## 4 Investigations Results
### Analyses of Properties of the Considered Networks
You can clearly see that there are large bursts of the power dissipated in the transistor when it is turned on and off. With an increase in the operating frequency of the switch, the average value of the power dissipated increases due to an increase in the number of power pulses per time unit. It is worth noting that the power dissipated in the transistor when it is turned on (conduction losses) is even a hundred times lower than the maximum power dissipated during the transistor switching. The presented \(p(t)\) waveforms justify the increasing dependence of the average value of power \(p_{avg}\) on the frequency observed in Figure 7a. The peak-to-peak values of temperature excess \(\Delta T_{ipp}\) obtained with load resistance \(R_{0}=40\ \Omega\) are low. In order to check the influence of load resistance on this temperature excess, some analyses of the considered system were carried out at other values of \(R_{0}\). Figure 9 shows the steady state \(\Delta T_{j}(t)\) waveforms computed for the system operating at frequency \(f=1\) kHz, loaded with the resistor of \(R_{0}=4\ \Omega\) and cooperating with the cooling system C.  
As can be seen, under the considered operating conditions, the transistor junction temperature ripple is characterized by the peak-to-peak value of as much as 27 K. This means that the peak-to-peak value corresponds to as much as 30% of the average value of this temperature excess. For the other cooling systems under the considered cooling conditions, the junction temperature exceeds the maximum allowable value for the transistor under consideration. Figure 10 shows the results of calculations of the waveforms of the junction temperature excess of the transistor operating in the switch circuit operating at frequency \(f=1\) kHz and load resistance \(R_{0}=8\ \Omega\). It can be seen that the shape of the \(\Delta T_{j}(t)\) waveforms is significantly different for both the cooling systems under consideration. The observed differences result from the strong differentiation of the thermal values of time constants in system B and small differences in the values of these parameters for system C. The peak-to-peak values of the junction temperature excess \(\Delta T_{j}\) reach up to 7 K.  
Figure 11 presents the waveforms of \(\Delta T_{j}\) obtained at frequency \(f=10\) kHz and load resistance \(R_{0}=25\ \Omega\) for all the considered cooling systems. As is visible, the obtained waveforms \(\Delta T_{j}(t)\) are nearly constant functions. The values of \(\Delta T_{j}\) change between 2 K for system C and 155 K for system A. The peak-to-peak value of the junction temperature changes from 1 mK for system A to 0.168 K for system B and is much lower than the average value of this temperature at the steady state. Figure 14: Waveform of the junction temperature at the steady state for the transistor working in the boost converter and cooperating with the cooling system C at frequency \(f=10\) kHz and load resistance \(R_{0}=15\ \Omega\). Figure 12 shows the influence of frequency on the average value of the power dissipated in the transistor working in the boost converter (Figure 12a), the average value of the internal temperature excess of this transistor \(\Delta T_{jmg}\) (Figure 12b) and the quotient of the peak-to-peak value to the average value of this temperature excess (Figure 12c). The calculations were performed with the load resistance of the converter equal to 200 \(\Omega\) and the operating frequency \(f\) in the range from 10 to 400 kHz. The value of the output voltage of the converter in these operating conditions is in the range from 408 to 435 V. This means that the power dissipated in the load is in the range from 832 W to 946 W.  
As can be seen, the shape of the dependences presented in Figure 12 is the same as the analogous relations presented in Figure 7. It is worth noting, however, that in the case of the boost converter, an increase in frequency causes much lower changes in the average power value and an increase in the junction temperature of the power MOSFET.