[align=left] Abstract: This paper introduces the working principle and design method of a practical auxiliary switching power supply for voltage-source inverters. Experiments show that the switching power supply operates stably, has low output ripple, no transformer overheating, and good system operating characteristics. Keywords: switching power supply; pulse width; modulation high-frequency transformer; ripple [b]0 Introduction[/b] The auxiliary switching power supply of a voltage-source inverter, a dual-transistor flyback switching power supply, can efficiently provide multiple DC outputs. All circuit components are composed of discrete components, with strong anti-interference ability and stable and reliable operation, thus meeting the high reliability requirements of voltage-source inverters. The following uses the switching power supply powered by the inverter control circuit as an example to introduce the design method of the flyback switching power supply. This switching power supply has been tested and put into operation. [b]1 Structure and Working Principle of Dual-Transistor Flyback Switching Power Supply[/b] The structural block diagram of the dual-transistor flyback switching power supply is shown in Figure 1. [b]2 Working Principle of Main Circuit[/b] The circuit of the flyback half-bridge converter using MOSFETs in the primary winding is shown in Figure 2. The primary winding of the high-frequency transformer is directly connected to the DC power supply Vs via MOSFETs, and the two MOSFETs need to be switched on and off simultaneously. Therefore, a small dual-winding output transformer is typically used to drive it through a signal with the same phase but mutual isolation. Like other flyback circuits, when the MOSFETs are turned on, energy is stored only in the magnetic circuit; when they are turned off, the magnetic circuit converts the energy into electrical energy and sends it to the load. The two diodes D and D are used to feed excess flyback energy back to the power supply Vs and clamp both MOSFETs at V (ignoring the forward voltage drop of the diodes). 3 Design of Switching Power Supply 3.1 Technical Specifications This design is a 75W flyback converter with 6 outputs. The specific technical requirements are: ① Input voltage Uac = 220 (1 ± 15) V; ② Operating frequency: 30kHz. The operating frequency has a significant impact on the size, weight, and circuit characteristics of the power supply. If the operating frequency is high, the size of the output filter inductor and capacitor will decrease, but the switching loss will increase, the heat will increase, and the heat sink size will increase; ③ 6 outputs: 5V/4A, 12V/2A, -12V/0.5A, 3 outputs: -15V/0.5A, -9V/0.5A; ④ Output ripple and noise: maximum 1%; ⑤ Reliable shielding measures are taken between the primary and secondary windings; ⑥ Power supply efficiency is 80%; ⑦ Operating temperature range: Ta=0～50℃. 3.2 Main Circuit of Switching Power Supply The main circuit switching transistor is a voltage-driven CMOS transistor, 2SK1317, which has the advantages of high frequency, simple drive control, and low drive power compared with the transistor in the traditional flyback self-excited switching power supply. In order to reduce the switching stress of the switching transistor, an RC buffer connected in parallel with the primary inductor is designed to absorb the energy of the turn-off overvoltage. In order to meet the requirement of low output ripple, multi-stage capacitor filtering is used for the output. 3.3 Transformer Design Compared with traditional linear transformers, high-frequency transformers have the advantages of small size and light weight. The design of the high-frequency transformer in this device is described below. The design flow is shown in Figure 3. For high-frequency transformers, the selection of the magnetic core is particularly important. Typically, high magnetic flux density and low magnetic flux loss are preferred. High Curie temperature and high permeability are the main technical indicators for evaluating the quality of the magnetic core. EE-type ferrite cores made of R2KB ferrite material are usually selected, as they offer advantages such as a wide variety of types, large lead space, convenient wiring, and low price. Considering the high voltage on the primary side of the high-frequency transformer, conductors with excellent insulation performance must be used on the primary side. Since a large current will flow through the secondary winding, copper discs can be considered to increase the current carrying capacity. The turns ratio of the primary and secondary windings is determined by the input voltage, output voltage, power, and switching frequency. The turns ratio should be chosen appropriately; if it is too large, the output voltage may not reach the design value even when the conduction time reaches its maximum; if it is too small, the primary losses will increase. From the above analysis, it can be seen that the core area and number of cores should be calculated according to the actual requirements first, and then the wire diameter of the primary and secondary sides should be calculated. At the same time, leakage inductance, line loss, and conductor capacitance can be determined by calculation or simulation. If the core loss is too large, the wire diameter or turns ratio needs to be adjusted repeatedly until the design requirements are met. The main design parameters of the high-frequency transformer are shown in Table 1. [b]4 Test Waveform Analysis[/b] The test waveform of the voltage V across the drain and source terminals of the switching transistor 2SK1317 is shown in Figure 4. After the output power supply is filtered by LC and regulated by the subsequent three-terminal fixed linear regulator 7815, the output voltage waveform when the power supply is outputting at rated power is shown in Figure 5. The peak-to-peak value of the output voltage ripple is about 100mV, which meets the design requirements. From the above two waveform diagrams, it can be seen that the working waveform of the switching transistor is relatively ideal, with no oscillation when turned off. The output +15V voltage is stable and relatively smooth. [b]5 Conclusion[/b] The switching power supply designed in this paper is stable, with small output ripple. The main circuit switching transistor has an ideal waveform with no oscillation or voltage spikes. The transformer does not overheat. The system has good working performance. References [1] Zhang Zhansong, Cai Xuansan. Principles and Design of Switching Power Supplies [M]. Beijing: Electronic Industry Press, 1998 [2] Chen Jian. Power Electronics [M]. Beijing: Higher Education Press, 2002 Click to download: Design of Auxiliary Switching Power Supply for Voltage-Type Inverter Based on IGBT