Energy-efficient integrated circuits for piezoelectric energy harvesting

Abstract

This thesis presents the design, circuit implementation, and measurement results of energy-efficient interface circuits for piezoelectric energy harvesting (PEH).

Chapter 1 introduces the background and motivation for this work. It begins by discussing various application scenarios for wireless sensors and emphasizes the critical need for a sustainable power supply to ensure their long-term operation. Energy harvesting systems are identified as a promising alternative to traditional batteries, with piezoelectric energy harvesting standing out as an ideal solution due to the ubiquitous presence of ambient vibrations in the environment. Since efficient energy conversion requires dedicated interface circuits, the chapter reviews typical circuit architectures and highlights three main challenges in the state-of-the-art: the trade-off between system size and rectifier efficiency, the sensitivity and complexity of maximum power point tracking (MPPT) algorithms, and low end-to-end efficiency due to cumulative energy losses in cascaded architectures.

Chapter 2 provides a comprehensive review of existing interface circuits commonly used in PEH systems. To enhance the output power efficiency of rectifiers, various active rectification techniques have been proposed, such as Synchronized Switch Harvesting on Inductor (SSHI) and Synchronized Switch Harvesting on Capacitor (SSHC). However, SSHI requires bulky inductors, while SSHC depends on multiple dedicated flying capacitors, increasing the system’s overall volume. The chapter also introduces two widely used MPPT techniques—Fractional Open-Circuit Voltage (FOCV) and Perturb and Observe (P&O). Both approaches have their respective drawbacks: FOCV requires open-circuit voltage sampling and flipping efficiency calibration, which results in discontinuous tracking and energy loss; P&O, on the other hand, relies on complex circuitry and consumes significant power. Finally, the chapter analyzes the issue of cascaded energy losses in current system architectures, which leads to relatively low end-to-end efficiencies, typically ranging from 50% to 80%.

Chapter 3 addresses the challenge of minimizing rectifier volume without compromising efficiency by proposing a synchronized switch harvesting rectifier that utilizes reusable storage capacitors. In this design, three capacitors are shared to function both as energy storage elements and as temporary flying capacitors during the energy harvesting and piezoelectric transducer (PT) voltage flipping phases. These capacitors are dynamically reconfigured into nine connection states during the flipping period, effectively replicating the functionality of conventional SSHC flying capacitors. This sharing and reconfiguration technique significantly reduces system size. Measurement results show a PT voltage flipping efficiency of 78%, demonstrating the design’s potential for compact, high-efficiency energy harvesting applications.

Chapter 4 proposes a duty-cycle-based (DCB) MPPT algorithm to overcome the limitations of the FOCV and P&O techniques. The DCB algorithm establishes a direct relationship between the rectifier’s on-off duty cycle and its maximum power point (MPP). Mathematical analysis shows that maintaining a 50% duty cycle allows the system to operate at its MPP. Unlike FOCV, this approach eliminates the need for open-circuit voltage sampling and flipping efficiency calibration. It also avoids the complex power computations and hardware overhead associated with P&O. In addition to its simplicity, the DCB method offers robust tracking performance. Experimental results demonstrate a peak MPPT efficiency of up to 98%, with an average tracking efficiency of 94%.

Chapter 5 presents a single-stage bias-flip rectifier to address the issue of cascaded energy loss in conventional PEH system architectures. This design transfers energy directly from the PT to the output capacitor, reducing intermediate losses. By fixing the rectifier’s on-off duty cycle at 50% to achieve MPPT, the need for a separate rectified capacitor is eliminated, resulting in a shorter startup time and faster MPPT response. Experimental results show an end-to-end efficiency of up to 92.5%, with energy extraction performance improved by a factor of 9.3× compared to a full-bridge rectifier (FBR).

Chapter 6 summarizes the main findings of the thesis and compares the proposed designs in Chapters 3, 4, and 5 with the current state-of-the-art. It also outlines potential directions for future work, including 1) the development of a fully capacitive rectifier with output regulation, 2) MPPT strategies under non-ideal sinusoidal excitation conditions, and 3) power limit analysis and corresponding optimization techniques.