Study On High Frequency Induction Heating Power Supply Based On Multilevel Converter Technology

Induction heating power supply is widely used in metallurgy, metal heating treatment field and has a powerful pushing force for national economy. With power electronics technology improving, research on the induction heating power supply especially on high frequency application is emphasis more than ever. For developing the technology of high frequency induction heating power supply and promoting the high frequency application of multilevel inverter, high frequency induction heating power supply based on multilevel technology is studied in this paper.It is analyzed that the work process of two H-bridges cascaded multilevel inverter in induction heating power supply with traditional driving signal. The conclusion is that reverse recovery current in diode could cause ill effect in inverter module in case of short turn-on time, and an asymmetry output voltage level of inverter could increase the complexity of harmonics control.A novel working mode of multilevel inverter in induction heating power supply is presented. The output voltage of multilevel inverter could be adjusted by setting driving signal of power switch; the relationships between the dead region of driving signals and the phase commutating points of load current are investigated to analyze the problem of load voltage commutating, the working conditions are given to avoid repetitious commutation of load voltage polarity in dead region; the operation process of multilevel inverter with novel working mode is analyzed in detail.A passive lossless snubber circuit is designed for restraining the reverse recovery current in diode and improving the turn-on condition of power switch. The operation process of snubber circuit is investigated in detail and corresponding method of parameters designing is given.The harmonics controlling means of cascaded multilevel inverter applicable to high frequency field are discussed for developing high frequency application of multilevel inverter in single-phase power system. In order to eliminate lower order harmonic in the multilevel inverter with different DC source ratio, the range of switching angles and fundamental voltage amplitude are given by solving the switching angles equation system. According to the relationship between the total harmonic distortion and fundamental voltage amplitude, the method of selecting module source ratio is presented.A method of eliminating specified harmonics based waveform resultant principle in cascaded multilevel inverter is given to avoid solution of the multi-variable high-order equation system, when the number of modules cascaded in inverter or the order of the harmonics to be eliminated increase. The switching angles can be easily obtained by simple trigonometry formula deduction for eliminating specified multi-harmonics. Basic principle and deduction procedure are explained.A harmonics control way of output voltage of cascaded multilevel inverter in high frequency induction hearting power supply is presented. According to the characteristic of output power in induction hearting power supply, the fundamental voltage content in output waveform of inverter should be increased as high as possible, which decreases total harmonic distortion ratio of output voltage, for improving the utilization ratio of inverter power supply. Based on the harmonics control way of multilevel inverter and the characteristic of output voltage of multilevel inverter in induction hearting power supply, basic procedure of eliminating specified harmonics in two modules cascaded multilevel inverter is deduced and the reasonable values of switching angles are advised.The results of simulation and experiment have verified the validity of design methods and the conclusions given by theoretically analyzing in this paper.