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

A Study on Energy-Efficient Inductor Current Controls for Maximum Energy Delivery in Battery-free Buck Converter

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 김재하.A discontinuous conduction mode (DCM) buck converter, which acts as a voltage regulator in battery-free applications, is proposed to maximize the ener-gy delivery to the load system. In this work, we focus the energy loss problem during start-up and steady-state operation of the buck converter, which severely limits the energy delivery. Especially, the energy loss problem arises from the fact that there is no constant power source such as a battery and the only a small amount of energy harvested from the ambient energy sources is available. To address such energy loss problem, this dissertation proposes optimal induc-tor current control techniques at each operation to greatly reduce the energy losses. First, a switching-based stepwise capacitor charging scheme is presented that can charge the output capacitor with constant inductor current during start-up operation. By switching the inductor with gradually incrementing duty-cycle ratios in a stepwise fashion, the buck converter can make the inductor current a constant current source, which can greatly reduce the start-up energy loss com-pared to that in the conventional capacitor charging scheme with a voltage source. Second, a variable on-time (VOT) pulse-frequency-modulation (PFM) scheme is presented that can keep the peak inductor current constant during steady-state operation. By adaptively varying the on-time according to the op-erating voltage conditions of the buck converter, it can suppress the voltage ripple and improve the power efficiency even with a small output capacitor. Third, an adaptive off-time positioning zero-crossing detector (AOP-ZCD) is presented that can adaptively position the turn-off timing of the low-side switch close to the zero-inductor-current timing by predicting the inductor current waveform without using a power-hungry continuous-time ZCD. To demonstrate the proposed design concepts, the prototype battery-free wireless remote switch including the piezoelectric energy harvester and the proposed buck converter was fabricated in a 250 nm high-voltage CMOS technology. It can harvest a total energy of 246 μJ from a single button press action of a 300-mm2 lead magnesium niobate-lead titanate (PMN-PT) piezoelectric disc, and deliver more than 200 μJ to the load, which is sufficient to transmit a 4-byte-long message via 2.4-GHz wireless USB channel over a 10-m distance. If such battery-free application does not use the proposed buck converter, the energy losses in-curred at the buck converter would be larger than the energy harvested, and therefore it cannot operate with a single button-pressing action. Furthermore, thanks to the proposed energy efficient buck converter, the battery-free wire-less remote switch can be realized by using a cheaper PZT piezoelectric source, which can achieve a 10× cost reduction.CHAPTER 1 INTRODUCTION 1 1.1 MOTIVATION 1 1.2 THESIS CONTRIBUTION AND ORGANIZATION 6 CHAPTER 2 OPERATION MODE AND OVERALL ARCHITECTURE 8 2.1 TOPOLOGY SELECTION 8 2.2 PRINCIPLE OF OPERATION 11 2.2.1 BASIC OPERATION IN CCM 12 2.2.2 BASIC OPERATION IN DCM 15 2.3 OPERATION MODE 17 2.4 OVERALL ARCHITECTURE 19 CHAPTER 3 OPTIMAL INDUCTOR CURRENT CONTROLS FOR MAXIMUM ENERGY DELIVERY 23 3.1 CONSTANT INDUCTOR CURRENT CONTROL WITH SWITCHING-BASED STEPWISE CAPACITOR CHARGING SCHEME 24 3.1.1 CONVENTIONAL CHARGING SCHEME WITH A SWITCH 24 3.1.2 ADIABATIC STEPWISE CHARGING 27 3.1.3 PROPOSED START-UP SCHEME 29 3.2 CONSTANT INDUCTOR PEAK CURRENT CONTROL WITH VARIABLE ON-TIME PFM SCHEME 35 3.2.1 BASIC OPERATION OF PFM BUCK CONVERTER 35 3.2.2 CONSTANT ON-TIME PFM SCHEME 39 3.2.3 VARIABLE ON-TIME PFM SCHEME 41 3.3 INDUCTOR CURRENT PREDICTION WITH ADAP-TIVE OFF-TIME POSITIONING ZCD (AOP-ZCD) 44 3.3.1 PREVIOUS SAMPLING-BASED ZCD 44 3.3.2 PROPOSED ADAPTIVE OFF-TIME POSITIONING ZCD 47 CHAPTER 4 CIRCUIT IMPLEMENTATION 49 4.1 CIRCUIT IMPLEMENTATION OF SWITCHING-BASED STEPWISE CAPACITOR CHARGER 49 4.1.1 VOLTAGE DETECTOR (VD) 50 4.1.2 DIGITAL PULSE WIDTH MODULATOR (DPWM) 52 4.1.3 PROGRAMMABLE DUTY-CYCLE CONTROLLER (DCC) 55 4.1.4 SWITCHED CAPACITOR (SC) STEP-DOWN CONVERTER 57 4.2 CIRCUIT IMPLEMENTATION OF VARIABLE ON-TIME PULSE GENERATOR 59 4.3 CIRCUIT IMPLEMENTATION OF ADAPTIVE OFF-TIME POSITIONING ZCD 64 4.3.1 ADAPTIVE OFF-TIME (AOT) PULSE GENERATOR 64 4.3.2 TIMING ERROR DETECTOR AND SHIFT-REGISTER 68 CHAPTER 5 MEASUREMENT RESULTS OF PROPOSED BUCK CONVERTER 70 5.1 SWITCHING-BASED STEPWISE CAPACITOR CHARGER 71 5.2 STEADY-STATE PERFORMANCE WITH VOT PULSE GENERATOR AND AOP-ZCD 74 CHAPTER 6 REALIZATION OF BATTERY-FREE WIRELESS REMOTE SWITCH 84 6.1 KEY BUILDING BLOCKS OF BATTERY-FREE WIRELESS REMOTE SWITCH 85 6.2 PIEZOELECTRIC ENERGY HARVESTER WITH P-SSHI RECTIFIER 86 6.2.1 ANALYSIS ON SINGLE-PULSED ENERGY HARVESTING 88 6.2.2 PROPOSED PIEZOELECTRIC ENERGY HARVESTER 91 6.2.3 CIRCUIT IMPLEMENTATION 93 6.3 MEASUREMENT RESULTS OF BATTERY-FREE WIRELESS SWITCH 96 CHAPTER 7 CONCLUSION 101 BIBLIOGRAPHY 103 초 록 110Docto

SUSTAINABLE ENERGY HARVESTING TECHNOLOGIES – PAST, PRESENT AND FUTURE

Chapter 8: Energy Harvesting Technologies: Thick-Film Piezoelectric Microgenerato

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

Contributions to the design of energy harvesting systems for autonomous sensors in low power marine applications

Tesi en modalitat de compendi de publicacionsOceanographic sensor platforms provide biological and meteorological data to help understand changes in marine environment and help to preserve it. Lagrangian drifters are autonomous passive floating platforms used in climate research to obtain surface marine data. They are low-cost, versatile, easy-to-deploy and can cover large extensions of the ocean when deployed in group. These deployments can last for years, so one of the main design challenges is the autonomy of the drifter. Several energy harvesting (EH) sources are being explored to reduce costs in battery replacement maintenance efforts such as solar panels. Drifters must avoid the impact of the wind because this may compromise proper surface current tracking and therefore, should ideally be mostly submerged. This interferes with the feasibility of solar harvesting, so other EH sources are being explored such as the oscillatory movement of the drifter caused by ocean waves. Wave energy converters (WEC) are the devices that turn this movement into energy. The motion of the drifter can principally be described by 3 oscillatory degrees of freedom (DoF); surge, heave and pitch. The heave motion includes the buoyancy’s response of the drifter, which can be explained by a mass-spring-damping model. By including the wave’s hydrodynamic load in this model, it is converted into a nonlinear system whose frequency response includes the wave’s frequency and the natural frequencies from the linear system. A smart option to maximize the captured energy is to design the inner WEC with a natural frequency similar to that of the drifter's movement. In this thesis, a 4 DoF model is obtained. This model includes the heave, the surge and the pitch motion of the drifter in addition to the inner pendulum motion relative to the buoy. Simultaneously, different pendulum-type WECs for small-size oceanic drifters are proposed. One of these converters consists of an articulated double-pendulum arm with a proof mass that generates energy through its relative motion with the buoy. Different experimental tests are carried out, with a prototype below 10 cm in diameter and 300 g of total mass, proving the capability of harvesting hundreds of microwatts in standard sea conditions EH sources require an additional power management unit (PMU) to convert their variable output into a constant and clean source to be able to feed the sensor electronics. PMUs should also ensure that the maximum available energy is harvested with a maximum power point tracking (MPPT) algorithm. Some sources, such as WECs, require fast MPPT as its output can show relatively rapid variations. However, increasing the sampling rate may reduce the harvested energy. In this thesis, this trade-off is analyzed using the resistor-based fractional open circuit voltage-MPPT technique, which is appropriate for low-power EH sources. Several experiments carried out in marine environments demonstrate the need for increasing the sampling rate. For this purpose, the use of a commercial PMU IC with additional low-power circuitry is proposed. Three novel circuits with a sampling period of 60 ms are manufactured and experimentally evaluated with a small-scale and low-power WEC. Results show that these configurations improve the harvested energy by 26% in comparison to slow sampling rate configurations. Finally, an EH-powered oceanographic monitoring system with a custom wave measuring algorithm is designed. By using the energy collected by a small-size WEC, this system is capable of transmitting up to 22 messages per day containing data on its location and measured wave parameters.Les plataformes d’observació oceanogràfiques integren sensors que proporcionen dades físiques i biogeoquímiques de l’oceà que ajuden a entendre canvis en l’entorn marí. Un exemple d’aquestes plataformes són les boies de deriva (drifters), que són dispositius autònoms i passius utilitzats en l’àmbit de la recerca climàtica per obtenir dades in-situ de la superfície marina. Aquests instruments són de baix cost, versàtils, fàcils de desplegar i poden cobrir grans superfícies quan s’utilitzen en grup. L’autonomia és un dels principals desafiaments en el disseny de drifters. Per tal d’evitar els costos en la substitució de bateries, s’estudien diferents fonts de captació d’energia com per exemple la solar. Els drifters utilitzats per l’estudi dels corrents marins superficials han d’evitar l’impacte directe del vent ja que afecta al correcte seguiment de les corrents i, per tant, cal que estiguin majoritàriament submergides. Això compromet la viabilitat de l’energia solar, fet que requereix l’estudi d’altres fonts de captació com el propi moviment de la boia causat per les onades. Els convertidors d’energia de les onades (WEC, wave energy converters) compleixen aquesta funció. El moviment dels drifters pot explicar-se bàsicament a través de 3 graus de llibertat oscil·latoris: la translació vertical i la horitzontal i el balanceig. La translació vertical inclou la flotabilitat del dispositiu, que es pot descriure mitjançant el model massamolla- amortidor. Incloure la càrrega hidrodinàmica de l’onada en aquest model el converteix en un sistema no lineal amb una resposta freqüencial que inclou la de l’onada i les naturals del sistema lineal. Una opció per maximitzar l’energia captada és dissenyar el WEC amb una freqüència natural similar a la del moviment de la boia. En aquesta tesis es proposa un model de 4 graus de llibertat per a l’estudi del moviment del drifter. Aquest inclou els 3 graus de llibertat de la boia i el moviment del pèndul relatiu a ella. En paral·lel, es proposen diferents WEC del tipus pendular per drifters de reduïdes dimensions. Un d’aquests WEC consisteix en un doble braç articulat amb massa flotant que genera energia a través del seu moviment relatiu al drifter. S’han dut a terme diferents proves experimentals amb un prototip inferior a 10 cm de diàmetre i 300 g de massa, les quals demostren la seva capacitat de captar centenars de microwatts en condicions marines estàndard. Utilitzar fonts de captació d’energia requereix incloure una unitat gestora de potència (PMU, power management unit) per tal de convertir la seva sortida variable en una font constant i neta que alimenti l’electrònica dels sensors. Les PMU també tenen la funció d’assegurar que es recull la màxima energia mitjançant un algoritme de seguiment del punt de màxima potència. Els WEC requereixen un seguiment d’aquest punt ràpid perquè la seva sortida consta de variacions relativament ràpides. Tanmateix, augmentar la freqüència de mostreig pot reduir l’energia captada. En aquesta tesi, s'analitza a fons aquesta relació utilitzant la tècnica de seguiment de la tensió en circuit obert fraccionada basada en resistències, que és molt adequada per a fonts de baixa potència. Diversos experiments realitzats en el medi marí mostren la necessitat d'augmentar la freqüència de mostreig, així que es proposa l'ús de PMU comercials amb una electrònica addicional de baix consum. S’han fabricat tres circuits diferents amb un període de mostreig de 60 ms i s’han avaluat experimentalment en un WEC de reduïdes dimensions. Els resultats mostren que aquestes configuracions milloren l'energia recollida en un 26% en comparació a PMU amb mostreig més lent. Finalment, s’ha dissenyat un sistema autònom de monitorització marina que inclou un algoritme de mesura d'ones propi. Aquest sistema és capaç de transmetre fins a 22 missatges al diaPostprint (published version