As more and more everyday objects are embedded with the capabilities of sensing, processing, and communication, the internet of things (IoT) has been developed. Conventional power supply approaches relying on batteries are no longer adequate due to the rapidly growing cost of battery recharging or replacement. Energy harvesting technology is a promising solution due to omnipresent energy in the environment such as thermal, light, and mechanical energy that can be converted to electrical energy by particular energy transducers. However, the output voltage and power of energy transducers do not match the supply voltage and power of load devices, so an interface circuit is required for voltage conversion and power management. This thesis focuses on research and design of high-efficiency en...[ Read more ]

As more and more everyday objects are embedded with the capabilities of sensing, processing, and communication, the internet of things (IoT) has been developed. Conventional power supply approaches relying on batteries are no longer adequate due to the rapidly growing cost of battery recharging or replacement. Energy harvesting technology is a promising solution due to omnipresent energy in the environment such as thermal, light, and mechanical energy that can be converted to electrical energy by particular energy transducers. However, the output voltage and power of energy transducers do not match the supply voltage and power of load devices, so an interface circuit is required for voltage conversion and power management. This thesis focuses on research and design of high-efficiency energy harvesting interface circuits and systems based on reconfigurable switching converters.

First, the thesis investigates an interface for thermoelectric generators (TEG) that transform thermal energy into electrical energy with a low output voltage (< 100 mV). Previous works usually assumed TEG to be a single-polarity dc source and their interfaces can only handle positive input voltages. Nevertheless, the polarity of TEG voltage depends on the temperature gradient, that is, TEG can be a dual-polarity dc source in practice. Therefore, this thesis presents an auto-polarity interface based on a reconfigurable boost/buck-boost converter for TEG. A collaborative efficiency-improving scheme of frequency selection and maximum power point tracking (MPPT) is implemented for a wide range of input power from 1 µW to 800 µW. An improved digital zero-current detection (ZCD) technique with fast searching is realized to turn off power switches accurately. In addition, dual-polarity cold startup is accomplished with a pair of cross-coupled Dickson charge pumps. This work is fabricated with a 0.13-μm CMOS process and achieves end-to-end efficiencies higher than 80% for input voltages from 90 mV to 0.4 V and from −110 mV to −0.4 V.

Next, the thesis investigates an interface capable of both energy harvesting and recycling. Power harvested from energy transducers is fluctuant or discontinuous due to unpredictable environment conditions, so an energy storage is required to accomplish power balance between the source and load. Therefore, the thesis presents a dual-frequency dual-input-dual-output interface based on a reconfigurable boost/buck converter. The dual-frequency operation decouples the rate of energy extraction at the input stage from the rate of energy distribution at the output stage. The system automatically makes transitions among two energy-harvesting modes and one energy-recycling mode according to the load condition. The low-frequency harvesting together with the high-frequency recycling allows the interface to maintain high efficiencies across a broad range of load power. Besides, a real-time-calculation ZCD technique turns off the power switches instantaneously and accurately to prevent synchronization loss. The interface is fabricated with a 0.13-µm CMOS process and achieves end-to-end efficiencies higher than 80% for load power from 1 μW to 10 mW with 100-μW harvested power.

Multiple different energy transducers working together can produce more stable power than a single energy transducer. Most previous multi-source interfaces apply to dc sources only. Nevertheless, interfaces compatible with both dc and ac sources allow a broader range of applications. Therefore, this thesis presents a vibration and light hybrid interface for a piezoelectric harvester (PEH) and a photovoltaic cell (PVC) which are a common ac source and a common dc source, respectively. The interface consists of a flipping-input synchronized-switch harvesting-on-capacitors (SSHC) rectifier and a modified dual-input boost converter. Contrast to flipping capacitors in conventional SSHC rectifiers, this work flips the input of the rectifier instead. Consequently, low-density metal-insulator-metal (MIM) capacitors can be replaced by high-density metal-oxide-semiconductor (MOS) capacitors. The rectified voltage of PEH and the output voltage of PVC are fed into the boost converter and regulated by a generic MPPT scheme. The interface is fabricated with a 0.18-µm CMOS process. The rectifier achieves a voltage flipping efficiency of 0.75 and the converter achieves a peak conversion efficiency of 95%.