Abstract: This paper introduces the design of an auxiliary power supply system for railway passenger cars, based on the TMS320F2407DSP as the control core. The design employs an advanced SVPWM control strategy, leveraging the powerful control capabilities of the TMS320F2407DSP chip and utilizing real-time algorithms to enhance the system's rapid response capability and improve the safety and reliability of the passenger car auxiliary power supply system. The composition of the auxiliary power supply system and the structure and working principle of the DC/DC and DC/AC module main circuits are briefly introduced. The control concept of the system is explained in detail, and the software programming flowchart is presented. Based on practical problems encountered in application, improvement methods and effects are introduced. Keywords: Auxiliary power supply; Digital signal processing; Space vector control Introduction With the development of the national economy, how to improve the environmental protection and service quality of railway passenger cars while ensuring safety has increasingly attracted people's attention. The main auxiliary equipment of passenger trains includes air conditioning systems, audio-visual systems, and interior lighting, etc. The installation of these devices can improve the riding environment and make the journey more comfortable. Currently, most auxiliary power supply systems for electrified railway passenger trains in my country adopt a DC 600V power supply system. This means the locomotive receives 25 kV AC power from an overhead line via a pantograph, which is then stepped down by a transformer and rectified to DC 600V before being supplied to each carriage via a busbar. The auxiliary power supply system described in this article is suitable for air-conditioned passenger cars with a DC 600V power supply system, as well as EMU trains with corresponding power supply systems. Its main circuit uses a high-frequency transformer to isolate the output from the input 600V busbar, and its control chip employs an advanced DSP controller. The DC/DC converter in the system uses a two-stage conversion: the first stage uses a Boost converter to control the system's output, and the second stage uses a resonant switch, improving the overall system efficiency. The system features real-time control, self-protection, self-diagnosis, self-recovery, CAN, and RS485/232 communication functions. The system has a powerful diagnostic system for faults such as short circuits, overvoltage, undervoltage, overcurrent, overheating, and grounding, thereby improving system safety and reliability, and also enhancing system maintainability. 1. Hardware and Software Design of Locomotive Auxiliary Power Supply System Based on DSP 1.1 Main Circuit The main circuit consists of a DC/DC converter and a three-phase inverter. The DC/DC converter uses charging technology with a high-frequency transformer; one output directly charges the battery, and the other outputs 600V to supply the three-phase inverter. The three-phase inverter uses space vector inverter technology. The main circuit of the system is shown in Figure 1. 1.2 Control Circuit Structure The control power supply is powered by a DC 110V power supply. The internal power module converts the DC 110V to DC 24V, DC ±15V, and DC 5V respectively, providing 5V power to the control chip, ±15V power to the measurement system, and 24V power to the drive circuit. When the system is working, the DSP automatically selects the working mode based on the acquired input voltage, current, output voltage, current, heat sink temperature, and frequency setting. The circuit structure of the control system is shown in Figure 2. 1.3 System Working Principle and Software Description 1.3.1 Working Principle of the DC/DC Module As shown in Figure 3, the DC/DC module consists of three parts: a boost circuit, a resonant circuit, and a rectifier circuit. During system startup, the duty cycles of S1 and S2 are maintained at 50%. The system gradually adjusts the duty cycles of S3 and S4 based on the detected battery charging current and battery voltage to stabilize the voltage output to the battery at the set value (generally DC 120 V). After startup, the system fixes the duty cycles of S3 and S4 and dynamically adjusts the duty cycles of S1 and S2 based on the battery charging current and battery voltage, 600 V input voltage, and battery temperature compensation characteristics to ensure that the battery voltage strictly conforms to the charging characteristic curve. The charging characteristic curve is provided by the battery manufacturer, and the required charging characteristic curves vary between different manufacturers. A simplified flowchart of the DC/DC module program is shown in Figure 4. 1.3.2 Working Principle of the Three-Phase Inverter The circuit diagram of the three-phase inverter is shown in Figure 1. The 600V input is provided by the DC/DC module. Because the DC/DC converter starts slowly, its output voltage also rises slowly, so the inverter input voltage (after rectification from the transformer output) also rises slowly. Therefore, this three-phase inverter saves the need for the pre-charge circuit in the traditional inverter stage. When the system is working, if the inverter detects a DC 600V input greater than or equal to 600V, the three-phase inverter starts according to the system output mode requirements (the mode determines the voltage and frequency), outputting three-phase 380V to supply the bus air conditioning. The three-phase inverter inverts the 600V DC based on the DC/DC input voltage using a space vector method. The space vector inverter method has the highest voltage utilization. A simplified flowchart of the three-phase inverter is shown in Figure 5. 2. Experiment and Improvement During actual vehicle testing, it was found that the system was mismatched with the locomotive's DC 600V rectified power supply, causing fluctuations in the 600V mains voltage and current during equipment startup, affecting the normal operation of the auxiliary power supply system. The test waveform is shown in Figure 6. In Figure 6, the vertical axis is 200V/diV, and the horizontal axis is 50ms/div. The upper curve shows the waveform of the DC 600V bus voltage when the auxiliary power supply starts up, with fluctuations exceeding 200V and a frequency of approximately 6Hz. The lower curve shows the waveform of the DC 600V bus current, with fluctuations approaching 40A. Furthermore, the phase difference between the voltage and current waveforms is close to 90°, indicating a very low power factor in the auxiliary power supply system. Such fluctuations cause the bus voltage to momentarily exceed the overvoltage and undervoltage values ​​of the auxiliary power supply system, triggering equipment protection and preventing normal operation. The reasons for this are twofold: firstly, the redundancy of the locomotive's DC 600V rectifier power supply is insufficient, resulting in a large current draw when equipment in each carriage starts simultaneously, causing DC 600V voltage fluctuations; secondly, the low power factor of the auxiliary power supply also affects the utilization rate of the DC 600V power supply. To address this issue, the locomotive's DC600V rectifier power supply was modified, and the control section of the auxiliary power system was also upgraded. PFC (Power Factor Correction) control was added to the software, making the system exhibit resistive load characteristics. This means that during operation, the DC600V voltage and current are almost in phase, and fluctuations are controlled within acceptable limits. This has fundamentally improved the system's startup and operation. The improved input voltage and current waveforms are shown in Figure 7. 3. Conclusion This system is a new type of DC600V passenger car auxiliary power supply system jointly developed by our company and SMA GmbH of Germany. It has been used on passenger trains on the Wuchang-Xi'an and Hankou-Shenzhen lines at the Wuchang Depot of the Zhengzhou Railway Bureau, and is currently operating well.