This work deals with increasing the switching frequency of transistors in power electronic converters. Higher switching frequencies lead to smaller passive components and this reduces the material cost and allows to meet space requirements more easily. In addition, faster converters respond more quickly to the changes of the controller signals. The facilitation of the increase of switching frequencies in power converters is therefore very important. First, wide-bandgap semiconductors are studied. These are semiconductor materials that allow to reduce the dimensions of switching components with respect to silicon components for the same power capabilities. Therefore, they can switch faster and be operated at higher frequencies. A cooperation with Imec was started. Imec developed gallium nitride HEMTs. Important electric characteristics of these devices are measured in this work and a figure of merit is determined in order to compare them with conventional components and with WBG-competitors. A new measuring circuit is developed in order to determine the dynamic on-resistance of HEMTs and MOSFETs. Another way to increase the switching frequency is the application of resonant techniques. As a result, the switching losses can be kept low and silicon components can be operated at high switching frequencies and fast rise and fall times. An LLC-converter is designed and developed. The necessary and sufficient conditions for zero voltage switching are analysed. The optimum values of the elements in the resonant tank are determined. The parasitic elements of the transformer are incorporated in the resonant tank. An accurate high-frequency model is developed for the transformer and it is examined whether the use of an air core has advantages. A fast gate-driver is developed and built, capable to control a half-bridge. The losses are simulated and determined for each element of the converter separately. The converter is also constructed and its efficiency is measured. In converters operating at high frequencies and with fast dv/dt's and di/dt's, the parasitics in the components and those of the copper PCB-tracks become important. Unwanted oscillations and overshoot can occur, and therefore, a method is used in this work to predict these phenomena: the Partial Element Equivalent Circuit method. This technique is extensively validated by comparison with measurement results and the results of other modelling techniques, and is subsequently applied to switching converters. Oscillations, overshoot and signal distortion are not the only problems caused by high frequencies and fast components, also the emission of electromagnetic fields is problematic. These fields may interfere with electronics in the vicinity of the converter. A tool is therefore developed in this work for calculating these fields. It is validated by applying it to systems and antenna topologies for which analytical solutions exist or for which results could be obtained with other numerical techniques such as the Method of Moments or the Finite Element Method. Then, the field calculation technique is used to obtain the radiated emissions in CISPR 22 EMC-tests of an inverted buck converter.