In recent years, the Internet of Things (IoT) era is growing in momentum. It is desirable that all IoT devices are designed to be in-situly energy autonomous via energy harvesting, since battery replacement in such astronomical scale is laborious and expensive. This thesis first reviews the state-of-the-art of microwatts energy harvesting sources and storage capacitors. Based on the electrical characteristics of different energy harvesting sources and storage capacitors, an integral multi-source energy harvesting scheme is proposed to power a multi-level supply voltage wireless sensor node at system level. Circuit level innovation associated with building the proposed system architecture, i.e. linear regulator, switched-inductor and switched-capacitor converters are then pursued. The ci...[ Read more ]

In recent years, the Internet of Things (IoT) era is growing in momentum. It is desirable that all IoT devices are designed to be in-situly energy autonomous via energy harvesting, since battery replacement in such astronomical scale is laborious and expensive. This thesis first reviews the state-of-the-art of microwatts energy harvesting sources and storage capacitors. Based on the electrical characteristics of different energy harvesting sources and storage capacitors, an integral multi-source energy harvesting scheme is proposed to power a multi-level supply voltage wireless sensor node at system level. Circuit level innovation associated with building the proposed system architecture, i.e. linear regulator, switched-inductor and switched-capacitor converters are then pursued. The circuit prototypes are implemented on a standard 0.13-μm CMOS process.

A shunt regulator is designed to provide voltage supervision, regulation and over-voltage protection, and balance individual storage capacitor in a stacked-capacitor system. The core circuit of the shunt regulator is a multi-threshold six-transistor Schmitt-Trigger, which is conventionally regarded as a digital logic cell that consumes minimal quiescent power. The measured mean output voltage is 1.118 V and standard deviation is 8 mV.

Conventional transformer-based boost converters commonly used to achieve autonomous low voltage start-up encounter low efficiency and potential start-up failures when energy harvesting source with high internal resistance is used. An improved design of transformer-based boost converter with bipolar output option is proposed to solve the start-up problem. Circuit techniques to enable maximum power point tracking feature of this boost converter are also investigated. Minimum start-up voltage of boost converter measured is 21 mV, requiring 5.8 μW of input power to generate 1 V output, with peak efficiency of 74%.

Instead of permitting shunt regulator routes unused power to ground, charge pump is connected in parallel to the shunt regulator, with the intention to transfer the excess power to secondary storage capacitor. Internal power loss mechanisms of charge pump circuits which can be utilized to supplement the shunt regulator are studied. Besides, a bipolar clock driven cross-coupled charge pump and the associated level shifter circuit are presented. Using thick oxide transistor, the proposed charge pump can generate up to 6 V output from 300 mV input.