Research On LLC Resonant Topology At High Frequency Solid State Induction Heating Power Supplies

Induction heating power supply is widely used in metal heating treatment industrial nowadays. It could satisfy almost all the requirements in heating treatment, which includes: through heating, melting, found, steel rolling, quench, backfire, anneal, seal, drying and some special applies like case sealing and optical fiber pulled crystal etc. Because of so many useful ways, it has a powerful pushing force for national economy. So research on the induction heating power supply especially on high frequency applied is emphasis more than ever. In China, for starting on researching the power supply not so long, the technology is a little lag than international leading level. We normally used the vacuum tube as the power elements in the circuit, which has lower efficiency in using electric power. To improve the whole system quality of high frequency induction heating power supply, based on using fully controlled power elements, starting with the improved LLC resonant topology, the characteristic of LLC resonant topology used in high frequency parallel power supply and how to improve the power capability have deeply analyzed and studied in the paper.Firstly, it presented briefly the international induction heating development achievement nowadays, some important concepts in it and it also introduced international induction heating status in quo and development direction in future from the aspects as: topologies of power supply, power regulation and how to expand the capability. According the existed problems in high frequency power supply development, it presented feasibility of fulfilling the high frequency power supply used the matured power electronics technology and phase loop lock control based on improved resonant topology LLC upgrade from LC resonant topology.Secondly, the theoretical analyses and experimental studies of the characteristic of LLC resonant topology have been studied deeply in paper based on anglicizing simply LC resonant topology firstly. According the characteristic of induction heating load and equivalent circuit of it, the paper has analyze the amplitude frequency and phase character between output current and output voltage, output current and load current, output voltage and capacitor voltage respectively based on how to get to the maxim output power in load. It presented the results of the ratios between load current and output current, capacitor voltage and output voltage, the output power size when the power supply worked nearly at resonant frequency. It also has given the rules of how to choose elements parameter and choose the phase offset between output voltage and capacitor voltage as control variable of the phase loop lock according the analysis results.Because of the some special induction-heating load with low quality factor Q caused the phase offset of output voltage and output current very big and the switch loss is too much, the paper introduced an improved LLC resonant topology to reduce the phase offset of output voltage and current through adding the auxiliary inductor to increase load quality factor. The theoretical and simulation results showed that the scheme could decrease the phase offset well, while not change the output power and resonant frequency.To improve the power level and redundancy of high frequency solid-state induction heating power supply, the paper used the LLC resonant topology in the parallel voltage type inverters through using to reference of parallel technology in UPS and the character of LLC topology. To reduce the switching loss in high frequency power supply, the inverter adopted the phase loop lock technologyto make the system work at resonant frequency, so the inverter control could not regulation output power and the LLC parallel power supply used open loop control. The paper has got that current balance of open loop system is not too bad through analyzing when the system working at DC voltage difference, output voltage phase difference and series inductor difference in detail. The result of experimental validated the correctness of theoretical analysis. And it also offers the support of how to design and research the power supply in next step.According the theoretical analyze above, it completed one experimental prototype. The prototype is a integrated system, and its main circuits included the SCR rectifier voltage regulator, the voltage type inverter, its control system included voltage regulate loop and phase loop control. The principles of control loop and protection are also given. The results of experiments showed that the whole power supply system could work stability at resonant frequency to ensure the system export the maxim power to load and meet current transportation requirement. It owned the very good adaptability in different loads with a higher efficiency. Finally, the summarization has been made and expectation of work has been given in the paper.