LLC resonant converters have been a popular choice for DC-DC converters due to their high efficiency, high power density, and hold-up capability in power supplies for communication systems, datacenters, consumer electronics, and automobiles. With the rapid development of wide-bandgap devices and novel magnetic materials, the push for higher switching frequencies to achieve higher power densities at lower costs is gaining traction. To demonstrate high efficiency and high power density, the Center for Power Electronics Systems (CPES) at Virginia Tech designed an 800W, 1MHz 400V/12V LLC converter for future datacenters, which could achieve a peak efficiency of 97.6% and a power density of 900 W/in3. However, with the ever-increasing demand for online services, the performance of power delivery must also be simultaneously improved to keep pace with the demand. The focus of this thesis is improving the performance of CPES’ previous 400V/12V LLC converter by investigating different aspects of its design and operation. Ultimately, design guidelines are proposed, and improvements are demonstrated to effectively achieve higher efficiency and higher power density than the previous CPES converter. Multiple aspects of the LLC converter’s design and structure are investigated to further improve its performance, and three main areas are the focus of this thesis. The output-side termination design of the planar transformer is investigated and modeled, and design guidelines for filter capacitor selection are provided for optimal efficiency. Next, the existing shielding technique for matrix transformers, which helps reduce common-mode (CM) noise without compromising on efficiency, is investigated for asymmetry and current-sharing issues, and modifications have been proposed to improve its efficiency. Thirdly, the LLC converter’s switching frequency is optimized to improve its performance over the previous CPES converter. Finally, the hardware results with the proposed improvements are demonstrated, and the converter’s performance is compared with the previous CPES converter as well as other recent proposed solutions. / M.S. / The electricity demand by datacenters has been growing exponentially over the past few decades, especially due to the boom of artificial intelligence in addition to other internet services. This has resulted in a requirement to continually improve the efficiencies of the power delivery from the grid, through the datacenter power architecture, and finally to the loads on the server racks. The overall datacenter power architecture has been improved over time to improve the total efficiency. However, the performance of each stage along the power architecture must be improved to keep in pace with the energy demand. The focus of this thesis is to improve the performance of the 400V/12V DC-DC stage for future datacenters. Previously, the Center for Power Electronics Systems (CPES) at Virginia Tech developed a 1MHz 800W 400V/12V LLC converter with 97.6% peak efficiency and 900W/in3 power density. However, the performance of the converter must be further improved to stay ahead of the competition and keep in pace with the increasing energy demand. Multiple aspects of the LLC converter’s design and structure are investigated to further improve its performance, and three main areas are the focus of this thesis. Firstly, the high-frequency termination design, or how different components are interconnected and arranged, is studied, and a capacitance selection guideline is proposed to maximize the efficiency. Next, the existing shielding technique for matrix transformers, which helps reduce common-mode (CM) noise without compromising on efficiency, is investigated for asymmetry and current-sharing issues, and modifications have been proposed to improve its efficiency. Thirdly, the LLC converter’s switching frequency is optimized to improve its performance over the previous CPES converter. Finally, the hardware results with the proposed improvements are demonstrated, and the converter’s performance is compared with the previous CPES converter as well as other recent proposed solutions.

The low-voltage dc-dc converter (LDC) in electrical vehicles (EVs) is to convert high dc voltage (270V~430V) from traction battery to low dc voltage (12.5V~15.5V) for the vehicle auxiliary systems. Galvanic isolation is required in the LDC due to safety considerations. Three challenges exist in the LDC design: (1) wide regulation range; (2) high output current; (3) thermal management. The single stage solutions, such as phase-shift full-bridge converter and LLC resonant converter, have been widely studied in the past. A matrix transformer is widely adopted in single-stage LDC design to deal with the large current. At last, the low-profile design allows large footprint area for high power density and ease of cooling design. However, the trade-off between wide regulation range and efficiency exists in single-stage LDC design. Recently, a two-stage solution is proposed to achieve high efficiency and wide regulation range at the same time. The fixed turn ratio LLC stage serves as a dc transformer (DCX) to meet the galvanic isolation requirements and PWM dc-dc stage regulates the output voltages. In this thesis, a variable ratio matrix transformer-based LLC converter is proposed for two-stage LDC efficiency improvement. The transformer secondary copper losses are reduced by taking advantage of the adaptive number of element transformers. In addition, the PWM dc-dc stage achieves better efficiency with variable intermediate bus voltage. The operation principle and design considerations are studied in this thesis. The proposed 1600W two-stage LDC prototype achieves 96.82% full load efficiency under 400V input condition which is 1.2% efficiency higher than the fixed ratio LLC based two-stage design. Last but not least, the prototype shows a comparable efficiency to the fixed ratio LLC based two-stage design even under the low input voltage (270V) condition. / M.S. / The electrical vehicle market is growing rapidly in recent years. However, the driving range is one of the bottlenecks which imperils market growth in the future. Thus, efficient power modules in electric vehicles are desired to extend the driving range. Low voltage dc-dc converter is one of the power modules in electric vehicles which is rated at several kilowatts and converts traction battery voltage for the vehicle auxiliary system, such as air conditioner, headlights, power steering and etc. In this thesis, a variable ratio matrix transformer-based LLC converter is proposed for the two-stage low-voltage dc-dc converter efficiency improvement. Consequently, the driving range of electric vehicles is further extended.

This thesis examines the development of DC-DC converters that are suitable for Module Integrated Converters, (MICs), in grid interactive photovoltaic (PV) systems, and especially concentrates on the study of the half bridge dual converter, which was previously developed from the conventional half bridge converter. Both hard-switched and soft-switched half bridge dual converters are constructed, which are rated at 88W each and transform a nominal 17.6Vdc input to an output in the range from 340V to 360Vdc. An initial prototype converter operated at 100kHz and is used as a base line device to establish the operational behaviours of the converter. The second hard-switched converter operated at 250kHz and included a coaxial matrix transformer that significantly reduced the power losses related to the transformer leakage inductance. The soft-switched converter operated at 1MHz and is capable of absorbing the parasitic elements into the resonant tank. Extensive theoretical analysis, simulation and experimental results are provided for each converter. All three converters achieved conversion efficiencies around 90%. The progressive increases in the operation frequency, while maintaining the conversion efficiency, will translate into the reduced converter size and weight. Finally different operation modes for the soft-switched converter are established and the techniques for predicting the occurrence of those modes are developed. The analysis of the effects of the transformer winding capacitance also shows that soft switching condition applies for both the primary side mosfets and the output rectifier diodes.