The paper discusses a new approach to the design of thermal management for high-frequency high-power transformers. Proposed thermal management concept is based on heat removal by conduction aided by advanced heat conducting elements (heat pipes). The heat is collected on dedicated surfaces called thermal surfaces where the heat exchange with the environment occurs. Several implementations of heat removal paths are proposed and core and winding structures are analysed in order to reach an optimal heat removal solution. Further, the proposed heat removal concept is evaluated with a focus on the design and evaluation process of the thermal management system. The paper is concluded with experimental results on a 50 kW transformer prototype that utilises the proposed heat removal concepts. Experimental results of a full power converter prototype are briefly discussed as well.

In this paper, the possibility of reaching high power densities in multikilowatt dc-dc converters with galvanic isolation is demonstrated and the main design issues are discussed. The issues related to converter topology, transformer design, and thermal management are addressed, and new conceptual solutions are proposed. Implementing zero-voltage-switching quasi-zero-current-switching topology, optimized transformer design with leakage layer, and thermal management based on conduction enhanced by heat pipes at critical places resulted in very high power density and efficiency. The power density reached by the converter prototype is 11.13 kW/L with water cooling and 6.6 kW/L with air cooling. In the same time, the measured efficiency exceeded 97% in a broad load range. The new design concepts are demonstrated on a 50-kW converter prototype that was successfully tested at full-load conditions.

High-frequency, high-power transformers are becoming of increasing importance. Such transformers often use foil windings because of their superior thermal properties. Transformers in resonant converters require a specific leakage inductance. The specific leakage inductance value can be obtained by implementation of so called leakage layer. The leakage layer might not influence only the leakage inductance value but also the losses in winding foils. The influence of the leakage layer geometry on the winding losses is discussed in this paper. FEM simulations are used to analyse the influence of several leakage layer configurations and the results are compared with the performance of transformer prototypes. The investigation showed that the leakage layer spanning the whole width of the winding window gives the best results with a relatively simple shape of the layer.

ZVS, quasi-ZCS topologies utilise a specific current waveform which demands specific properties from IGBT power modules. This paper compares the performance of three selected IGBT types under zero-voltage-switching, quasi-zero-current-switching. The main criterion of the evaluation is the generated power loss. The investigated IGBTs are implemented in a 50kW test converter for power loss measurements. The power loss is evaluated for turn-on, on-state and turn-off time intervals and for different load conditions. The investigation identifies strong and weak points of the compared IGBTs and selects a type which is the most suitable for ZVS, quasi-ZCS applications. The paper is concluded by presenting the practical efficiency measurements on the converter prototype with "the best" IGBT implemented.

This thesis is concerned with the possibilities of increasing the power density of high-power dc-dc converters with galvanic isolation. Three cornerstones for reaching high power densities are identified as: size reduction of passive components, reduction of losses particularly in active components and thermal management. In addition to the cornerstones, the spatial integration of converter components is considered as it is also important for high power density. The size reduction in passives is obtained by increasing the operating frequency. On the other hand, an increase of operating frequency yields also higher losses in passive components themselves. Therefore, the thesis addresses the power loss in windings of magnetic components with special attention to the transformer windings. Integration of several passive components into a single component is also considered as the means to increase the power density. The main challenge is the integration of components at the considered high power levels. A high operating frequency results in a substantial increase of switching losses in active components. The higher losses yield reduced efficiency and require larger heatsinks which consequently reduce the effect of the higher operating frequency on the overall power density. In this thesis, the power loss in active components is reduced by applying so called Zero-Voltage-Switching Quasi-Zero-Current-Switching topology. The heat generated inside converter components must be removed to prevent them from overheating. Performing this task becomes more difficult as the power density increases because of higher power dissipated in a smaller volume. The thesis considers thermal management of a power converter on component, converter and system level. Each of the levels is addressed separately and adequate heat removal methods and concepts are proposed. Spatial integration of converter components is important to obtain a high power density as well. The key to the successful integration of components is to make design choices which result in components of compatible dimensions and shapes. Basic guidelines that should serve as an aid in the design of high power density high-power converters are discussed in this thesis. The operating conditions and requirements vary with the power level processed by a power converter. Therefore, the scalability of the design concepts and approaches is important for their practical implementation in the wide range of considered applications. This thesis briefly discusses scaling up with respect to power density and performance of the converter components. The ability of achieving high power densities in high power is demonstrated on a 50 kW converter prototype. The reached power density is in order of 11.2 kW/litre with water cooling and 6.6 kW/litre with forced air cooling. It is also shown that by designing for high power density, the efficiency might not need to suffer. The measured efficiency of the final converter prototype is as high as 97.5 % in a broad load range.