Contents I. Introduction
Piezoelectric energy harvesters (PEHs) are used to extract energy from mechanical vibrations. In order to convert the energy from ac to dc, track the maximum-power-point (MPP) of the PEH, and store maximum energy on the storage node, an energy-harvesting interface circuit, including a rectifier and a dc–dc converter is usually adopted, as shown in Fig. 1. Different types of PEH rectifiers have been developed, like a full-bridge rectifier (FBR) [1], a synchronous electric charge extraction (SECE) rectifier [2], and a bias-flip rectifier, including its inductor implementation, synchronized-switch harvesting-on-inductor (SSHI) [3], [4] and capacitor implementation, synchronized-switch harvesting-on-capacitor (SSHC) [5]. To compare their performance, a figure of merit (FOM), which is also called energy-extraction improvement, is defined in [6] \begin{equation*} {\mathrm{ FOM}}={P_{\mathrm{ REC}} / \left ({C_{P} \cdot V_{OC} ^{2} \cdot f}\right )}\tag{1}\end{equation*} where , , , and stand for rectifier output power, PEH parasitic capacitance, PEH open-circuit voltage, and vibration frequency. This FOM represents the energy-extraction capability of each rectifier by normalizing their output power with the ideal FBR. The FBR has a low energy-extraction capability with an FOM < 100% due to compensating the energy on the PEH capacitor every half-cycle. An SECE has a medium FOM with a reported value of 420% [2] because it still needs to charge the PEH capacitor from zero voltage. Instead of compensating the energy on the capacitor or charging the capacitor, the bias-flip rectifier flips the voltage on every half-cycle using an inductor or capacitors, so the current generated from the PEH can directly go through the rectifier, which improves the energy-extraction capability and makes it have the highest performance among different rectifiers. The reported highest FOM for parallel-SSHI is 681% [3] and 970% for SSHC [5].
Block diagram of a general piezoelectric energy-harvesting system.
