Study On Three-level Inverter Arc Welding Power Supply Controlled By A Digital Signal Processor

The application of the inverter technology in the welding power supplycontributes to an obvious improvement of the Welding apparatus in theminiaturization and highly effective aspect. At the same time, the good dynamiccharacteristic of the arc-welding inversion power supply provides an abundantplatform for the welding craft controlling. With the improvement of power electronicstechnology, especially the emergence of the power element IGBT, arc weldinginverters are going to be the most prospective welding power supply in the weldingfield. They have many good qualities, such as small volume, lightness, high efficiencyand rapid dynamic response.We can increase frequency of the power elements (IGBT) to decrease the volumeand weight of the arc welding power supply. Though higher frequency can bring moregood qualities for the arc welding power supply inverter, there are another newquestions, such as great loss, inductive switch-off, capacitive switch-on and diode'sreverse current as switching elements are forced to be on or off by conventional PWMto control arc welding power supply. The switch loss increases with the improvementof inverter frequency, so it is hard to improve inverter frequency further. Softswitching PWM technique can resolve this problem existing in conventional PWMtechnique, so it becomes one focus of welding field more and more.Conventional input supply of arc-welding is rectified by diodes, and filtered bybig capacitors. That results in low power factor, high harmonic current, pollutingelectrified wire line. And it dose not satisfy the international standardization. In orderto restrain harmonic and enhance power factor, power factor correction is introduced.At the same time, power factor correction enhances the output voltage that the rearpower semiconductor must endure high voltage and then the expense increases.Three-level inverter, whose power components endure half voltage stress, can reducetransistors' voltage-stress and has low harmonic, so it fits to be the rear circuit of thethree phase power factor correction. And in order to lighten switching loss, softswitching is introduced to thee level converter.AbstractThis paper first simply introduces the three-level inverter topology and thecontrol strategy, and summarizes the three-level flying capacitor inverters circuitstructure and the strategy of phase-shifting pulse wide modulation controlling. On thebasis of this, the paper analyzes the principle of ZVZCS-PWM three-level converterin detail, gives out the condition of soft switching and the parameter. This papersecond introduces the control principles of three-level inverter with flying capacitorand uses control theory to analyze the system and gets the system transfer functionmodel. The application of the Digital Signal Processor (DSP) controller to three-levelinverter not only simplifies the hardware and improves the performance of the system,but also realizes the system optimized control, which makes it possible to apply someoptimizing algorithm. This paper selects TI Corporation's chip TMS320LF2407ADSP,with the interior SVPWM generator that can be used to complete the power factorcorrection. DSP has two event managers that can output 16 PWM waves. At the sametime, the surplus PWM waves may be used to control power factor of the three-levelarc welding inverter power supply. The paper introduces the inverter main powerelectric circuit, the power components driving electric circuit, the capacitor chargingcushion electric circuit, the current and voltage checking electric circuit andprotection electric circuit with photocoupler, which can protect the transistors and hasgood EMI performance. Thus the drive circuit separates the control circuit frompower circuit.The digital control system of the arc welding inverter based on DSP is designedin this paper. In this system, the type of DSP TMS320LF2407A has high speed(40MHz) and precision (16bits), and provides the ability to realize the digital controlsystem. In this paper, some primary parts of the digital control system are designed,including the hardware system and software system. The closed-loop control systemfor voltage and current is designed. The feedback signal is sampled, then the analogsignal is converted to digital signal by A/D, and DSP process the data, then the PWMwaveform is output to control IGBT. In the software program, the intelligent digital PIalgorithm is adopted. The simulation model is established about PI algorithm, credibleand applied parameters are provided. The program is written using compile toimprove the speed of the program. At the same time, we enhanced the system'sreliability using the technique of anti-jamming in the program design. Three-levelinverter based on DSP control system is set up.To reduce the influence of voltage and current disturbance on the output, theregulator and corrector are reasonably selected. Selecting suitable parameters, acontrol system with high stability, good dynamic characteristic and high staticaccuracy are gotten. Based on the parameter calculation and simulation analysis, anarc-welding inverter prototype is designed, whose maximal output current is120A,varies from 20A to 120A, no load voltage varies from 65V to 70V. Theexperiment results indicate the principle analysis, parameters calculation andsimulation to ZVZCS-PWM three-level converter are correct. In this converter, itsleads are switched at the state of zero voltage, and lag legs is at the state of zerocurrent, and the endure voltage is half of input.The ZVZCS-PWM three-level circuit has all sort advantages of soft-switchPWM technology. The three-level suits the requirement of the rear circuit of powercorrection and high input voltage, because the switch power elements endure only halfvoltage stress. The control system meets the requirement of the arc welding inverter.