A Quasi-Resonant Bidirectional Converter with Soft-Switching Operation for Energy Storage Applications

The increased penetration of renewable energy power systems to produce clean and sustainable energy has led to the increased usage of various types of energy storage devices, such as high power density battery technologies, flywheel energy storage and super-capacitors. Energy storage devices are essential in any renewable generation systems to ensure providing uninterruptible and reliable power. Typically, a power electronic converter is required to serve as the intermediary between the common grid in a renewable energy system and the energy storage device. To be specific, the power converter must be able to facilitate bidirectional power flow between the grid and the energy storage device. Since the voltage level of the energy storage device is often much lower than the grid voltage level, the bidirectional converter must ensure that the voltage level can be stepped up or down efficiently as per the system requirements depending on the direction of the power flow. In this thesis, a unique quasi-resonant bidirectional converter topology is proposed for energy storage application. The proposed circuit only requires two switches to achieve bidirectional power flow. Hence, compared to the conventional dual-active bridge (DAB) based bidirectional converter topologies that require 8 switches, the total number of active switching devices required the proposed topology is greatly reduced. In addition, both switches in the proposed topology are able to achieve zero voltage switching (ZVS) turn-on and zero current switching (ZCS) turn-off to minimize the switching power losses without using additional auxiliary circuits. The operating principles and design equations of the proposed circuit will be discussed in details in this thesis. An extended version of the proposed topology that employs a modular design structure for high power application is also presented and discussed. Simulation results and experimental works on a proof-of-concept hardware prototype are given to highlight the performance of the proposed bidirectional converter

IMPROVEMENT STUDY ON SOFT-SWITCHED QUASI-RESONANT DC/DC BOOST CONVERTER

This report describes a novel soft-switched quasi-resonant DC/DC boost converter. Recently, remarkable efforts have been made in soft-switched quasi-resonant DC/DC converters to reduce losses and improve power efficiency. This project presents a new technique and it had improved the performance of the most recent study on soft-switched quasi-resonant DC/DC boost converter, which is presented in Ba-Thunya and Prasad's study [1]. The proposed converter employs an active snubber circuit with an auxiliary switch in series with a clamp capacitor to reduce powerlosses in Ba-Thunya and Prasad's original an active snubber circuit with an auxiliary switch and a clamp diode to reduce power losses in Ba-Thunya and Prasad's original converter. The energy from the snubber inductor after the auxiliary switch turn-off is returned to the input or delivered to the output via the active snubber circuit, thus the voltage stress onthe main switch is reduced and switching losses are minimized. Furthermore, the proposed converter reduces the reverse-recovery-related losses of the boost rectifier by controlling the di/dt rate of the rectifier current with the snubber inductor. This report describes the principle of operation of the new soft-switched quasi-resonant DC/DC boost converter. The feasibility study of the proposed converter is investigated using PSPICE program

A Class-E-Based AC-DC converter for PFC applications

Connection of nonlinear utility load har increased through resent years and is expected to continue increasing. Nonlinear utility load injects harmonic content into the grid and reduces voltage quality for nearby consumers. To limit harmonic content from nonlinear load, the International Electrotechnical Commission requires power supplies to be designed according to IEC 61000-3-2. Fulfilling this standard for nonlinear load is done by power factor correction (PFC). Conventionally, pulse width modulation (PWM) converter has been used for PFC converters as they provide high efficiency with a simple control technic. However, as PWM converters switch by hard-switching, that limits the switching frequency through switching loss and generates EMI, resonant converters has become more attractive. Resonant converters operate at soft-switching where the voltage across and/ or current through is zero in the switching moment. This reduces switching loss and EMI, and allow for high switching frequency. High switching frequency is desired as it enables high power density. Through this thesis, two resonant converters using high switching frequency has been proposed. These converters are based on a Class-E converter as it has low noise and high efficiency when switching at high frequency. The thesis includes a mathematical model for both converts, simulation and experimental testing result. Result from testing differs from calculated and simulated values, and troubleshooting for one of the converters has been conducted. Through troubleshooting and a second test with changed parameters, the performance of the converter increased compared to the first test. Due to lack of time, the debugging process was not completed and will be a part of future work

Design of a low-voltage low-power dc-dc HF converter

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 230-234).Many portable electronic applications could benefit from a power converter able to achieve high efficiency across wide input and output voltage ranges at a small size. However, it is difficult for many conventional power converter designs to provide wide operation range while maintaining high efficiency, especially if both up-and-down voltage conversion is to be achieved. Furthermore, the bulk energy storage required at contemporary switching frequencies of a few megahertz and below limits the degree of miniaturization that can be achieved and hampers fast transient response. Therefore, design methods that reduce energy storage requirements and expand efficient operation range are desirable. This thesis focuses on the development of a High Frequency (HF) dc-dc SEPIC converter exploiting resonant switching and gating with fixed frequency control techniques to achieve these goals. The proposed approach provides high efficiency over very wide input and output voltage ranges and power levels. It also provides up-and-down conversion, and requires little energy storage which allows for excellent transient response. The proposed design strategies are discussed in the context of a prototype converter operating over wide input voltage (3.6 - 7.2V), output voltage (3 - 9V) and power (0.3 - 3W) ranges. The 20MHz converter prototype, utilizing commercial vertical MOSFETs, takes advantage of a quasi-resonant SEPIC topology and resonant gating technique to provide good efficiency across the wide operating ranges required. The converter efficiency stays above 80% across the entire input voltage range at the nominal output voltage. The closed-loop performance is demonstrated via an implementation of a PWM on-off control scheme, illustrating the salient characteristics in terms of additional control circuitry power dissipation and transient response.by Jingying Hu.S.M

Highly Efficient SiC Based Onboard Chargers for Plug-in Electric Vehicles

Grid-enabled plug-in electrified vehicles (PEVs) are deemed as one of the most sustainable solutions to profoundly reduce both oil consumption and greenhouse gas emissions. One of the most important realities, which will facilitate the adoption of PEVs is the method by which these vehicles will be charged. This dissertation focuses on the research of highly efficient onboard charging solutions for next generation PEVs. This dissertation designs a two-stage onboard battery charger to charge a 360 V lithium-ion battery pack. An interleaved boost topology is employed in the first stage for power factor correction (PFC) and to reduce total harmonic distortion (THD). In the second stage, a full bridge inductor-inductor-capacitor (LLC) multi-resonant converter is adopted for galvanic isolation and dc/dc conversion. Design considerations focusing on reducing the charger volume, and optimizing the conversion efficiency over the wide battery pack voltage range are investigated. The designed 1 kW Silicon based charger prototype is able to charge the battery with an output voltage range of 320 V to 420 V from 110 V, 60 Hz single-phase grid. Unity power factor, low THD, and high peak conversion efficiency have been demonstrated experimentally. This dissertation proposes a new technique to track the maximum efficiency point of LLC converter over a wide battery state-of-charge range. With the proposed variable dc link control approach, dc link voltage follows the battery pack voltage. The operating point of the LLC converter is always constrained to the proximity of the primary resonant frequency, so that the circulating losses and the turning off losses are minimized. The proposed variable dc link voltage methodology, demonstrates efficiency improvement across the wide state-of-charge range. An efficiency improvement of 2.1% at the heaviest load condition and 9.1% at the lightest load condition for LLC conversion stage are demonstrated experimentally. This dissertation proposes a novel PEV charger based on single-ended primary-inductor converter (SEPIC) and the maximum efficiency point tracking technique of an LLC converter. The proposed charger architecture demonstrates attracting features such as (1) compatible with universal grid inputs; (2) able to charge the fully depleted battery pack; (3) pulse width modulation and simplified control algorithm; and (4) the advantages of Silicon Carbide MOSFETs can be fully manifested. A 3.3 kW all Silicon Carbide based PEV charger prototype is designed to validate the proposed idea

A Comprehensive Review of DC-DC Converters for EV Applications

DC-DC converters in Electric vehicles (EVs) have the role of interfacing power sources to the DC-link and the DC-link to the required voltage levels for usage of different systems in EVs like DC drive, electric traction, entertainment, safety and etc. Improvement of gain and performance in these converters has a huge impact on the overall performance and future of EVs. So, different configurations have been suggested by many researches. In this paper, bidirectional DC-DC converters (BDCs) are divided into four categories as isolated-soft, isolated-hard, non-isolated-soft and non-isolated-hard depending on the isolation and type of switching. Moreover, the control strategies, comparative factors, selection for a specific application and recent trends are reviewed completely. As a matter of fact, over than 200 papers have been categorized and considered to help the researchers who work on BDCs for EV application

Design and implementation of synchronous buck converter based PV energy system for battery charging applications

The Photo Voltaic (PV) energy system is a very new concept in use, which is gaining popularity due to increasing importance to research on alternative sources of energy over depletion of the conventional fossil fuels world-wide. The systems are being developed to extract energy from the sun in the most efficient manner and suit them to the available loads without affecting their performance. In this project, synchronous buck converter based PV energy system for portable applications; especially low power device applications such as charging mobile phone batteries are considered. Here, the converter topology used uses soft switching technique to reduce the switching losses which is found prominently in the conventional buck converter, thus efficiency of the system is improved and the heating of MOSFETs due to switching losses reduce and the MOSFETs have a longer life. The DC power extracted from the PV array is synthesized and modulated by the converter to suit the load requirements. Further, the comparative study between the proposed synchronous buck converter and the conventional buck converter is analysed in terms of efficiency improvement and switching loss reduction. The proposed system is simulated in the MATLAB-Simulink environment and the practical implementation of the proposed converter is done to validate the theoretical results. Open-loop control of synchronous buck converter based PV energy system is realised through ICs and experimental results were observed

Data Center Power System Emulation and GaN-Based High-Efficiency Rectifier with Reactive Power Regulation

Data centers are indispensable for today\u27s computing and networking society, which has a considerable power consumption and significant impact on power system. Meanwhile, the average energy usage efficiency of data centers is still not high, leading to significant power loss and system cost. In this dissertation, effective methods are proposed to investigate the data center load characteristics, improve data center power usage efficiency, and reduce the system cost. First, a dynamic power model of a typical data center ac power system is proposed, which is complete and able to predict the data center\u27s dynamic performance. Also, a converter-based data center power emulator serving as an all-in-one load is developed. The power emulator has been verified experimentally in a regional network in the HTB. Dynamic performances during voltage sag events and server load variations are emulated and discussed. Then, a gallium nitride (GaN) based critical conduction mode (CRM) totem-pole power factor correction (PFC) rectifier is designed as the single-phase front-end rectifier to improve the data center power distribution efficiency. Zero voltage switching (ZVS) modulation with ZVS time margin is developed, and a digital variable ON-time control is employed. A hardware prototype of the PFC rectifier is built and demonstrated with high efficiency. To achieve low input current total harmonic distortion (iTHD), current distortion mechanisms are analyzed, and effective solutions for mitigating current distortion are proposed and validated with experiments. The idea of providing reactive power compensation with the rack-level GaN-based front-end rectifiers is proposed for data centers to reduce data center\u27s power loss and system cost. Full-range ZVS modulation is extended into non-unity PF condition and a GaN-based T-type totem-pole rectifier with reactive power control is proposed. A hardware prototype of the proposed rectifier is built and demonstrated experimentally with high power efficiency and flexible reactive power regulation. Experimental emulation of the whole data center system also validates the capability of reactive power compensation by the front-end rectifiers, which can also generate or consume more reactive power to achieve flexible PF regulation and help support the power system