Modern wide-bandgap devices, such as SiC or GaN based devices, feature significantly reduced switching losses and the question arises if soft-switching operating modes are still beneficial. For most semiconductor devices only limited informa- tion is available to estimate the switching losses. Especially if a wide operating range is desired, excessive measurements have to be performed to determine the switching losses for arbitrary operating points. Therefore, in this paper a fast calculation method to determine the switching losses based on the charge equivalent approximation of the MOSFET capacitances, relying only on datasheet parameters, is presented. In addition, the turn- off losses at high switching currents are investigated and an analytical expression to estimate the maximum current range for which the MOSFET can be turned-off with negligible switching losses is proposed.

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 Swiss Federal Institute of Technology (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 (400 V/48 V). There, the characteristic of the power density, the efficiency, and the volume distribution between the components as functions of frequency are discussed. For the operating points with maximal power density, the loss distribution is also presented. Furthermore, 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/L and a switching frequency of ca. 130 kHz are obtained.

A fast and accurate model of the transformer leakage inductance is crucial for optimisation-based design of galvanically isolated converters. Analytical models are fastly executable and therefore specially relevant for such optimisations. This paper compares several analytical leakage inductance per unit length models with respect to accuracy and computational effort. The considered models are applicable to E-Core and U-Core transformers. 2D FEM simulations are used as benchmark to evaluate the model accuracy whereas the computation time is extracted as indicator for computational effort. Six different transformer prototypes provide the geometries for the comparison. Based on the conducted comparisons, Roth's model is the most accurate. Rogowski's model is the fastest low-error model. Margueron's model is the most versatile as it takes the finite permeability of the core into account. The conducted comparisons lay the foundation for accurate and fast Double-2D modelling of transformer leakage inductance as it is executed for the two main cross sections of E-core and U-core transformers: inside the transformer window and outside the transformer window.

Due to the increasing integration of renewable energies into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low frequency transformer and a fractionally rated power electronic converter, hybrid transformers can be applied to ensure a high power quality by controlling voltage, active and reactive power dynamically. For the application in conventional grids, hybrid transformers have to withstand considerable overvoltages and overcurrents during voltage surges or short circuits. Although hybrid transformers recently gained increasing research interest, protection requirements and concepts have not been discussed yet. These aspects are, however, essential to evaluate the practical potential of hybrid transformers. In this paper, protection requirements are derived and a protection concept is developed and verified by simulations and experiments for a 100kVA hybrid distribution transformer. Although the protection concept has a considerable impact on the design and performance of the converter, it increases the total losses and volume of the hybrid transformer only insignificantly. An efficiency of 98.64 % is achievable for a hybrid transformer including protection, which is only slightly lower than the efficiency of 98.48 % for a comparable conventional transformer. This renders the HT a promising solution for today's and future AC grids.

In this paper, an analytical method to model the thermal resistance of windings made either of solid or litz wire is presented and validated by measurements. In addition, an extended numerical approach for litz wire windings, which also considers the twist pitch, is shown. With the presented model it is possible to describe the thermal resistance of arbitrary wire arrangements. This approach can be used in straight forward thermal designs of magnetic devices, or it can be integrated in optimization procedures to improve the thermal designs of magnetic components. The analytical models are verified by measurements with non-potted and epoxy-potted solid wire test setups and test setups for litz wire, which all show a good matching between calculated and measured values. Besides the models for the thermal resistance between the winding layers, also a method to simplify the thermal network of multi-layer windings is presented. Finally, a thermal equivalent circuit of a test transformer has been calculated, which shows a good match between the measured and the predicted temperature distribution.

In this paper, two control strategies for voltage balancing in a modular serial parallel resonant converter modulator system, which is used in a high voltage pulsed power application, are presented and verified by simulations and measurements. The system is based on two series parallel resonant converter modules, forming an input series output parallel stack. To obtain the given output voltage pulse of 115 kV, 9 of these input series output parallel stacks are connected in parallel at the input and in series at the output, forming an input parallel output series system. A robust input voltage balancing by an auxiliary circuit and an output voltage balancing by control is introduced and proofen by measurements. In addition, an alternative approach, where the input and output voltage balancing is achieved purley by control, is given and verified by simulations. For designing the control of this system, a large and a small signal model is derived and the influence of component tolerances is investigated.