## 5 Conclusions  
This paper describes a new method of computing the waveforms of the junction temperature of discrete power SiC-MOSFETs operating in switched networks at the steady state. It is proved that this method makes it possible to obtain the correct results at a much shorter computation time than for the classical method of transient analysis. On the other hand, the use of the method using one non-physical thermal time constant makes it possible to obtain the correct average value of the junction temperature at the steady state, but the peak-to-peak value of this temperature can be much underestimated. It is worth emphasizing that in the electrothermal analysis of switching circuits, the power of switching losses should not be ignored as this may lead to a significant underestimation of the junction temperature. For two typical applications of power electronic networks some electrothermal computations were performed for different cooling systems of the transistor. The classical switch with a power SiC-MOSFET and a boost converter with this transistor were considered. The computer simulations show that switching losses in the transistor are an important component of power losses. Their part in the total losses is an increasing function of switching frequency and can be as high as 95%. As the switching frequency increases, the junction temperature \(T_{j}\) of the transistor also increases in proportion to the value of thermal resistance. The variable wave of the power dissipated in the transistor causes ripples in the \(T_{j}(t)\) waveform. The peak-to-peak value of these ripples depends on the switching frequency, load resistance and transient thermal impedance of the transistor characterizing the dynamic properties of the cooling system used. The peak-to-peak value of temperature \(T_{j}\) ripples is a decreasing function of frequency and load resistance. The considered ripples are the biggest for system C with very short thermal time constants. If the period of the transistor control signal is close to the values of thermal time constants, the peak-to-peak values of ripples of the transistor junction temperature may exceed even 25 K. If this period is bigger, these ripples can be much bigger. The observed peak-to-peak values of the transistor junction temperature ripples may affect the reliability of such transistors. For example, at very low switching frequency and a high value of the dissipated power, the value of the junction temperature can change between the room temperature and the welding temperature of the soldering alloy. Such an operation mode can result in a very short lifetime of the used transistor. It is worth noting that these ripples are the biggest for the system, in which the most effective cooling system of the transistor case is used, while the peak-to-peak value of these ripples is negligible for the classical passive transistor cooling systems characterized by a high value of thermal resistance and long thermal time constants. The presented investigation results may be useful for designers of cooling systems for transistors working in power supply systems. They show that the indispensable efficiency of a cooling system depends strongly on the operating frequency of the analyzed circuit. On the other hand, it is also important that for cooling systems characterized by longer thermal time constants it is possible to obtain a lower smaller peak-to-peak value of the transistor junction temperature. Therefore, free cooling systems could be better than forced cooling systems for power transistors operating in switching circuits. The obtained results will also be used to develop more accurate and faster methods of the electrothermal analysis of switching circuits. Conceptualization, K.G. ; methodology, K.G. and P.G. ; validation, K.G. and P.G. ; investigation, K.G. ; writing--original draft preparation, K.G. and P.G. ; writing--review and editing, K.G. and P.G. ; visualization, K.G. and P.G. ; supervision, K.G. All authors have read and agreed to the published version of the manuscript.