Rapid expansion of the information technology (IT) sector, market size and consumer interest for off-line power supply continue to rise, particularly for computers, flat-panel TVs, servers, telecom, and datacenter applications. Normal components of an off-line power supply include an electromagnetic interference (EMI) filter, a power factor correction (PFC) circuit, and an isolated DC-DC converter. For off-line power supply, an isolated DC-DC converter offers isolation and output voltage adjustment. For an off-line power supply, it takes up significantly more room than the rest; thus, an isolated DC-DC converter is essential for enhancing the overall performance and lowering the total cost of an off-line power supply. In contrast, data center server power supplies are the most performance-driven, energy-efficient, and cost-aware of any industrial application power supply. The full extent of data centers' energy consumption is coming into focus. By 2030, it is anticipated that data centers will require around 30,000 TWh, or 7.6% of world power usage. In addition, with the rise of cloud computing and big data, the energy consumption of data centers is anticipated to continue rising rapidly in the near future. In data centers, isolated DC-DC converters are expected to supply even higher power levels without expanding their size and with much greater efficiency than the present standard, which makes their design even more challenging. LLC resonant converters are frequently utilized as DC-DC converters in off-line power supply and data centers because of their high efficiency and hold-up capabilities. LLC converters may reduce electromagnetic interference because the primary switches and secondary synchronous rectifiers (SRs) both feature zero-voltage-switching (ZVS) and zero-current-switching (ZCS) for the SRs. Almost every state-of-the-art off-line power supply uses LLC converters in their DC-DC transformations. However, LLC converters face three important challenges. First, the excessive core loss caused by the uneven flux distribution in planar magnetics, owing to the huge size and high-frequency operation of the core. These factors led to the observation of dimensional resonance within the core and an excessive amount of eddy current circulating within the core, which resulted in the generation of high eddy loss within the ferrite material. This was normally assumed to be negligible for small core sizes and lower frequencies. This dissertation proposes methods to help redistribute the flux in the core, particularly in the plates where the majority of core losses are concentrated, and to provide more paths for the flux to flow so that the plates' thickness can effectively be reduced by half and core losses, particularly eddy loss, are reduced significantly. Second, the majority of power supplies in the IT sector are needed to deliver high-current output, but the transformer is cumbersome and difficult to build because of its high conduction losses. In addition, establishing a modular solution that can be scaled up to greater power levels while attaining a superior performance relative to best practices is quite difficult. By increasing the switching frequency to several hundred kilohertz using wide-band-gap (WBG) transistors, printed circuit board (PCB) windings may include magnetics. This dissertation offers a modular and scalable matrix transformer structure and its design technique, allowing any number of elemental transformers to be integrated into a single magnetic core with significantly reduced winding loss and core loss. It has been shown that the ideal power limitations per transformer for PCB-based magnetics beat the typical litz wire design in all design areas, in addition to the unique advantages of PCB-magnetics, such as their low profile, high density, simplicity, and automated construction. Alternatively, shielding layers may be automatically put into the PCB windings between the main and secondary windings during the production process to reduce CM noise. A method of shielding is presented to reduce CM noise. The suggested transformer design and shielding method are used in the construction of a 3 kW 400V/48 V LLC converter, with a maximum efficiency of 99.06% and power density of 530W/in3. Thirdly, LLC converters with a matrix transformer encounter a hurdle for extending greater power, including the number of transformers needed and the magnetic size. In addition to the necessity of resonant inductors, which increase the complexity and size of the magnetic structure, there is a need for a resonant inductor. By interconnecting the three-phases in a certain manner, three-phase interleaved LLC converters may lower the circulating energy, but they have large and numerous magnetic components. In this dissertation, a new topology for three-phase LLC resonant converters is proposed. Three-phase systems have the advantage of flux cancellation, which may be used to further simplify the magnetic structure and decrease core loss. In addition, a study of the various three-phase topologies is offered, and a criterion for selecting the best suitable topology is shown. Compared to the single-phase LLC, the suggested topology has less winding loss and core loss. In addition, three-phase transformers have a lower volt-second rating, and smaller core sizes may be used to mitigate the impact of eddy loss in the ferrite material. In contrast, three-phase systems offer superior EMI performance, which is shown in the loss and size of the EMI filter, and much less output voltage ripple, which is reflected in the size of the output filter. Finally, several methods of integrating resonant inductors into transformer magnetics are presented in order to accomplish a simple, compact, and cost-effective magnetic architecture. By increasing the switching frequency to 500 kHz, all six transformers and six inductors may be achieved using four-layer PCB winding. To decrease CM noise, additional 2-layer shielding may be implemented. A 500 kHz, 6-8 kW, 400V/48V, three-phase LLC converter with the suggested magnetic structure achieves 99.1% maximum efficiency and a power density of 1000 W/in3. This dissertation addresses the issues of analysis, magnetic design, expansion to higher power levels, and electromagnetic interference (EMI) in high-frequency DC/DC converters used in off-line power supply and data centers. WBG devices may be effectively used to enable high-frequency DC/DC converters with a hundred kilohertz switching frequency to achieve high efficiency, high power density, simple yet high-performance, and automated manufacture. Costs will be minimized, and performance will be considerably enhanced. / Doctor of Philosophy / The IT industry, market size, and customer interest in off-line power supply continue to grow quickly, especially for computers, flat-panel TVs, servers, telecom, and datacenter applications. Off-line power supplies usually have a DC-DC converter, an EMI filter, and a PFC circuit. A DC-DC converter is needed for an off-line power supply. An isolated DC-DC converter makes an off-line power supply work better and cost less, even though it takes up more space than the rest. But power supplies for data center servers are the most performance-driven, energy-efficient, and cost-conscious industrial applications. It's becoming clear how much energy data centers use. By 2030, data centers will use 7.6% of the world's power, or 30,000 TWh. With the rise of cloud computing and big data, energy use in data centers is likely to go up by a lot. In data centers, isolated DC-DC converters are expected to have much more power without getting bigger and to be much more efficient than the current standard. This makes their design even harder. LLC resonant converters are often used as DC-DC converters in data centers and off-line power supplies because they are very efficient and easy to control. LLC converters may have less electromagnetic interference because both the primary switches and the secondary synchronous rectifiers (SRs) have zero-voltage-switching (ZVS) and zero-current-switching (ZCS). Almost every modern off-line power supply uses LLC converters for DC-DC stage. LLC converters have to deal with three big problems. Due to the large size of the core and the high frequency of operation, the uneven distribution of flux in planar magnetics causes too much core loss. This dissertation suggests ways to redistribute flux in the core, especially in the plates where most core losses are concentrated and provide more flux paths to reduce plate thickness by half and core losses, especially eddy loss. Second, most IT power supplies need to put out a lot of current, but transformers are bulky and hard to build because they lose a lot of current. It is hard to make a modular solution that can scale up to higher levels of power and perform better than best practices. With wide-band-gap (WBG) transistors, the switching frequency can be raised to several hundred kilohertz so that magnetics can be added to PCB windings. This dissertation describes a modular and scalable matrix transformer structure and design method that lets any number of elemental transformers be put into a single magnetic core with much less winding loss and core loss. PCB-based magnetics have a low profile, a high density, are easy to build, and can be built automatically. Their ideal power limits per transformer beat the typical litz wire design in every way. Shielding layers can be added automatically between the main and secondary PCB windings to cut down on CM noise. CM noise is lessened by shielding. The suggested transformer design and shielding method are used to build a 3 kW 400V/48 V LLC converter with a maximum efficiency of 99.06% and a power density of 530W/in3. Third, LLC converters with matrix transformers can't get more power without more transformers and a bigger magnetic size. Resonant inductors, which add to the size and complexity of a magnetic structure, are also needed. By connecting the three phases, three-phase interleaved LLC converters use less energy, but they have a lot of magnetic parts. In this paper, a three-phase LLC resonant converter topology is proposed. In three-phase systems, flux cancellation makes magnetic structures easier to understand and reduces core loss. There is also a study of three-phase topologies and a set of criteria for choosing one. Compared to the single-phase LLC, the topology cuts down on winding and core loss. Three-phase transformers have a lower volt-second rating, and ferrite material eddy loss can be reduced by making the core smaller. The size and loss of the EMI filter show that three-phase systems have less output voltage ripple and better EMI performance. Finally, several ways of putting resonant inductors into the magnetics of a transformer are shown to make a magnetic architecture that is simple, small, and cheap. At 500 kHz, all six transformers and all six inductors can be wound on a four-layer PCB. CM noise can be cut down with 2-layer shielding. With the suggested magnetic structure, a 500 kHz, 6-8 kW, 400V/48V, three-phase LLC converter can reach 99.1% maximum efficiency and 1000 W/in3. This dissertation presents analysis, magnetic design, expanding to higher power levels, and electromagnetic interference (EMI) in high-frequency DC/DC converters used in off-line power supplies and data centers. WBG devices can be used to make high-frequency DC/DC converters with a switching frequency of a few hundred kilohertz that are powerful, easy to use, and can be automated. Both cost and performance will get better.