Energy-efficient Interfaces for Vibration Energy Harvesting

Ultra low power wireless sensors and sensor systems are of increasing interest in a variety of applications ranging from structural health monitoring to industrial process control. Electrochemical batteries have thus far remained the primary energy sources for such systems despite the finite associated lifetimes imposed due to limitations associated with energy density. However, certain applications (such as implantable biomedical electronic devices and tire pressure sensors) require the operation of sensors and sensor systems over significant periods of time, where battery usage may be impractical and add cost due to the requirement for periodic re-charging and/or replacement. In order to address this challenge and extend the operational lifetime of wireless sensors, there has been an emerging research interest on harvesting ambient vibration energy. Vibration energy harvesting is a technology that generates electrical energy from ambient kinetic energy. Despite numerous research publications in this field over the past decade, low power density and variable ambient conditions remain as the key limitations of vibration energy harvesting. In terms of the piezoelectric transducers, the open-circuit voltage is usually low, which limits its power while extracted by a full-bridge rectifier. In terms of the interface circuits, most reported circuits are limited by the power efficiency, suitability to real-world vibration conditions and system volume due to large off-chip components required. The research reported in this thesis is focused on increasing power output of piezoelectric transducers and power extraction efficiency of interface circuits. There are five main chapters describing two new design topologies of piezoelectric transducers and three novel active interface circuits implemented with CMOS technology. In order to improve the power output of a piezoelectric transducer, a series connection configuration scheme is proposed, which splits the electrode of a harvester into multiple equal regions connected in series to inherently increase the open-circuit voltage generated by the harvester. This topology passively increases the rectified power while using a full-bridge rectifier. While most of piezoelectric transducers are designed with piezoelectric layers fully covered by electrodes, this thesis proposes a new electrode design topology, which maximizes the raw AC output power of a piezoelectric harvester by finding an optimal electrode coverage. In order to extract power from a piezoelectric harvester, three active interface circuits are proposed in this thesis. The first one improves the conventional SSHI (synchronized switch harvesting on inductor) by employing a startup circuitry to enable the system to start operating under much lower vibration excitation levels. The second one dynamically configures the connection of the two regions of a piezoelectric transducer to increase the operational range and output power under a variety of excitation levels. The third one is a novel SSH architecture which employs capacitors instead of inductors to perform synchronous voltage flip. This new architecture is named as SSHC (synchronized switch harvesting on capacitors) to distinguish from SSHI rectifiers and indicate its inductorless architecture

A fully integrated split-electrode synchronized-switch-harvesting-on-capacitors (SE-SSHC) rectifier for piezoelectric energy harvesting with between 358% and 821% power-extraction enhancement

Along with the development of the Internet of Everything (IoE), miniaturized piezoelectric vibration-energy harvesters have drawn significant recent interest as a means of harvesting ambient kinetic energy to power wireless sensors. As the energy generated by a piezoelectric transducer (PT) cannot be directly used, an interface circuit is needed to rectify the generated power and provide a stable supply. Full-bridge rectifiers (FBR) are widely used due to their simplicity despite their low energy efficiency. Recently, various interface circuits have been reported [1-5] to improve power efficiency, such as the SSHI (Synchronized Switch Harvesting on Inductor) rectifier. However, most of these reported circuits require large inductors to achieve good performance, and these inductors significantly increase the system volume, counter to the requirement for system miniaturization. Although a flipping-capacitor rectifier was proposed in [2] to flip voltages using on-chip capacitors, it was designed for high frequency (>100kHz) ultrasonic energy transfer applications and does not work with PTs with a large internal capacitor CP since the values of the capacitors required are too large for on-chip implementation. Another inductorless circuit, named SSHC (synchronized switch harvesting on capacitors), was recently proposed in [1] (Fig. 8.9.1); however, the required switched-capacitor (SC) values must equal CP to achieve optimal performance and this limits the on-chip implementation for PTs with large CP capacitance