It harnesses energy from inherent vibrations in the system (e.g., engine-powered applications) by modulating the parallel-plate distance of a variable capacitor and channeling the resulting change in charge into a secondary Li-lon micro-battery. The varactor, in essence, behaves like a vibration-dependent current source. Simulations how that a 100-to-1 pF variable plate capacitor subjected to vibrations with a period of 15 mu s produces an average harvesting current of 40.8 mu A, an energy gain of 569 pJ per cycle, and a net average power gain of 38 mu W.