30.3 A Bias-Flip Rectifier with a Duty-Cycle-Based MPPT Algorithm for Piezoelectric Energy Harvesting with 98% Peak MPPT Efficiency and 738% Energy-Extraction Enhancement

Synchronized bias-flip rectifiers, such as synchronized switch harvesting on inductor (SSHI) rectifiers, are widely used for piezoelectric energy harvesting (PEH) [1], which can replace the use of batteries in many loT applications, thus reducing both system volume and maintenance cost. However, the output power extracted by such rectifiers strongly depends on the impedance matching between the piezoelectric transducer (PT) and the circuit. To maximize this, two maximum power point tracking (MPPT) algorithms are often used. As shown in Fig. 30.3.1 (left), the Perturb & Observe (P&O) (a.k.a. hill-climbing) algorithm adjusts the rectified output power in a stepwise manner towards the maximum power point (MPP), thus establishing robust and continuous MPPT. However, accurately sensing the rectified output power often requires complex and power-hungry hardware [1], [2]. Another simpler algorithm is based on the fractional open-circuit voltage (FOCV) and involves periodically measuring the PT's open-circuit voltage amplitude $(\mathrm{V}_{\text{OC}})$ and regulating the rectified voltage $(\mathrm{V}_{\text{REC}})$ to a level $(\mathrm{V}_{\text{MPP}})$, which corresponds to the MPP [3–6]. However, the PT must be periodically disconnected from the rectifier to measure $\mathrm{V}_{\text{OC}}$, resulting in wasted energy, while the inherent delay in sensing $\mathrm{V}_{\text{OC}}$ variations reduces the overall tracking efficiency. Furthermore, a calibration step is usually necessary to determine $\mathrm{V}_{\text{MPP}}$, since this depends on the actual PT voltage flip efficiency $(\eta_{\mathrm{F}})$ of the bias-flip rectifier.