This paper presents a polymer-bonded magnetic core (PBMC) for high-frequency converters. The material of the magnetic core is composed of polymer matrices and special magnetic powder. With these materials, lightweight, nonbrittle, low-cost magnetic cores can be produced with flexible shapes and different dimensions. The magnetic field of an EI-type PBMC used for fly-back converter is obtained using two dimensional (2-D) finite element analysis (FEA). The detailed energy conversion process is studied with 2-D, direct-coupled field-circuit, and time-stepping FEA. The electromagnetic power conversion of the converter is illustrated with circuit and field analysis result. A 100 W fly-back converter is implemented with a novel-developed PBMC. The analysis is verified by experimental results, and the application feasibility is validated with the PBMCs for power conversion.