Tiny Piezoelectric Harvesters: Principles, Constraints, and Power Conversion

Wireless microsystems can add intelligence to hospitals, homes, and factories that can save money, energy, and lives. Unfortunately, tiny batteries cannot store sufficient energy to sustain useful microsystems for long, and replacing or recharging the batteries of hundreds of networked nodes is costly and invasive in the case of the human body. Thankfully, shocks and vibrations are prevalent in many applications, so ambient kinetic energy can continually replenish batteries to extend the life of the systems they support. And since tiny devices produce minimal damping effects on motion, they can draw as much power as the microelectronics allow. Unfortunately, uncollected charge, breakdown voltages, and energy losses limit how much power harvesting microsystems can generate. This is why this paper reviews how tiny transducers generate power and how state-of-the-art diode bridges and switched inductors and their derivatives draw and output as much power as possible. Of prevailing technologies, in fact, the recycling bridge pre-damps the transducer at the highest voltage possible all the time to output the highest power. But because it still needs a regulating charger to stay at its maximum power point, other pre-damping switched inductors suffer lower losses and require less space. Although the pre-damping bridgeless solution pre-damps every other half cycle, it generates comparable power with only two switches. No harvester, however, escapes the limits that power losses and breakdown voltages impose, so output power is always finite, and in the case of miniaturized systems, not very high.