## 1 Introduction  
Mobile devices, which include mobile phones, laptops, portable energy stores, or selected wearable devices, require electrical energy stored in built-in batteries in order to operate [1, 2, 3, 4]. A promising solution for charging various mobile devices is the Universal Serial Bus Power Delivery (USB PD) standard in version 3.1. It provides a maximum power output of 240 W and uses a USB Type-C connector to connect to the mobile device. The standard specifies that the charging systems are supplied with a maximum voltage of 48 V and a maximum load current of 5 A. Such high voltage and current values make it possible to charge the battery packs of various mobile devices using a USB Type-C connector [5, 6]. The USB PD charging standard works with the Quick Charge 4.0 and 4+ fast charging technology. The Quick Charge technology allows mobile device batteries to be charged quickly, and the charging time for the 20 to 80% State of Charge (SoC) range does not exceed 30 min for a 3 Ah battery capacity [6]. In order to charge a system with the USB PD technology, isolated Flyback converters are used [7, 8]. In these converters, Junction Field Effect Transistors (JFETs) made with the Gallium Nitride (GaN) or Silicon (Si) technology are used as switching devices [9, 10]. Most of the design solutions are mains-powered, which requires an active power factor correction (PFC) circuit. To control it, a negative feedback loop is used, where the source signal is voltage at the output of the Flyback converter [11, 12]. In the literature, many solutions can be found for controlling transistors operating in power supplies using the Flyback topology. There are also different solutions for negotiating different output voltages during a fast charging process. The next part of this section contains a literature review of such control circuits. In the literature, there are many different control methods described that are dedicated to semiconductor devices used in Flyback converters, which are very often used as voltage conditioning systems supplied from the power grid. In the paper [13], the topology of a Flyback converter is presented, the output of which is connected to inputs of several Buck converters. The solution of such a charging system topology allows several output voltage levels to be obtained, for several independent USB Type-C connectors. The paper [14] presents a Flyback-type charger architecture dedicated to high-performance USB PD-type chargers; what distinguishes this architecture is the control of an Active Clamp Flyback (ACF) converter requiring only two dedicated ICs for full converter operation in the Flyback configuration. This configuration allows for operation at high power density (up to 32.5 W/inch3). The Variable Frequency Peak Current Mode (VFPCM) control method is used in Flyback converters. This method is widely used in dedicated USB PD charging circuits with an output power less than 100 W. The presented control network ensures high converter efficiency even at low load resistances by reducing the switching frequency of semiconductor devices contained in the Flyback converter [15]. In the literature, different topologies can be found that are dedicated to charging circuits, divided into single-input and multi-output circuits incorporating both a Flyback converter and a Buck converter in their structure. The Buck converter provides a multi-output network dedicated to charging circuits used to power supply and charge mobile devices [16; 17; 18; 19]. A dedicated charging circuit in which the negotiation and communication protocol between the charging circuit and the mobile device was developed using a Field Programmable Gate Array (FPGA) is described in [20]. The difference with respect to the standard charging circuits is that the use of dedicated ICs negotiating voltage and current values between the charging circuit and the mobile device is bypassed.