Investigation and Design of Integrated Magnetics in High-power High-frequency Soft-switching DC/DC Converters for Battery Charger Applications

Abstract

Electric vehicles (EVs) have become more and more popular from higher fuel prices and growing global warming concerns in recent years. These vehicles rely on rechargeable battery packs that can be charged by external power sources. Despite their benefits, widespread adoption of EVs faces challenges like on-board chargers (OBC) with lighter weight, higher efficiency and higher power. There are two charging approaches for EVs: on-board charging for daily use and off-board fast charging for energy refill within an hour. The majority of EVs operate with 400V battery platforms and are equipped with 6.6 kW or 11 kW OBCs. In contrast, some higher end vehicles with battery capacities exceeding 100 kWh use 22kW OBCs, though these are limited by standard residential voltage and current constraints. To overcome these limitations, some manufacturers are shifting toward 800V battery architectures, which reduce charging time and lower overall system costs. This shift necessitates the research and innovation in 800V OBCs with enhanced efficiency and power density. Additionally, EVs can be charged using OBCs in a grid-to-vehicle power transfer mode and can transfer power from the batteries back to household appliances or grid (vehicle-to-grid) for camping or as part of the smart grid. Therefore, bidirectional function is needed for OBCs nowadays. Progress in power electronics has been largely driven by advancements in power semiconductor devices. Wide bandgap (WBG) semiconductors have introduced a great leap beyond traditional silicon-based devices. These WBG devices enable higher efficiency and power density, and they support operation at higher switching frequencies. Increasing the switching frequency reduces the volt-second across transformers, which allows for the use of printed circuit board (PCB)-based windings instead of traditional Litz wire. This transition simplifies manufacturing, reduces cost and improves control over parasitic elements. Moreover, integrating resonant inductors into PCB-based transformers further reduces the total number of transformers and inductors needed and boosts power density. This dissertation investigates the high-power resonant converters in bidirectional battery chargers, with a focus on achieving higher power density and efficiency using scalable magnetics building blocks (SMBs). It outlines the specific contributions made toward improving manufacturing processes and enhancing system performance. First, a dual channel flux analysis method is proposed to analyze the flux distribution in complicated integrated magnetics. The utilization of the method is carried out in all the chapters to aid the arrangement of the complicated structures. Second, two SMBs are proposed and compared for different powers. Both structures can be utilized in both single phase CLLC resonant converters and three phase CLLC resonant converters. The applications of the SMBs for different power levels are compared and analyzed. Third, a single phase 22-kW on-board charger structure for 800 V batteries using one SMB structure is proposed. Various planar transformer candidates are presented and thoroughly compared for 22-kW OBCs. Integration's impact on the transformer characteristics is analyzed, including the coupling between the windings, inductance, and flux distributions. Different numbers of parallel windings are also compared. The most promising candidate is chosen for the application. A general integrated transformer with good scalability is introduced and analyzed. The current sharing can be controlled without additional complexity so that better thermal balance is realized between the parallel windings. The resonant inductance is also integrated and controlled. The exposed windings give thermal management design more possibilities with a simple core structure. Comprehensive analysis and comparison of the proposed matrix transformers' arrangements is conducted. Detailed flux analysis and loss comparison are carried out. The guidelines with the smallest core loss are given. A fully PCB packaged SiC-based single-phase CLLC resonant converter for 22-kW OBC applications was created using the proposed integrated transformer structure, achieving a power density of 11.6 kW/L with maximum efficiency of 98.5% under 250 kHz switching frequency. Fourth, a 3PCLLC 22-kW OBCs using another kind of SMB structure with linearly control in the leakage inductance is discussed and proposed. The inductance calculation model includes integration using unbalanced windings and integration using pure inductors. Integrating using pure inductors can reduce the winding loss with a smaller footprint. So, it is more beneficial in high power applications with more leakage inductance integration. The flux is analyzed in detail. The benefit of three phases is discussed, which is flux cancellation between the three phases. Various structures are compared, and a more rigorous and time-efficient optimization and design process is proposed. Fifth, to enhance efficiency further, different winding structures are proposed for 3PCLLC resonant converters for 22-kW OBCs. 13% winding loss reduction is found using the optimized winding structure. To fully utilize the PCB layers, parallel windings are adopted for the inductors. Different parallel patterns are analyzed. The current is found to concentrate on the top layers for direct parallel windings. Three structures with perfect current sharing are compared and discussed. Using symmetrical structures can achieve perfect current sharing, but will decrease the power density. Using twisting method can achieve good current sharing without sacrificing power density, but the distribution of inside the copper is not guaranteed. Using PCB Litz can have evenly distribution in the lateral direction with a smaller AC resistance over DC resistance, but the DC resistance will be larger. After comparison and discussion, the twisting method is better is the first and second layer. PCB Litz wire is better in the third layer and above. Sixth, for 30 kW 1PCLLC converters in fast chargers, the stress on PCB windings will be too high to carry. Also, the requirement of power density for fast chargers is not as strict as OBCs. Litz wire can be utilized for these applications. However, designing a transformer for high power and high frequency with litz wire is still challenging because the strands number is large for high current. Therefore, parallel windings is needed to handle the current stress. Different winding patterns are compared, and a current distribution model for Litz wire transformers is proposed. Finally, two SMBs using litz wire are found to have perfect sharing based on the model and simulation results. Different resonant inductance integration methods are also compared and discussed. Integrating the resonant inductance in the air is found to be suitable for smaller inductance values if the strand AWG keeps the same. Integrating the resonant inductance in the core is found to be suitable for large inductance values. Finally, a 30-kW hardware is built to prove the concept with a peak efficiency of 99.2%.