As the core component of the isolated DC-DC converter, high-frequency transformers isolate different voltage levels, significantly determine the reliability and safety of the DC-DC converter. Among high-frequency transformers, planar high-frequency transformers show good repeatability and modularity. PCB, commonly used as the winding of planar transformers, has the potential of being used in high insulation applications with the electrical strength of 35 kV. However, planar transformers are rarely used in high-voltage occasions. Therefore, this paper explores the possibility of using planar transformers in high isolation voltage standards (>10 kV). The major concerns during the insulation design process are fully included. The design process of high-voltage-resistance planar transformers is also introduced. A specific prototype design is analyzed to verify the design and optimization methods. The power density of the transformer reaches up to a high level (14 kW/L) and the insulation withstand voltage fulfills the high-voltage requirement (>10 kV).