Module-integrated photovoltaic (PV) systems show high robustness against mismatching of the PV-generator, e.g. reasoned by partial shading. Thus, the surface potential for PV can be increased by the unevenly irradiated surfaces using module-integrated PV. Consequently, the important, already sealed surfaces in urban areas can be utilized to contribute to a sustainable energy supply.Different module-integrated system concepts are reviewed and compared. The parallel module-integrated converter concept is identified as the most flexible, safe and cost effective solution. It consists of modules with module-integrated DC-DC converters, feeding into one DC-distribution line in parallel. Since grid connection and metering is performed in a central unit, the critical DC-DC converter is designed to minimum functionality, i.e. maximum power point tracking, safety and efficiency. The system is flexible and scalable for arbitrary modules and can be combined with classical string or central systems.In this work a multi-resonant LLCC-type converter providing galvanic isolation is identified as the best topology, regarding soft-switching of semiconductor devices, high part-load efficiency and controllability. Detailed investigations are carried out for three research foci of the DC-DC converter. With the goal of high converter efficiency, first design rules are derived in a holistic approach for the resonant tank comprising five degrees of freedom. As second step, the integration of the magnetic resonant tank elements into one transformer-inductor device is analyzed. It is identified and included in the design rules, that an important design parameter of the previous step is only a function of geometry and material of the transformer-inductor device. Finally, the design and analysis of single- and three-phase LLCC-type converter solutions are considered and experimentally verified. Conclusions are given and detailed aspects on how to make use of single-phase and three-phase DC-DC converter advantages are qualified.A single-phase multi-resonant module-integrated prototype converter is presented. Corresponding to one of the design rules, i.e. that the converter has to be designed to the specified operation region, the transferable power is limited by the resonant tank elements, not by hot spots. The critical part-load efficiency reaches values up to 97.5 % at power levels around 100 W.