21.2 A 4µW-to-1mW parallel-SSHI rectifier for piezoelectric energy harvesting of periodic and shock excitations with inductor sharing, cold start-up and up to 681% power extraction improvement

A piezoelectric energy harvester (PEH) converts vibration-induced mechanical stress into electrical charge. This conversion is optimized for cantilever-based generators when excited continuously in resonance. However, this is rarely achieved using ambient vibrations, where changes in excitation frequencies (fEX) and magnitudes, or shock excitations are more common [1]. The implemented system (Fig. 21.2.1) is a fully autonomous energy-harvesting interface based on the parallel-synchronized-switch harvesting-on-inductor (SSHI) technique, also known as bias-flip [2], and can work with periodic as well as shock excitations. It allows enhanced ambient energy extraction by operating at an ideal rectified voltage (VBUF) [2] set by means of an optimal power point circuit (OPP). It operates with different harvesters and a wide variation of accelerations and excitation frequencies. The system achieves cold start-up and provides a configurable output voltage (VLDO) that can power systems such as wireless sensor nodes, biomedical or hand-held devices. It can use a single inductor by means of an inductor-sharing block (IS), and has over-voltage protection (OVP).