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

New Three Phase Photovoltaic Energy Harvesting System for Generation of Balanced Voltages in Presence of Partial Shading, Module Mismatch, and Unequal Maximum Power Points

The worldwide energy demand is growing quickly, with an anticipated growth rate of 48% from 2012 to 2040. Consequently, investments in all forms of renewable energy generation systems have been growing rapidly due to growth rate and climate concerns. Increased use of clean renewable energy resources such as hydropower, wind, solar, geothermal, and biomass is expected to noticeably alleviate many present environmental concerns associated with fossil fuel-based energy generation. In recent years, wind and solar energies have gained the most attention among all other renewable resources. As a result, both have become the target of extensive research and development for dynamic performance optimization, cost reduction, and power reliability assurance. The performance of Photovoltaic (PV) systems is highly affected by environmental and ambient conditions such as irradiance fluctuations and temperature swings. Furthermore, the initial capital cost for establishing the PV infrastructure is very high. Therefore, it is essential that the PV systems always harvest the maximum energy possible by operating at the most efficient operating point, i.e. Maximum Power Point (MPP), to increase conversion efficiency to reach 100% and thus result in lowest cost of captured energy. The dissertation is an effort to develop a new PV conversion system for large scale PV grid-connected systems which provides 99.8% efficacy enhancements compared to conventional systems by balancing voltage mismatches between the PV modules. Hence, it analyzes the theoretical models for three selected DC/DC converters. To accomplish this goal, this work first introduces a new adaptive maximum PV energy extraction technique for PV grid-tied systems. Then, it supplements the proposed technique with a global search approach to distinguish absolute maximum power peaks within multi-local peaks in case of partially shaded PV module conditions. Next, it proposes an adaptive MPP tracking (MPPT) strategy based on the concept of model predictive control (MPC) in conjunction with a new current sensor-less approach to reduce the number of required sensors in the system. Finally, this work proposes a power balancing technique for injection of balanced three-phase power into the grid using a Cascaded H-Bridge (CHB) converter topology which brings together the entire system and results in the final proposed PV power system. The developed grid connected PV solar system is evaluated using simulations under realistic dynamic ambient conditions, partial shading, and fully shading conditions and the obtained results confirm its effectiveness and merits comparted to conventional systems. The resulting PV system offers enhanced reliability by guaranteeing effective system operation under unbalanced phase voltages caused by severe partial shading

Model Predictive Control Methods for Photovoltaic Electrical Energy Conversion Systems

Solar photovoltaic energy systems (PV) have had a consistently increasing market penetration over the past seven years, with a total global installed capacity of over 500 GW. A PV installation must harvest the maximum possible electrical energy at the lowest cost to be economically justifiable. This presents many engineering challenges and opportunities within power electronics amongst which include low-cost power converter implementation, high reliability, grid-friendly integration, fast dynamic response to track the stochastic nature of the solar resource, and disturbance rejection to grid transient and partial shading. This dissertation investigates the controls of the power electronic interface with the objective to reduce cost, increase reliability, and increase efficiency of PV energy conversion systems. The overall theme of this dissertation involves exploring the theory of model predictive control (MPC) within a range of applications for PV systems. The applications within PV energy conversion systems are explored, ranging from cell to grid integration. MPC-based maximum power point tracking (MPPT) algorithm is investigated for the power electronics interface to maximize the energy harvest of the PV module. Within the developed MPC based MPPT framework, sensorless current mode and adaptive perturbation are proposed. The MPC framework is expanded further to include inverter control. The control of a single-phase H-bridge inverter and sub-multilevel inverter are presented in this dissertation to control grid current injection. The multi-objective optimization of MPC is investigated to control the dc-link voltage in microinverters along with grid current control. The developed MPC based MPPT controller is shown to operate with a single-stage impedance source three-phase inverter with PID based grid-side control