Maximum power-point extraction of small switched-inductor piezoelectric harvesters

Piezoelectric harvesters are popular today because they typically draw more power from kinetic energy in motion than electrostatic and electromagnetic systems. Still, tiny transducers only derive a small fraction of what is available. Thankfully, raising the damping force with which transducers draw power increases that fraction, except overinvesting battery energy for that purpose can overdamp the system. This is why harvesters monitor output power, and current, which normally requires fast and accurate circuits that consume substantial power. This paper, however, presents a low-loss alternative. The idea is to sense how output power changes by monitoring the time that the switched inductor requires to drain its energy. This way, with readily available parameters, a piezoelectric harvester can estimate the investment that will keep the system within 2.5% of its maximum power point.