## 4 Investigations Results
### Analyses of Properties of the Considered Networks  
Using the proposed method, some computations illustrating the influence of the used cooling system, switching frequency and load resistance of the tested networks were performed. Selected results of these computations are presented in Figures 7-14. Figures 7-11 show the steady-state characteristics of a transistor switch, and Figures 12-14 show a boost converter. The investigations were performed for the SIC-MOSFET situated in different cooling systems. The calculations, the results of which are shown in Figure 7, were made with load resistance \(R_{0}=40\)\(\Omega\). As can be seen in Figure 7a, an increase in frequency causes a practically linear increase in the value of the average power dissipated in the transistor. This increase results from an increase in the switching losses at practically constant conduction losses. In the considered change in frequency from 1 to 400 kHz, the value of \(p_{avg}\) increases by up to 10 times. It is also visible that the transistor cooling system has practically no effect on the \(p_{avg}(f)\) course. The choice of the cooling system only affects the maximum allowable operating frequency of the circuit under consideration. An excess in the junction temperature above the ambient temperature \(\Delta T_{j}\) is also a linearly increasing function of frequency. The value of this excess for the fixed frequency is proportional to the value of thermal resistance \(R_{th}\). It can be seen that for system A already at frequency \(f=100\) kHz, the temperature excess \(\Delta T_{j}\) reaches 190 K, while for system C at \(f=400\) kHz, this increase is much smaller and it does not exceed 10 K.  
In Figure 7c it can be observed that the peak-to-peak value of the considered temperature excess \(\Delta T_{ipp}\) is a decreasing function of frequency, and the highest values are obtained for system B characterized by the thermal time constant of the lowest value. Figure 8 shows the waveforms of the power dissipated in the transistor cooperating with system C and working in the switch circuit loaded with a resistor of \(R_{0}=40\)\(\Omega\). Figure 6: Computed waveforms of the junction temperature of the transistor operating as a switch with the cooling system C at frequency \(f\) equal to 1 kHz (**a**) and 100 kHz (**b**). Figure 8: Waveforms of the power dissipated at the steady state in the transistor operating in the switch circuit and cooperating with the cooling system C at selected frequency values. Figure 7: Dependence of the average power value (**a**), of the junction temperature excess (**b**) and the peak-to-peak value of the junction temperature ripples (**c**) on frequency for the transistor operating in the switch circuit cooperating with various cooling systems. Figure 11: Waveforms of the junction temperature at the steady state for the transistor operating in the switch circuit and cooperating with all the considered cooling systems at frequency \(f=10\) kHz and load resistance \(R_{0}=25\)\(\Omega\). Figure 10: Waveforms of the junction temperature at the steady state for the transistor operating in the switch circuit and cooperating with the cooling systems B and C at frequency \(f=1\) kHz and load resistance \(R_{0}=8\)\(\Omega\). Figure 9: Waveform of the junction temperature at the steady state for the transistor operating in the switch circuit and cooperating with the cooling system C at frequency \(f=1\) kHz and load resistance \(R_{0}=4\)\(\Omega\). Figure 12: Dependence of the average power value (**a**), the excess of the junction temperature (**b**) and the quotient of the peak-to-peak value of ripples to the average value of the junction temperature (**c**) on frequency for the transistor operating in the boost converter and cooperating with different cooling systems. Figure 13: Waveforms of the power dissipated at the steady state in the transistor operating in the boost converter and cooperating with system C at selected frequency values.