With the growth of renewable energy, electric vehicles, and the smart transformer market, isolated DC-DC converters have become indispensable as they provide galvanic isolation for protection and voltage transformation. High-frequency transformers play an important role in the size, power density, efficiency, and cost of the DC-DC converters. Conventional line-frequency power transformers are not well suited for high-power isolated DC-DC converters in renewable energy systems, EV fast chargers, and smart transformers as they have substantial volume and weight, and extremely high cost of installation, routine maintenance, and repair. On the other hand, the power handling capability of a single existing commercial high-frequency transformer is capped at around 20 kW, therefore, multiple high-frequency transformers are needed for parallel connections to satisfy higher power requirements. This thesis presents the development of a novel compact modular high-frequency transformer structure design, seamlessly integrated with SiC MOSFETs, for use in isolated DC-DC converters. Addressing the limitations of traditional line-frequency transformers and conventionally structured high-frequency transformers, this research introduces an innovative annular-shaped high-frequency transformer with multiple discrete pairs of U cores forming a radial arrangement with scalable size and power handling capability. The high-frequency transformers utilise this novel annular structure to achieve significant reductions in volume, weight, and cost without compromising efficiency. Another critical advancement is the integration of high-frequency transformer and SiC semiconductor devices, accommodating SiC semiconductor devices directly within the high-frequency transformer package. This integration not only notably reduces the overall footprint and weight of the whole DC-DC converter but also enhances its power density, establishing a superior alternative to existing solutions. The high-frequency transformer's unique design, combined with an appropriate winding configuration, can achieve sufficient leakage inductance without the need for physical auxiliary inductors for phase-shift full-bridge DC-DC converters. By eliminating external auxiliary inductors, which is a particularly advantageous feature, the phase-shift full-bridge DC-DC converters allow further advancement in overall compactness. Experimental validations for prototypes using soft ferrite and amorphous alloy cores of various power ratings, including 360 W, 1.5 kW, 11 kW, and 60 kW, supported by comprehensive 3D finite element method-based electromagnetic modelling and simulations in high-frequency transformers, confirm the feasibility and effectiveness of the proposed high-frequency transformer design. This research work also underscores the benefits of the novel design, including improved compactness and power density, and reduced weight and cost for a high-frequency transformer as a component and as an integrated DC-DC converter.