**5. Conclusions**

In this study, a new electrothermal averaged model of the diode transistor switch—including IGBT and a rapid switching diode—is proposed. This model makes it possible to compute non-isothermal (taking into account self-heating phenomena) characteristics of DC–DC converters operating both in CCM and DCM. Values of voltages, currents and internal temperatures of semiconductor devices at the thermally steady state can be computed using this model. The proposed model uses piecewise linear approximation of current–voltage characteristics of IGBT and the diode operating in on–state.

Practical usefulness of the proposed model was verified for the boost converter operating both in CCM and DCM at different values of duty cycle of the control signal in a wide range of load resistance. It was shown that the new electrothermal averaged model makes it possible to obtain good accuracy of modeling dependences Vout(d), Vout(R0), η(R0), η(d) and Vout(iout) in both operating modes of the converter. This accuracy is much higher than accuracy obtained with the use of the considered literature models, particularly in DCM.

The proposed model makes it also possible to compute values of internal temperature of both semiconductor devices operating in the tested DC–DC converter. The obtained results of computations of internal temperature of the diode are convergen<sup>t</sup> with the results of measurements in all considered modes of operation of the DC–DC converter. In contrast, the results of computations of internal temperature of the IGBT fit well the results of measurements only for the DC–DC converter operating in CCM. In DCM, these differences can exceed even 20 ◦C. It was also shown that changes in the value of ambient temperature practically do not influence on the DC–DC converter operating at fixed values of duty cycle and load resistance. These changes cause changes in internal temperatures of IGBT and the diode. These changes are very important from the point of view of estimation life time of the tested converter. As it is known from the literature [39], an increase in value of device internal temperature by 8 ◦C causes even twice decrease in life time of such devices.

Computations performed with the use of the proposed model illustrate influence of load resistance and duty-cycle of the control signal on selected characteristics of the boost converter. The obtained results of computations make it possible to estimate the range of admissible values of the mentioned parameters, for which internal temperature of the diode and IGBT does not exceed the admissible value given by the producer.

In further investigations, the authors will attempt to find a cause of these differences and will propose some modifications of the model. The model will be also tested in a wide range of frequency of the control signal and for different cooling conditions of semiconductor devices operating in the DC–DC converter, as well as at different values of input voltage, load current and ambient temperature.

**Author Contributions:** Conceptualization (P.G. and K.G.); computations (P.G. and K.G.), methodology (P.G. and K.G.); experimental verification (P.G.); writing text of the article (K.G. and P.G.); review and editing (K.G. and P.G.); visualization (P.G. and K.G.); supervision (K.G.). All authors have read and agreed to the published version of the manuscript.

**Funding:** The scientific work is a result of the project No. 2018/31/N/ST7/01,818 financed by the Polish National Science Center.

**Conflicts of Interest:** The authors declare no conflicts of interest.
