Introduction The TMS320LF2407 is a low-cost and widely used DSP chip with an instruction cycle of up to 25ns (40MHz). Compared with microcontrollers, the TMS320LF2407 has the following advantages: strong data processing capability; high computing speed; high PWM resolution; real-time completion of complex calculations and control; and shorter sampling cycle. It has two event manager modules, EVA and EVB, which can realize: 16-channel A/D conversion; three-inverter control; symmetrical and asymmetrical PWM waveforms; dead-time elimination; and 6-channel PWM output, making it very suitable for motor control and inverter control. To improve the working conditions of electric locomotive drivers' cabs, air conditioners are generally installed. Due to the extremely harsh working conditions in locomotive drivers' cabs, including large vibrations, high ambient temperatures, and poor power quality, the installed air conditioners should be equipped with dedicated inverter power supplies suitable for locomotive air conditioners. On electric locomotives, poor power quality is usually manifested in two aspects. (1) The voltage of the power grid supplying electric locomotives in my country fluctuates greatly. Its rated voltage is 25kV, while the actual voltage fluctuates in the range of 18 to 31kV, and sometimes even more. The grid voltage is reduced to single-phase AC 396V by the transformer inside the locomotive, and the corresponding fluctuation range is 285 to 492V. (2) Electric locomotives are supplied with power in sections. When switching from one power supply section to another, that is, crossing the phase break section, the locomotive loses power for a few seconds. The periodic power outages inside the locomotive will cause the compressor to start frequently, which is extremely detrimental to the air conditioning compressor. Therefore, when crossing the phase break section, the air conditioner must be powered by the 110V battery pack inside the locomotive through the inverter power supply. According to technical requirements, within the range of single-phase AC voltage fluctuation of -30% to +24% and battery pack voltage fluctuation of -30% to +20%, the inverter power supply must provide the air conditioner with a three-phase 380V AC voltage with voltage fluctuation not exceeding ±5%, frequency within ±1%, and total harmonic content less than 15% of the fundamental effective value. The rated output capacity of this inverter power supply is 5kVA, the conversion efficiency is greater than 90%, and it can operate reliably in an environment of 0℃ to 60℃. 1. System Hardware Structure and Working Principle The overall structural block diagram of the electric locomotive air conditioner inverter power supply is shown in Figure 1. It consists of a control section and a main circuit section. The control section uses the DSP chip TMS320LF2407 as its core, plus detection circuits, display circuits, control button input circuits, logic control output circuits, etc. Figure 1. Overall structural block diagram of the air conditioning inverter power supply for electric locomotives. 1.1 Main circuit. The principle of the main circuit of the air conditioning inverter power supply for electric locomotives is shown in Figure 2. It consists of a rectifier, filter, boost circuit, push-pull circuit, and inverter bridge. The 396V single-phase AC voltage is rectified by the rectifier bridge composed of D1 to D4 and filtered by L1, C1, and C2 before being sent to the boost circuit S1, L2, and D10. To obtain a three-phase 380V AC voltage, the output voltage of the boost circuit must not be lower than 540V within the fluctuation range of the single-phase AC voltage. Due to the large fluctuation range of the single-phase AC voltage, it is difficult for the output voltage of the boost circuit to be stabilized at 540V. When the single-phase AC voltage fluctuates in the positive direction, even if the boost circuit is not working, the output voltage after rectification and filtering will be higher than 540V. Figure 2. Main circuit diagram of the air conditioning inverter power supply for electric locomotives. The push-pull circuit consists of S2, S3, T, and D5 to D9, and its output DC voltage can be slightly lower than 540V. The boost and push-pull circuits use IGBTs as switching transistors, and the inverter bridge employs a six-unit intelligent power module (IPM) integrating drive, detection, and protection. The IPM is an advanced hybrid integrated power device, composed of high-speed, low-power IGBT chips and optimized gate drive and protection circuits. The use of IGBT chips capable of continuously monitoring the power device current enables efficient overcurrent and short-circuit protection. Furthermore, the integrated overheat and undervoltage lockout protection circuits further enhance system reliability. Within the power supply section, the inverter is powered by AC 396V; when crossing phase-splitting sections, it is powered by a 110V battery bank. Since the battery bank provides limited current and cannot operate continuously, once the electric locomotive enters a phase-splitting section, the inverter begins to reduce its voltage and frequency to 32Hz. In this mode, the inverter stops operating if the operating time exceeds 15 seconds or the battery voltage drops below 82V. Whether starting or stopping, the inverter operates in frequency conversion mode, and the frequency conversion speed automatically changes according to the operating status. Considering the impact of switching frequency on current waveform quality and switching losses, the switching frequency of the IGBT in the boost circuit is set to 10kHz, the switching frequency of the IGBT in the push-pull circuit is set to 7kHz, and the switching frequency of the IPM in the inverter bridge is set to 2kHz. Contactors K1 to K3 are controlled by the TMS320LF2407 subsystem according to timing. M1 to M3 are the air conditioner compressor, indoor fan, and outdoor fan, respectively. 1.2 DSP TMS320LF2407 Subsystem The DSP TMS320LF2407 subsystem mainly completes the generation of inverter bridge PWM drive signals, signal detection and fault handling, fault display, control of boost and push-pull circuits, key detection, and logic control. The PWM control scheme used in this inverter power supply is third harmonic injected PWM (THIPWM). In this way, the fundamental amplitude of the inverter output line voltage can reach the DC bus voltage of the inverter bridge, which improves the DC bus voltage utilization rate by 15% compared with sinusoidal PWM. THIPWM utilizes the ability of the line voltage in a three-phase system to automatically eliminate 3k (k=1, 2, 3...) harmonics in the phase voltage. It artificially injects a certain component of the 3k harmonic into the three-phase modulation reference sine wave, thereby reducing the peak value of the modulation wave and avoiding overmodulation. Signal detection includes whether a phase segment has been crossed, the inverter bridge DC bus voltage, whether the battery voltage is below 82V, and various fault signals. Phase segment crossing detection determines whether there is single-phase 396V. The single-phase AC 396V is rectified, filtered, and compared by a small isolation transformer and then sent as a switching signal to the DSP. Once the electric locomotive is detected entering the phase segment, the DSP immediately starts the push-pull circuit. A voltage sensor detects the inverter bridge DC bus voltage, and the obtained analog signal is sent to the A/D converter of the DSP TMS320LF2407 to adjust the THIPWM modulation ratio M, stabilizing the inverter output voltage. When entering the phase segment, if the battery voltage is detected to be below 82V, the inverter immediately stops working. Various fault signals include IPM fault output, inverter bridge overcurrent, inverter bridge DC bus voltage overvoltage/undervoltage, boost diode short circuit/overcurrent, push-pull diode short circuit/overcurrent, 110V overvoltage/undervoltage, and inverter power supply overheating. All these fault signals are sent to the DSP for LED display. The inverter bridge DC bus voltage overvoltage/undervoltage and 110V overvoltage/undervoltage are handled by the DSP. The boost and push-pull diode fault signals, after processing by the DSP, determine the on/off state of the boost and push-pull circuits. Other fault signals are sent to the DSP's power drive protection interrupt input pin PDPINT. Controlling the boost and push-pull circuits means controlling the on/off state of the boost and push-pull diodes. Since the DSP2407 can output up to 16 PWM signals, an external PWM controller is no longer needed, greatly simplifying the system hardware design and reducing costs. The control signal for boost diode S1 is T3PWM, with a switching frequency set to 10kHz; the control signals for push-pull diodes S2 and S3 are T4PWM, with a switching frequency set to 7kHz. The control buttons mainly include start/stop selection, automatic/manual selection, ventilation/cooling selection, etc. According to the operation selection and the working process of the air conditioner, the DSP TMS320LF2407 performs different logic control. 1.3 Digital implementation of THIPWM waveform It is very easy to digitally implement the THIPWM waveform by using the comparison unit in the EV module of the DSP TMS320LF2407. Set the counting mode of the general timer to the continuous increment/decrement counting mode to generate a symmetrical PWM waveform. The counting process of the general timer forms a "virtual" triangular wave, as shown in Figure 3. The expression of the triangular reference wave is listed in equations (1) to (3). Where: TxPR/2 corresponds to the inverter bridge output voltage being zero; M is the modulation ratio. vu, vv, and vw are sent to the comparison registers CMPR1, CMPR2, and CMPR3 (or CMPR4, CMPR5, and CMPR6) respectively. The value of the comparator register CMPRX determines the intersection of the triangular reference wave and the carrier triangular wave. At the intersection, the output states of PWMX and PWMX+1 are changed, thus obtaining the switching mode of the THIPWM inverter. Figure 3 shows the counting process of the general-purpose timer Tx. 2. System Software Design The control software of the electric locomotive inverter power supply mainly consists of five parts: the system main program, the PWM interrupt service routine, the timer overflow interrupt service routine, the PDPINT interrupt service routine, and the A/D conversion completion interrupt service routine. The main program primarily handles parameter setting, initialization of various functional components, detection of control buttons, overvoltage and undervoltage handling, and logic control of the air conditioner. The PWM interrupt service routine mainly calculates the three-phase PWM width, adjusts the modulation ratio M, and processes the frequency conversion process. The timer overflow interrupt service routine primarily handles the timing required for the air conditioner's logic control, including timings of 50s, 30s, 15s, and 10s. The PDPINT interrupt service routine is used for handling and displaying major faults. The A/D conversion completion interrupt service routine is used to perform A/D conversion and filtering of the inverter bridge DC bus voltage. 3. Conclusion Experiments have proven that this inverter power supply meets all design requirements. Using a power harmonic analyzer, the output voltage fluctuation is measured to be no more than ±2%, and the total harmonic content is no more than 2%. Two years of stable and reliable operation on electric locomotives proves that the design is reasonable, has strong anti-interference capabilities, and can meet the operating requirements of electric locomotives under harsh conditions. The application of the TMS320LF2407 and IPM intelligent power modules, suitable for motor control, simplifies the design of this inverter, resulting in a more compact structure, optimized performance, and higher reliability.