UC Santa Cruz

Power conditioning circuit is an essential element for energy harvesting applications. Designed for input rectifying, voltage conversion and output regulation, power conditioning circuit transforms harvested energy from AC to DC and from an unregulated, usually low amplitude input voltage to a specified, well-regulated DC output to power the intended load.

Drastically different from conventional power systems, energy harvesting devices usually collect energy from weak power sources and produce low amplitude AC outputs. These devices typically have high output impedance with pronounced reactance in comparison with a regular power supply. The electrical characteristics of the energy harvesting devices make it difficult or even impractical to use existing power conditioning solutions for energy harvesting applications. For example, the forward voltage drop of a typical rectifier diode will prevent those harvested energies with voltage amplitudes below this threshold to be rectified and delivered to their loads. In fact, many recent energy harvesting work are hampered by the lack of adequate power conditioning circuits and fail to prove their practical purposes and values of their energy harvesting techniques.

Inspired by the early work on piezoelectric wind energy harvesting, this research work mainly focuses on low voltage power conditioning circuits for energy harvesting applications and covered both low voltage rectifier circuits and voltage boosting circuits that are essential for demonstrating the usefulness of energy harvesting techniques. By examining the device physics and operation principle of the existing current rectifying solutions, this work proposed a new low-voltage, high-efficiency hybrid rectifier circuit with low in-line rectifying voltage loss. This new circuit is prototyped and demonstrated to have the highest rectifying efficiency in comparison with other passive rectifier circuits from the literature.

To achieve low-voltage and low-power voltage boosting, this work explored three most representative voltage boosting approaches for energy harvesting applications. It first studied the feasibility of passive voltage boosting method, which uses the reactive nature of the energy harvesting device to enhance its output voltage. By introducing an inductive load to a capacitive piezoelectric energy harvesting device, this research work studied the feasibility, advantages and limitations of the passive voltage boosting technique via transformer circuit, and presented bench measurement data for both voltage conversion ratio and power conversion efficiency.

This work also studied two representative active voltage boosting circuits- charge pump type and inductive boost converter type. Both circuits are bench studied for their minimal input voltage range, voltage boosting capabilities and power conversion efficiencies. Based on these results, a low input voltage power conditioning circuit with hybrid input rectifier is developed and prototyped. Bench test on the prototype circuit showed that it has high power conversion efficiency around 60% at 10mW and low start-up voltage of approximately 0.435V VRMS which is better than others work from the literature. This new power conditioning circuit is tested in two energy harvesting applications (thermoelectric and piezoelectric) and both showed great energy harvesting results.