Energy-efficient and Power-dense DC-DC Converters in Data Center and Electric Vehicle Applications Using Wide Bandgap Devices

The ever increasing demands in the energy conversion market propel power converters towards high efficiency and high power density. With fast development of data processing capability in the data center, the server will include more processors, memories, chipsets and hard drives than ever, which requires more efficient and compact power converters. Meanwhile, the energy-efficient and power-dense converters for the electric vehicle also result in longer driving range as well as more passengers and cargo capacities. DC-DC converters are indispensable power stages for both applications. In order to address the efficiency and density requirements of the DC-DC converters in these applications, several related research topics are discussed in this dissertation. For the DC-DC converter in the data center application, a LLC resonant converter based on the newly emerged GaN devices is developed to improve the efficiency over the traditional Si-based converter. The relationship between the critical device parameters and converter loss is established. A new perspective of extra winding loss due to the asymmetrical primary and secondary side current in LLC resonant converter is proposed. The extra winding loss is related to the critical device parameters as well. The GaN device benefits on device loss and transformer winding loss is analyzed. An improved LLC resonant converter design method considering the device loss and transformer winding loss is proposed. For the DC-DC converter in the electric vehicle application, an integrated DC-DC converter that combines the on-board charger DC-DC converter and drivetrain DC-DC converter is developed. The integrated DC-DC converter is considered to operate in different modes. The existing dual active bridge (DAB) DC-DC converter originally designed for the charger is proposed to operate in the drivetrain mode to improve the efficiency at the light load and high voltage step-up ratio conditions of the traditional drivetrain DC-DC converter. Design method and loss model are proposed for the integrated converter in the drivetrain mode. A scaled-down integrated DC-DC converter prototype is developed to verify the design and loss model

Magnetic Integration Techniques for Resonant Converters

This thesis sets out a series of new transformer topologies for magnetic integration in different resonant converters. Resonant converters like LLC converters require a high number of magnetic components, leading to low power density and high cost. These magnetic components can usually be integrated into a single transformer to increase power density, efficiency, manufacturing simplicity and to reduce cost. This strategy is known as integrated transformer (IT). The work described in this thesis has sought to deliver improvements in implementing this strategy. The benefits of resonant converters compared to pulse-width-modulated (PWM) converters are discussed. To show the drawbacks of PWM converters, two hard-switched DC-DC converters and two soft-switched DC-DC converters using state-of-the-art wide bandgap (WBG) gallium nitride devices are constructed and investigated. The LLC resonant converter is fully discussed for unidirectional and bidirectional applications. The different techniques for magnetic integration that can be applied to the LLC resonant converter are reviewed. Amongst these techniques, the inserted-shunt integrated transformers, which have gained popularity recently, are made a focus of the thesis. In general, the important challenges concerning the inserted-shunt integrated transformers are the need for bespoke material for the shunt, unwanted high leakage inductance on the secondary side, and that integrated magnetics are not usually suitable for bidirectional converters such as CLLLC converters. Two new topologies (IT1 and IT2) for inserted-shunt integrated transformers are presented that do not need bespoke material for the shunt and can be constructed from materials available commercially in large and small quantities. However, the manufacturing of these proposed topologies is challenging since magnetic shunts are made by joining several smaller magnetic pieces to form a segmented piece. A further new topology (IT3) is presented that not only does not need bespoke material for the shunt but also benefits from simple manufacturing. However, inserted-shunt integrated transformer, including all three proposed topologies (IT1-IT3), still suffer from increased leakage inductance on the secondary side, leading the control and design of the resonant converters to difficulty. Another topology (IT4) is therefore proposed that can be constructed easily with commercially available materials and does not increase the leakage inductance on the secondary side. However, all four proposed topologies (IT1-IT4) and other topologies with an inserted-shunt are not suitable for use in bidirectional LLC-type resonant converters when different primary and secondary leakage inductances are needed, such as where variable gain is required. Finally, a topology (IT5) is proposed that can be used in bidirectional LLC-type converters while it still benefits from simple manufacturing and using commercially available materials. All the proposed topologies (IT1-IT5) are discussed in detail and their design guidelines and modelling are provided. The theoretical analysis is confirmed by finite-element (FEM) analysis and experimental results. A unidirectional LLC resonant converter and a bidirectional CLLLC resonant converter are implemented to investigate the performance of the proposed integrated transformers (IT1-IT5) in practice. It is shown that the converters can operate properly while all their magnetic components are integrated into the proposed transformers

Optimization of LLC resonant converters

Usualmente, na área da eletrónica de potência, tem que existir um trade off entre densidade de potencia e o rendimento, por forma a desenhar dispositivos que sejam pequenos o suficiente, para ocupar o mínimo espaço, mas ao mesmo tempo altamente eficientes, por forma a maximizar a energia consumida em trabalho resultante, especialmente em veículos elétricos, onde existem várias etapas de conversão de energia. O presente trabalho visa estudar os conversores ressonantes e as suas topologias associadas, continuando o estudo realizado pela Mestre Maria Ruxandra Luca em parceria com a Universidade de Oviedo, tendo como principal objetivo a otimização de um conversor ressonante LLC de 4.2 para carregamento de baterias. Este tipo de conversor é mais vantajoso quando comparado com os conversores tradicionais, devido à utilização do conceito de ressonância e de técnicas Soft Switching, como o Zero Current Switch (ZCS) e Zero Voltage Switch (ZVS). Estar em ressonância significa, ter um comportamento resistivo pelo facto da soma de todas as impedâncias do tanque de ressonante ser nula. Isto leva a que a corrente esteja em fase com a tensão, permitindo o mínimo de perdas, para uma situação em que o ganho do conversor é unitário. Porém, para alterar o valor da tensão da saída do conversor, este ganho tem que ser alterado (com a modulação de frequência), levando o conversor a trabalhar fora da sua zona de ressonância, com um desfasamento entre tensão e corrente, aumentando significativamente as perdas nos semicondutores comutadores. O uso de técnicas Soft Switching, como o Zero Current Switch (ZCS) e Zero Voltage Switch (ZVS), permite a minimização de perdas de comutação quando o conversor trabalha fora de ressonância, utilizando mecanismos como a equalização da corrente no transformador (entre corrente magnetizante e corrente série) e Dead-Time para fazer com que as comutações sejam feitas quando a corrente e a tensão estão a zero. Devido á menor taxa de perdas nas comutações, o uso de frequências mais elevadas é possível, obtendo assim conversores com uma maior densidade de potência, mantendo uma operação com elevada eficiência. Neste trabalho é apresentado um breve capítulo do estado da arte, em que diversos modos de conversão DC-DC são apresentados, comparando as suas vantagens e desvantagens, seguido de uma análise às arquiteturas e topologias mais utilizadas nos conversores ressonantes. Com o objetivo de aumentar a eficiência, são descritos os andares do conversor onde existem mais perdas, com as suas causas, e possíveis soluções como o uso de transístores de alta mobilidade de eletrões, (do Inglês High Electron Mobility Transitors HEMT) combinados com materiais wide band-gap, que permitem operar de forma mais eficiente quando comparados com semicondutores de silício, a utilização de air-gap distribuído, bobines entrelaçadas e o fio de Litz, para minimizar as correntes de Eddy produzidas no transformador, e ainda a utilização de retificação síncrona em substituição aos díodos retificadores. De seguida, num terceiro capítulo, é apresentada a configuração base do conversor LLC ressonante para o carregamento de baterias de iões de lítio, detalhando cada um dos blocos associados, acompanhado de uma análise teórica por forma a permitir compreender o funcionamento do conversor, quais os principais fatores mais importantes, e qual o impacto da frequência de comutação no comportamento do conversor. Neste capítulo é ainda apresentado o processo de desenho deste conversor discriminando quais os parâmetros iniciais necessários, com uma análise detalhadas das perdas associadas ao design base, finalizando com o estudo, das diferentes arquiteturas do conversor nos andares de conversão AC-DC e DC-AC, e da retificação síncrona com a utilização de HEMTs, na eficiência do conversor. Simulações serão então conduzidas posteriormente utilizando modelos reais dos componentes presentes no conversor, com o uso do software LTSpice, comparando de forma detalhada o design base, com os designs otimizados previamente obtidos, de forma a observar o impacto das alterações propostas. Inicialmente foi previsto construir o conversor apresentado em [1] e o conversor otimizado mais eficiente, testá-los experimentalmente, mas devido à situação atual da pandemia Sars-Cov (Covid 19), o mesmo não foi possível, a tempo de entregar este trabalho, sendo este, um dos trabalhos futuros. Este trabalho foi desenvolvido em parceria com a Universidad de Oviedo, com o grupo de investigação LEMUR na Escuela Politécnica de Ingeniería de Gijón, onde foram feitas as analises teóricas e simulações do conversor de ressonância LLC

A Comprehensive Review on Planar Magnetics and the Structures to Reduce the Parasitic Elements and Improve Efficiency

Due to the need for highly efficient and compact power electronic converters to operate at higher frequencies, traditional wire-wound magnetics are not suitable. This paper provides a comprehensive review of planar magnetic technologies, discussing their advantages as well as associated disadvantages. An extensive review of the research literature is presented with the aim of suggesting models for planar magnetics. Several strategies are proposed to overcome the limitations of planar magnetics, including winding conduction loss, leakage inductance, and winding capacitance. The goal of this study is to provide engineers and researchers with a clear roadmap for designing planar magnetic devices

Analysis and validation of a multiple output series resonant converter

In this paper a novel bidirectional multiple port dc/dc transformer topology is presented. The novel concept for dc/dc transformer is based on the Series Resonant Converter (SRC) topology operated at its resonant frequency point. This allows for higher switching frequency to be adopted and enables high efficiency/high power density operation. The feasibility of the proposed concept is verified on a 300W, 700 kHz three port prototype with 390V input voltage and 48V and 12V output voltages. A peak overall efficiency of 93% is measured at full load. A very good load and cross regulation characteristic of the converter is observed in the whole load range, from full load to open circuit. The sensitivity analysis of the resonant capacitance is also performed showing very slight deterioration in the converter performances when a resonant capacitor is changed ±30% of its nominal value

Impedance Control Network Resonant Step-Down DC-DC Converter Architecture

In this paper, we introduce a step-down resonant dc-dc converter architecture based on the newly-proposed concept of an Impedance Control Network (ICN). The ICN architecture is designed to provide zero-voltage and near-zero-current switching of the power devices, and the proposed approach further uses inverter stacking techniques to reduce the voltages of individual devices. The proposed architecture is suitable for large-step-down, wide-input-range applications such as dc-dc converters for dc distribution in data centers. We demonstrate a first-generation prototype ICN resonant dc-dc converter that can deliver 330 W from a wide input voltage range of 260 V – 410 V to an output voltage of 12 V.MIT Skoltech InitiativeMIT Energy InitiativeNational Science Foundation (U.S.) (Award 1307699)Texas Instruments Incorporated (Graduate Women's Fellowship for Leadership in Microelectronics

Automated tool for 3D planar magnetic temperature modelling: application to EE and E/PLT core-based components

International audienceThermal performance of power converters is a key issue for the power integration. Temperatures inside active and passive devices can be determined using thermal models. Modelling the temperature distribution of high frequency magnetic components is quite complex due to diversity of their geometries and used materials. This paper presents a thermal modelling method based on lumped elements thermal network model, applied to planar magnetic components made of EE and E/PLT cores. The 3D model is automatically generated from the component's geometry. The computation enables to obtain 3D temperature distribution inside windings and core of planar transformers or inductors, in steady state or in transient case. The paper details the proposed modelling method as well as the automated tool including the problem definition and the solving process. The obtained temperature distributions are compared with Finite Element simulation results and measurements on different planar transformers

Study of a Symmetrical LLC Dual-Active Bridge Resonant Converter Topology for Battery Storage Systems

A symmetrical LLC resonant converter topology with a fixed-frequency quasi-triple phase-shift modulation method is proposed for battery-powered electric traction systems with extensions to other battery storage systems. Operation of the converter with these methods yields two unique transfer characteristics and is dependent on the switching frequency. The converter exhibits several desirable features: 1) load-independent buck-boost voltage conversion when operated at the low-impedance resonant frequency, allowing for dc-link voltage regulation, zero-voltage switching across a wide load range, and intrinsic load transient resilience; 2) power flow control when operated outside the low-impedance resonance for integrated battery charging; 3) and simple operational mode selection based on needed functionality with only a single control variable per mode. Derivation of the transfer characteristics for three operation cases using exponential Fourier series coefficients is presented. Pre-design evaluation of the S-LLC converter is presented using these analytical methods and corroborated through simulation. Furthermore, the construction of a rapid-prototyping magnetics design tool developed for high-frequency transformer designs inclusive of leakage inductance, which is leveraged to create the magnetic elements needed for this work. Two 2kW prototypes of the proposed topology are constructed to validate the analysis, with one prototype having a transformer incorporating the series resonant inductance and secondary clamp inductance into the transformer leakage and magnetizing inductance, respectively. A test bench is presented to validate the analysis methods and proposed multi-operational control scheme. Theoretical and experimental results are compared, thus demonstrating the feasibility of the new multi-mode operation scheme of the S-LLC converter topology

An Integrated Single-phase On-board Charger

With the growing demand for transportation electrification, plug-in electric vehicles (PEVs), and plug-in hybrid electric vehicles (PHEVs), cumulatively called electric vehicles (EVs) are drawing more and more attention. The on-board charger (OBC), which is the power electronics interface between the power grid and the high voltage traction battery, is an important part for charging EVs. Besides the OBC, every EV is equipped with another separate power unit called the auxiliary power module (APM) to charge the low voltage (LV) auxiliary battery, which supplies all the electronics on car including audio, air conditioner, lights and controllers. The main target of this work is a novel way to integrate both units together to achieve a charger design that is not only capable of bi-directional operation with high efficiency, but also higher gravimetric and volumetric power density, as compared with those of the existing OBCs and APMs combined. To achieve this target, following contributions are made: (i) a three-port integrated DC/DC converter, which combines OBC and APM together through an innovative integration method; (ii) an innovative zero-crossing current spike compensation for interleaved totem pole power factor correction (PFC) and (iii) a new phase-shift based control strategy to achieve a regulated power flow management with minimum circulating losses

Analysis, Design and Implementation of a Resonant Solid State Transformer

This thesis discusses the design of a full-bridge resonant LLC Solid State Transformer. The proposed topology uses a high-frequency transformer which helps minimizing its cost and size, and enables operating at varying load conditions. By using a resonant circuit, soft switching is achieved. Commutation techniques are discussed, namely ZVS and ZCS. Both concepts are applied on different legs of the H-bridge. Pulse frequency modulation (PFM) and Phase Shifting Modulation (PSM) are utilized to control this resonant converter. One of the requirements of this work is to achieve a tightly regulated DC bus voltage. This was shown to be achieved using the proposed controller. An experimental setup was assembled and the controller was tested, the results match the simulation and calculation results. The SST setup was tested for two different power levels. The outputs confirm the validity of the controller in feeding the load and regulating the voltage within the desired switching frequency interval, while maintaining soft switching. A thermal analysis was conducted to calculate losses, and a conversion efficiency of 97.18% was achieved. Using a high frequency transformer, a reduction in size and cost is achieved as compared to conventional low frequency transformers that usually are large in size and require more material to be assembled (copper and iron). Design requirements and limitations, the proposed control scheme, modeling and implementation, and test results are provided in this thesis