Design and implementation of efficient energy harvesting circuits for ultra low power and impact energy applications

DESIGN AND IMPLEMENTATION OF EFFICIENT ENERGY HARVESTING CIRCUITS FOR ULTRA LOW POWER AND IMPACT ENERGY APPLICATIONS By Tao Feng The battery-powered electronic systems, such as wearable and mobile device, wireless sensors, implantable bio-medical devices, etc., have widely appeared in our daily life to improve the life quality and work efficiency. However, the dependence on batteries has brought great challenges to current electronic devices in many aspects, such as system miniaturization, massive deployment, device lifetime and environmental pollution, etc. To address these issues, the idea of harvesting the energy from the ambient environment instead of the battery has been proposed. Various energy harvesting technologies have been implemented as the alternative solutions for the battery-powered electronic devices. However, low PCE (power conversion efficiency) prevents these energy harvesting technologies from being widely adopted in practical applications. Another major concern for energy harvesting technologies is the power threshold which is the minimum energy that can be harvested by the energy harvesting systems. Actually the power threshold sets the lower limit of the capability for energy harvesting systems, therefore, overcoming the threshold effect could significantly improve the efficiency of the energy harvesting systems. The objective of this dissertation is to investigate the power conversion efficiency and the power threshold of energy harvesting technologies, and then propose several solutions to overcome these issues. Firstly, the RF energy harvesting front-end which consists of an antenna, matching network and rectifier is examined due to pervasive wireless power in the environment. The conventional matching network in the near field RF energy harvesting front-end needs to trade-off between the coupling coefficient and Q-factor. In chapter 2, a high-Q series resonant matching network is proposed so that both of high coupling coefficient and high-Q matching network can be obtained at the same time, therefore the power conversion efficiency of the near filed RF energy harvesting can be significantly improved. Besides the matching network, the rectifier also plays a key role in RF energy harvesting systems. The major bottleneck of the RF rectifier is nearly zero power transfer efficiency when the input power is below the power threshold. Such operating region is called "dead zone" for the RF rectifier. To free the dead zone, a hybrid rectification technique which utilizes both of RF power and vibration power to efficiently harvest the energy below the threshold level is proposed in chapter 3. This technique utilizes low-frequency and low-power piezoelectric signal as the biasing circuit of the RF rectifier so that the threshold voltage of the RF rectifier can be optimized and thus effective power threshold is significantly reduced. In addition to the RF energy harvesting, vibration energy harvesting is also crucial, especially for the sensors embedded inside the structural and/or underneath the ground. Compared to continuous RF energy, the vibration energy is impulsive so that it is difficult for the harvester to capture the whole energy during a short pulse period. However, if the impulsive energy can be stretched in the time domain while keeping the total energy constant, the harvesting duration will become longer so that it is possible for the harvester to harvest the whole energy. Based on this idea, a nonlinear compressive circuit, named "time-dilation circuit", is proposed in chapter 4. This technique can quickly respond to the impulsive energy so that the dynamic harvesting range is extended. Comparing to electromagnetic propagation, acoustic propagation has less attenuation in the conductive media. Thus, the ultrasonic energy harvesting attracts more interest for the sensors embedded inside composite materials or human body. Furthermore, the use of ultrasound also allows miniaturization of the embedded telemetry system by relaxing the transducer size. In chapter 5, a CMOS system-on-chip (SOC) ultrasonic receiver tag is implemented to investigate the ultrasonic energy harvesting and telemetry. This system can not only harvest its operational energy directly from the acoustic interrogation signal, but also achieve bi-directional communication using acoustic back-scattering, thus it is successfully shown that the ultrasonic energy harvesting and telemetry is a proper solution for such applications. Finally, the contributions of this dissertation and open problems for future work are summarized in chapter 6. This dissertation is dedicated to my family: my dear mother Ying Liu, my passed father Yucai Feng, my wife Ling Sun and my lovely son Kevin Jinyuan Feng as well as my parents-in-law Yuanbi Sun and Yurong Ma.