The subject of this thesis is measurement of virtual junction temperature the insulated gate bipolar transistors (IGBTs) in operating conditions. The objective is to develop a new method for measurement of virtual junction temperature in insulated gate bipolar transistors (IGBTs) in operating conditions that could be performed in operating conditions and at the driver potential. The purpose of the thesis is to develop a measurement method that can be used for over temperature protection and for real-time measurement of virtual junction temperature in operating conditions. The thesis begins with a description of IGBT fundamental structure and phenomenology. The purpose of the description is to enable a better understanding of the principles of IGBT operation and its dynamic characteristics. The IGBT control and the production of inverse layer and conductive channels are described. An explanation is given of the threshold voltage, which is used in this thesis as a temperature-dependent parameter for determination of virtual junction temperature. Current and voltage wave shapes in IGBT during switching are presented. Fundamental concepts of driver are considered. Some effects of driver on IGBT voltage and current relations during switching are also presented. The influence of stray inductances on switching losses is analysed. Chapter three deals with IGBT modelling. The thesis presents IGBT models that can be applied rather to converter design and development than to the production of IGBT itself. Among the presented models there are the simple ones, which can be used in analytical methods, more complex ones, which are used in various simulators, and even the most complex numerical ones. The objective of this chapter is to present various possibilities of modelling, what is important for choice of IGBT current class and for thermal computation. Electric, thermal and electrothermal IGBT models are presented. Electrothermal modelling based on transient thermal impedance and equivalent RC network is analysed. In chapter four an outline of the existing methods for measurement of virtual junction temperature is given. Recent methods for real-time measurement of virtual junction temperature of IGBT based on measurements and calculations preformed using digital microprocessors for rapid signal processing. In computation, mathematical models are used that describe the behaviour of certain converter structures. An application of such a method for implementation of circuits that are dependent on the virtual junction temperature is presented as well. The same chapter deals with temperature dependence of threshold voltage. Also measurement results are presented that were obtained on various types of power IGBTs (current class from 100 to 1000 A), on various samples of the same type, but on IGBTs of different manufacturers. The measurement results agree with the results found in literature. Temperature dependence is linear, and the results obtained both by IGBT cooling and by heating are the same. The subject of chapter five is the development of a method for measurement of equivalent temperature of IGBT silicon based on temperature dependence of threshold voltage that can be implemented on the driver potential and can be used in operating conditions. A description is given of the switch on which the investigations were performed. IGBT current and voltage wave shapes during switching are presented. The voltage between the gate and the emitter, as the threshold voltage, is measured at the moment when the voltage drop appears on the stray inductance between the control and power connection of the emitter appearing during the IGBT conduction time. Various time relations are presented. A condition for voltage measurement on this stray inductance was introduced in order to improve the measurement accuracy. By this condition the measurement in the range of voltage zero crossing between the gate and the emitter. As an additional condition, the value of the voltage between the gate and the emitter higher exceeding two volts is introduced. This enables adjustment of the voltage drop detection on the above-mentioned stray inductance to the values that are negligibly higher than zero (several tens of mVs). In chapter six the application of newly developed method for implementation of the protection against excessive virtual junction temperature is described. The protection is implemented at the driver potential of the IGBT emitter. The protection actuation signal is transferred galvanically separated to the superordinated control electronics. Besides the protection, also the current limitation in dependence on the virtual junction temperature is described. The signal that the virtual junction temperature has reached the set value is transmitted to the superordinated control electronics, where the current limitation algorithm is implemented. The subject of chapter seven is the application of the new method for real-time measurement of virtual junction temperature in operating conditions. Hardware and software were implemented and used for the measurement of virtual junction temperature during the switching of IGBT to the preset temperature of the heated IGBT. The measurement results were compared with the temperatures obtained by thermopairs installed in the vicinity of IGBT. The measurement accuracy is analysed. The possibilities of data storage by means of hardware and also of data transfer to the master computer are also implemented. In conclusion, the possibilities for application of this new method of measurement of virtual junction temperature of IGBT are presented. This method enables implementation of the protection against excessive virtual junction temperature, of current limitation in dependence on the virtual junction temperature and also the real-time measurement of virtual junction temperature in operating conditions. All of these possibilities can have considerable effects on the price, reliability of operation, weight and dimensions of converters with power IGBTs.