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For the generation of short high-power pulses in many applications, power modulators based on capacitor discharge are used, where the peak power is drawn from the input capacitor bank. In order to continuously recharge the energy buffer during operation at a lower average power, usually, power supplies connected to the mains are used. Due to the worldwide variation in mains voltages and the desired ability to adapt the capacitor voltage of the modulator, the power supply has to support a wide input and output voltage range, whereby the supply should draw a sinusoidal current from the mains due to EMI regulations. Additionally, depending on the modulator concept, a galvanic isolation also has to be provided. In order to achieve the mentioned specifications for the considered power supply, a combination of an ac-dc and a dc-dc converter is proposed, whereas the mains voltage is rectified by a three-phase buck-boost converter to 400 Vdc, and thereafter, an isolated dc-dc converter charges the input capacitor bank of the power modulator up to 3.5 kV. This paper focuses on the basic operation and the design of the 3.5-kV/11-kW isolated dc-dc converter, which includes transformer design, efficiency-volume optimization, and component selection. In this paper, compared with the well-known flyback converter, the proposed full-bridge-based topology results in a much higher efficiency and power density.