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The demand for decreasing costs and volume leads to a constantly increasing power density of industrial converter systems. In order to improve the power density further different aspects, like thermal management and electromagnetic effects must be considered in conjunction with the electrical design. Therefore, a comprehensive optimization procedure based on analytical models for minimizing volume of DC-DC converter systems has been developed at the Power Electronic Systems Laboratory of the ETH Zurich. Based on this procedure three converter topologies - a phase shift converter with current doubler and with capacitive output filter and a series-parallel resonant converter - are optimized with respect to power density for a telecom supply (400V/48V). There, the characteristic of the power density, the efficiency and the volume distribution between the components as function of frequency is discussed. For the operating points with maximal power density also the loss distribution is presented. Further more, the sensitivity of the optimum with respect to junction temperature, cooling and core material is investigated. The highest power density is achieved by the series-parallel resonant converter. For a 5 kW supply a density of approximately 12 kW/ltr. and a switching frequency of ca. 130 kHz results.