Electronic DC transformer with high power density

TL;DR: In this paper, the authors considered the possibility of increasing the power density of high-power dc-dc converters with galvanic isolation, and three cornerstones for reaching high power densities are identified: size reduction of passive components, reduction of losses particularly in active components and thermal management.

Abstract: 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.