For many years, increasing the efficiency while reducing the size has been the max-im in the development of power electronic converters. The introduction of novel wide-bandgap power semiconductors opens up new possibilities in the design of the converters, for example higher switching frequencies can be achieved or the converter can be operated at higher ambient temperatures. For the computer-aided design optimization as exact as possible loss models of the components are needed. However, the high switching speeds of the power semiconductors generate previously unknown problems for measurements. Therefore, different methods for the measurement of switching losses of power semiconductors as well as winding and core losses of magnetic components such as inductors and transformers are investigated in this thesis. Particular attention is paid to the effects of time shifts between current and voltage during power measurement. As an alternative measurement method, a calorimetric power measurement is investigated. This offers the advantage of being able to carry out the power measurement completely independently of the electrical system. A disadvantage is the “inertia” of the thermal system, which leads to long measurement periods, which, however, is compensated in part by a temperature control. Finally, the design of power electronic converters with wide-bandgap power semiconductors is discussed in several typical applications.