This paper proposes a design approach of a partial power processing(PPP) based dc-dc converter for electric vehicle extreme fast charging stations. The partial power converter is made up of two stages: the front-end stage is an input-series output-parallel(ISOP) LLC converter, which acts as a dc-dc transformer(DCX). The rear-end stage is a three-phase interleaved buck converter to realize flexible constant current/voltage(CC/CV) charging control. With the proposed topology structure, only less than one fifth of the total charging power is processed. Thus, compared with full rated charging converters, its efficiency and power density are significantly improved. The operation principles with modulation strategy is presented in depth. In addition, a simple control strategy is proposed to realize current-sharing control of the three-phase interleaved buck converter. The system design approach is provided in detail. Finally, simulation and experimental tests on a 300-kW prototype validate its appropriate performance.