Three terminal piezoelectric energy harvester based on novel MPPT design

Piezoelectric energy harvesters (PEHs) have been regarded as a feasible solution for microwatt power generators as they produce sufficient power to drive low-power electronic devices such as smart wireless sensors nodes [1]. In ultra-low power applications, such as battery-free sensor nodes based on vibration energy harvesting, the level of power to be transferred is often so low that it is difficult to design a Maximum Power Point Tracking (MPPT) circuitry efficient enough to consider its implementation worthwhile [2]. A vibrating piezoelectric element can be considered as an AC source in parallel with its internal capacitance [2]. It then needs to be rectified at a desired DC voltage level before the harvested energy is stored. Therefore, it is necessary to design an energy efficient PEH circuit, minimizing the nonscalable losses in the circuitry, especially when dedicated to low-power applications. The currently available adaptive circuits that can operate independent of the piezoelectric parameters and device loads have certain limitations such as the requirement of a complex electronic interface, like a dedicated microprocessor and an analog-to-digital (A/D) converter [3], [4] which inevitably dissipate a large part of the harvested energy [5]. To make MPPT worthwhile in ultra-low power energy harvesting applications, a negligible portion of the harvested power has to be accounted for the implementation of these functions. Since this may represent a tough challenge for IC designers, we propose a three terminal piezoelectric energy harvester with one of the terminals exclusively dedicated to sense the open circuit voltage (Voc) of the PEH. This approach can be advantageous as long as the Voc sensing cell is designed with area occupancy negligible compared to the main harvester. From a system point of view this loss in power and the presence of an extra pin is compensated by various advantages including the simplification of the circuit architecture associated to MPPT functions (e.g. pre-regulation, sampling, series switch and logic) and the reduction of the power absorption. With this new concept of piezoelectric energy harvester, the power management IC can be greatly simplified to a simple ultra-low power comparator, used to compare the voltage provided by the main harvester with the voltage provided by the Voc sensing cell.