27.3 A Piezoelectric Energy-Harvesting Interface Using Split-Phase Flipping-Capacitor Rectifier and Capacitor Reuse Multiple-VCR SC DC-DC Achieving 9.3× Energy-Extraction Improvement

Piezoelectric energy harvesters (PEH) exhibit promising features to scavenge the ambient vibration energy for ubiquitous miniaturized internet of things (IoT) devices. For the traditional PEH using a full-bridge rectifier (FBR), the inherent capacitance (C
_P
) limits the extractable AC-DC electrical power. By extending the damping duration, PEH interfaces employing non-linear techniques, such as the parallel-synchronized-switch harvesting-on-inductor (P-SSHI) [1], can increase the harvestable energy. However, they typically require bulky external high-Q inductors to enhance the extracted power. Recently, various inductor-less PEH interfaces exploiting only capacitors for flipping the PEH voltage during the zero-crossing of the PEH current (I
_P
) have been reported [2-4]. However, their achievable energy extraction improvement depends highly on the number of flipping phases. This work proposes a split-phase flipping-capacitor rectifier (SPFCR) implementation entailing only 4 capacitors, while achieving the highest number of phases (21) when compared to prior art [2-4]. To resolve the lack of input power (P
_in
) adaptability in [2-4], this work proposes a capacitor-reuse multiple voltage-conversion-ratio (VCR) switched-capacitor (SC) DC-DC converter to reduce the charge redistribution loss. Maximum power point tracking (MPPT) is also accomplished using the fractional FBR open circuit voltage (V
_OC,FBR
) for relaxed voltage tolerance, while raising the PEH extracted energy. This work demonstrates a measured 9.3× energy-extraction improvement when compared to a conventional FBR interface.