Energy Harvesting and Power Management Integrated Circuits for Self-Sustaining Wearables

Harvesting energy from ambient sources can provide power autonomy to energy efficient electronics and sensors. The last decade has seen a multitude of ways to scavenge energy from various sources like solar, thermal, electromagnetic, electrostatic, piezo-electric and many more. Thermal energy from human body heat is ubiquitous and can be harnessed seamlessly across day and night. Micropower generation from human body heat using thermoelectric generators (TEG) can replace battery to power miniaturized, unobtrusive, energy-efficient wearable devices for preventive health care and vital body signs monitoring and make them self-sustainable. This thesis is focused in realizing such a system and presents different integrated power management circuit techniques to solve the primary challenges associated with energy harvesting from human body heat. The first part of the thesis demonstrates an on-chip electrical cold-start technique to achieve low-voltage and fast start-up of a boost converter for autonomous thermal energy harvesting from human body heat. Improved charge transfer through high gate-boosted switches by means of cross-coupled complementary charge pumps enables voltage multiplication of the low input voltage during cold start. The start-up voltage multiplier operates with an on-chip clock generated by an ultra-low-voltage ring oscillator. The proposed cold-start scheme implemented in a general-purpose 0.18 µm CMOS process assists an inductive boost converter to start operation with a minimum input voltage of 57 mV in 135 ms, while consuming only 90 nJ of energy from the harvesting source, without using additional sources of energy or additional off-chip components. A single-inductor, self-starting and efficient low-voltage boost converter is described next, suitable for TEG-based body-heat energy harvesting. In order to extract maximum energy from a thermoelectric generator (TEG) at small temperature gradient, a loss-optimized maximum power transfer (LO-MPT) scheme is proposed that enables the harvester to achieve high end-to-end efficiency at small input voltages. The boost converter is implemented in a 0.18 µm CMOS technology and achieves above 75% efficiency for a matched input voltage range of 15 mV-100 mV, with a peak efficiency of 82%. Enhanced power extraction enables the converter to sustain operation at an input voltage as low as 3.5 mV. In addition, the boost converter self-starts in 252 ms with a minimum input voltage of 50 mV utilizing a dual-path architecture and a one-shot cold-start mechanism. The final section demonstrates a self-sustainable system where a low-power signal conditioning front-end with a unique dynamic threshold tracking loop is designed to decode heart beats from a noisy ECG signal and is powered by human body heat utilizing an autonomous DC-DC converter embedded in the same chip and an off-chip centimeter-scale TEG

Nano-Power Integrated Circuits for Energy Harvesting

The energy harvesting research field has grown considerably in the last decade due to increasing interests in energy autonomous sensing systems, which require smart and efficient interfaces for extracting power from energy source and power management (PM) circuits. This thesis investigates the design trade-offs for minimizing the intrinsic power of PM circuits, in order to allow operation with very weak energy sources. For validation purposes, three different integrated power converter and PM circuits for energy harvesting applications are presented. They have been designed for nano-power operations and single-source converters can operate with input power lower than 1 μW. The first IC is a buck-boost converter for piezoelectric transducers (PZ) implementing Synchronous Electrical Charge Extraction (SECE), a non-linear energy extraction technique. Moreover, Residual Charge Inversion technique is exploited for extracting energy from PZ with weak and irregular excitations (i.e. lower voltage), and the implemented PM policy, named Two-Way Energy Storage, considerably reduces the start-up time of the converter, improving the overall conversion efficiency. The second proposed IC is a general-purpose buck-boost converter for low-voltage DC energy sources, up to 2.5 V. An ultra-low-power MPPT circuit has been designed in order to track variations of source power. Furthermore, a capacitive boost circuit has been included, allowing the converter start-up from a source voltage VDC0 = 223 mV. A nano-power programmable linear regulator is also included in order to provide a stable voltage to the load. The third IC implements an heterogeneous multisource buck-boost converter. It provides up to 9 independent input channels, of which 5 are specific for PZ (with SECE) and 4 for DC energy sources with MPPT. The inductor is shared among channels and an arbiter, designed with asynchronous logic to reduce the energy consumption, avoids simultaneous access to the buck-boost core, with a dynamic schedule based on source priority

Power Management Circuits for Energy Harvesting Applications

Energy harvesting is the process of converting ambient available energy into usable electrical energy. Multiple types of sources are can be used to harness environmental energy: solar cells, kinetic transducers, thermal energy, and electromagnetic waves. This dissertation proposal focuses on the design of high efficiency, ultra-low power, power management units for DC energy harvesting sources. New architectures and design techniques are introduced to achieve high efficiency and performance while achieving maximum power extraction from the sources. The first part of the dissertation focuses on the application of inductive switching regulators and their use in energy harvesting applications. The second implements capacitive switching regulators to minimize the use of external components and present a minimal footprint solution for energy harvesting power management. Analysis and theoretical background for all switching regulators and linear regulators are described in detail. Both solutions demonstrate how low power, high efficiency design allows for a self-sustaining, operational device which can tackle the two main concerns for energy harvesting: maximum power extraction and voltage regulation. Furthermore, a practical demonstration with an Internet of Things type node is tested and positive results shown by a fully powered device from harvested energy. All systems were designed, implemented and tested to demonstrate proof-of-concept prototypes

Energy efficient control for power management circuits operating from nano-watts to watts

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 163-172).Energy efficiency and form factor are the key driving forces in today's power electronics. All power delivery circuits, irrespective of the magnitude of power, basically consists of power trains, gate drivers and control circuits. Although the control circuits are primarily required for regulation, these circuits can play a crucial role in achieving high efficiency and/or minimizing overall system form-factor. In this thesis, power converter circuits, spanning a wide operating range- from nano-watts to watts, are presented while highlighting techniques for using digital control circuits not just for regulation but also to achieve high system efficiency and smaller system form-factor. The first part of the thesis presents a power management unit of an autonomous wireless sensor that sustains itself by harvesting energy from the endo-cochlear potential (EP), the 70-100mV electrochemical potential inside the mammalian inner ear. Due to the anatomical constraints, the total extractable power from the EP is limited to 1.1-6.3nW. A low switching frequency boost converter is employed to increase the input voltage to a higher voltage usable by CMOS circuits in the sensor. Ultra-low power digital control circuits with timers help keep the quiescent power of the power management unit down to 544pW. Further, a charge-pump is used to implement leakage reduction techniques in the sensor. This work demonstrates how digital low power control circuit design can help improve converter efficiency and ensure system sustainability. All circuits have been implemented on a 0.18[mu]m CMOS process. The second part of the thesis discusses an energy harvesting architecture that combines energy from multiple energy harvesting sources- photovoltaic, thermoelectric and piezoelectric sources. Digital control circuits that configure the power trains to new efficient system architectures with maximum power point tracking are presented, while using a single inductor to combine energy from the aforementioned energy sources all at the same time. A dual-path architecture for energy harvesting systems is proposed. This provides a peak efficiency improvement of 11-13% over the traditional two stage approach. The system can handle input voltages from 20mV to 5V and is also capable of extracting maximum power from individual harvesters all at the same time utilizing a single inductor. A proposed completely digital timebased power monitor is used for achieving maximum power point tracking for the photovoltaic harvester. This has a peak tracking efficiency of 96%. The peak efficiencies achieved with inductor sharing are 83%, 58% and 79% for photovoltaic boost, thermoelectric boost and piezoelectric buck-boost converters respectively. The switch matrix and the control circuits are implemented on a 0.35pm CMOS process. This part of the thesis highlights how digital control circuits can help reconfigure power converter architectures for improving efficiency and reducing form-factors. The last part of the thesis deals with a power management system for an offline 22W LED driver. In order to reduce the system form factor, Gallium Nitride (GaN) transistors capable of high frequency switching have been utilized with a Quasi-Resonant Inverted Buck architecture. A burst mode digital controller has been used to perform dimming control and power factor correction (PFC) for the LED driver. The custom controller and driver IC was implemented in a 0.35[mu]m CMOS process. The LED driver achieves a peak efficiency of 90.6% and a 0.96 power factor. Due to the high power level of the driver, the digital controller is primarily used for regulation purposes in this system, although the digital nature of the controller helps remove the passives that would be normally present in analog controllers. In this thesis, apart from regulation, control circuit enabled techniques have been used to improve efficiency and reduce system form factor. Low power design and control for reconfigurable power train architectures help improve the overall power converter efficiency. Digital control circuits have been used to reduce the form factor by enabling inductor sharing in a system with multiple power converters or by removing the compensator passives.by Saurav Bandyopadhyay.Ph.D

Robustness and durability aspects in the design of power management circuits for IoT applications

With the increasing interest in the heterogeneous world of the “Internet of Things” (IoT), new compelling challenges arise in the field of electronic design, especially concerning the development of innovative power management solutions. Being this diffusion a consolidated reality nowadays, emerging needs like lifetime, durability and robustness are becoming the new watchwords for power management, being a common ground which can dramatically improve service life and confidence in these devices. The possibility to design nodes which do not need external power supply is a crucial point in this scenario. Moreover, the development of autonomous nodes which are substantially maintenance free, and which therefore can be placed in unreachable or harsh environments is another enabling aspect for the exploitation of this technology. In this respect, the study of energy harvesting techniques is increasingly earning interest again. Along with efficiency aspects, degradation aspects are the other main research field with respect to lifetime, durability and robustness of IoT devices, especially related to aging mechanisms which are peculiar in power management and power conversion circuits, like for example battery wear during usage or hot-carrier degradation (HCD) in power MOSFETs. In this thesis different aspects related to lifetime, durability and robustness in the field of power management circuits are studied, leading to interesting contributions. Innovative designs of DC/DC power converters are studied and developed, especially related to reliability aspects of the use of electrochemical cells as power sources. Moreover, an advanced IoT node is proposed, based on energy harvesting techniques, which features an intelligent dynamically adaptive power management circuit. As a further contribution, a novel algorithm is proposed, which is able to effectively estimate the efficiency of a DC/DC converter for photovoltaic applications at runtime. Finally, an innovative DC/DC power converter with embedded monitoring of hot-carrier degradation in power MOSFETs is designed

Ultra-Low Power Transmitter and Power Management for Internet-of-Things Devices

Two of the most critical components in an Internet-of-Things (IoT) sensing and transmitting node are the power management unit (PMU) and the wireless transmitter (Tx). The desire for longer intervals between battery replacements or a completely self-contained, battery-less operation via energy harvesting transducers and circuits in IoT nodes demands highly efficient integrated circuits. This dissertation addresses the challenge of designing and implementing power management and Tx circuits with ultra-low power consumption to enable such efficient operation. The first part of the dissertation focuses on the study and design of power management circuits for IoT nodes. This opening portion elaborates on two different areas of the power management field: Firstly, a low-complexity, SPICE-based model for general low dropout (LDO) regulators is demonstrated. The model aims to reduce the stress and computation times in the final stages of simulation and verification of Systems-on-Chip (SoC), including IoT nodes, that employ large numbers of LDOs. Secondly, the implementation of an efficient PMU for an energy harvesting system based on a thermoelectric generator transducer is discussed. The PMU includes a first-in-its-class LDO with programmable supply noise rejection for localized improvement in the suppression. The second part of the dissertation addresses the challenge of designing an ultra- low power wireless FSK Tx in the 900 MHz ISM band. To reduce the power consumption and boost the Tx energy efficiency, a novel delay cell exploiting current reuse is used in a ring-oscillator employed as the local oscillator generator scheme. In combination with an edge-combiner PA, the Tx showed a measured energy efficiency of 0.2 nJ/bit and a normalized energy efficiency of 3.1 nJ/(bit∙mW) when operating at output power levels up to -10 dBm and data rates of 3 Mbps. To close this dissertation, the implementation of a supply-noise tolerant BiCMOS ring-oscillator is discussed. The combination of a passive, high-pass feedforward path from the supply to critical nodes in the selected delay cell and a low cost LDO allow the oscillator to exhibit power supply noise rejection levels better than –33 dB in experimental results

Compact Micro scale Multi- Source (Solar and Thermal) Energy harvesting IC with regulated Multi load Power Management scheme

Power is required for all man-made systems to work and perform their corresponding activi-ties.Generation of power in large scale is carried out from power grids and supplied to systems that need high power whereas systems requiring less power are supplied from batteries. Batteries need to be replaced after their lifetime which seem to be a less attractive option in applications where systems are placed out of reach for humans as WSN nodes or in some of biomedical systems which are kept inside human body. The need for a self-supporting system i.e. a system that produces energy by itself and supports all the modules by powering them by itself is increased. A harvesting system that harvests energy from the ambience and converting that energy into electrical form and supplying the power to loads or storing in a battery is the solution for all the problems mentioned above. Solar, thermal, vibration and RF are the sources in the ambience from which energy can be harvested and supplied to load or charged into a battery. Availability of a single energy source(thermal,vibration,solar,RF) at all instances cannot be guaranteed creating a situation of insufficient supply of power to loads or unable to charge the load capacitor to the required voltage. Usage of multiple sources for harvesting energy is a prominent solution to the above mentioned problem. Designing a microscale energy harvesting from multiple sources is the main motto behind the current work.TEG and piezo are compatible to be used alternatively in the system because of their close resemblance in their energy densities. Therefore TEG and piezo have been used as two input sources for the system. The other modules in the system include design of a buck boost power converter that switches between buck and boost modes depending on the source connected to the system and the input voltage available at the input. The other module being a digital controller that generates clocks for power switches, signals that decide if TEG or PV need to be connected to the system.Intra source selection block for TEG array where multiple TEG sources switch between series or parallel depending on the voltage available across each TEG source so as to increase the net power from the TEG. The load is a capacitor that needs to be charged to 1.8V where the system stops working once the capacitor gets charged to the desired voltage. Idea of sharing a single inductor between two different sources without using two power converters for individual sources is implemented. Using the dead time of inductor current of TEG for PV source is the main thought behind the development of the current system. System is designed considering all the specifications, constraints, functionalit

180 nm CMOS Cold-Start Energy-Aware Switching Circuit for Energy Management in WPT Receiver

Modern electronic devices offer high level performance at low power consumption. This opens a possibility to have battery-less electronic devices. Energy harvesting and wireless power transfer are popular methods to power such devices. Both methods require energy management. The focus of this paper is on energy management for receiver circuit in a wireless power transfer system. More specifically, the paper focuses on a cold-start energy-aware switching circuit which is a key building block of energy management in WPT receiver. Proposed integrated circuit is designed to operate in discontinuous mode and can supply power to load circuits which require higher voltage and current levels than available from the WPT receiver. Unlike most similar solutions who are fully integrated, the proposed integrated circuit uses two external trimmer resistors to adjust required voltage levels and the power consumption. External trimmer resistors also allow to compensate for the process variations of IC fabrication. Developed circuit is fabricated in 180 nm TSMC CMOS technology and evaluated through laboratory measurement. Cold-start functionality and energy-aware switching are verified through standalone measurements and measurements with WSN node powered through developed circuit. The power consumption of cold-start switching circuit is measured less than 1 µW