## 5 Conclusions  
This paper presents the results of experimental studies on the characteristics of the charging system of different mobile devices operating in the USB Power Delivery 3.1 ERP standard and using a USB Type-C connector for charging. During the studies, the tested system was powered by voltage from the power grid with an RMS value V\({}_{\text{inRMS}}\) = 230 V and operating at three values of output voltage V\({}_{\text{out}}\) equal to 5, 9, and 20 V, respectively. The effect of the SoC level of selected mobile devices on selected operating parameters of the dedicated charging system was also measured in the range from 10 to 90% of the SoC value. The experimental studies were conducted using the developed measurement set-up. The measured characteristics confirm the strong influence of the tested switched-mode power supply system made in the ACF topology on the quality of electricity drawn from the power grid, as the recorded values of the input current harmonic distortion factor THD\({}_{\text{I}}\) for the rated RMS value of the supply voltage V\({}_{\text{inRMS}}\) = 230 V exceed the value of even 95%, which may be the result of interference introduced by the complex control system, common to the converter operating in the Flyback topology and to the driver system in USB Power Delivery technology. High values of the current harmonic distortion factor THD\({}_{\text{I}}\) exceeding even 95% occurred during the operation of a dedicated charging system loaded with Smartphone. It should also be noted that the value of output voltage V\({}_{\text{out}}\) on the value of the power grid voltage harmonic distortion factor THD\({}_{\text{V}}\) in a wide range of load resistance R\({}_{0}\) changed from 1 \(\Omega\) to 10 k\(\Omega\). The change in the THD\({}_{\text{V}}\) coefficient value did not exceed 5%. During the experiments, the waveforms of the current drawn from the power grid were also recorded. The level of distortion of these waveforms corresponds to high values of the harmonic content factor THD\({}_{\text{I}}\) of the current drawn from the power grid, whose courses significantly deviate from the sinusoidal shape. The tested switching power supply system in the ACF topology has a very high energy efficiency \(\eta\) in the range of small load resistances. It is as high as 91.2% for load resistance R\({}_{\rm o}\) = 4 \(\Omega\) at an RMS value of the supply voltage V\({}_{\rm inRMS}\) = 230 V. It can be seen that the highest value of energy efficiency \(\eta\), as high as 99.6%, was obtained for a dedicated charging system loaded with Smartphone for the battery SoC level = 70%. The above results demonstrated not only the efficiency of the operation of the considered charging system, but also analyzed the influence of this system on the power grid. It was shown that the considered charging system at typical operating conditions is characterized by a high value of THDI and high energy efficiency. From the point of view of the power grid, the lowest value of THDI can be obtained at low values of the output voltage and high values of SoC. The results presented in this paper may be useful for designers of power supply and charging systems for mobile devices equipped with different electrical energy storage systems whose charging power does not exceed 140 W. The aim of further studies will be to prepare a model of a selected class of power electronic systems operating in a wide range of changes in output voltage V\({}_{\rm out}\) and with linear and nonlinear load in the form of batteries used in mobile devices. The developed model should also take into account the SoC charge level of the electrical energy storage devices used. In order to perform such investigations, models of different kinds of batteries should be elaborated. Such models will describe the dynamic properties of these batteries. Investigations should also be performed for higher values of the output power. Conceptualization (P.P., T.L. and K.G. ); methodology (P.P. and K.G. ); validation (P.P., T.L. and K.G. ); investigation (P.P.