Research On Single-stage LLC Resonant Converter Based On SEPIC Circuit

As a new generation of green light source, LED has many advantages, such as high luminous efficiency, long working life and quick response. has been widely used in street lamps, automotive lighting and other fields.The conventional LED drive system has a two-stage actuating structure, and this kind of structure can achieve higher PF and lower THD. But there are many drawbacks, such as big size, complex control method and high cost. Therefore, research on the single-stage converter has become the main focus of LED drive system because of lower cost and higher reliability.At present, the single-stage AC-DC converters mostly work in hard switching condition, and the switch loss is relatively large. Therefore, they cannot be used in the high-power LED drive system.Under the condition of the integrated topological constraints, a novel single-stage converter structure is proposed by integrating switching devices in both SEPIC circuit and half-bridge LLC resonant circuit. Because switch devices of the two stages are integrated together, electric devices used in the system are less. This kind of structure can not only reduce the system cost less, but also enhance the system reliability. Because the LLC resonance circuit in the system still has the soft switching characteristics after integration, the system switch loss is lower. Meanwhile, the system bus voltage is lower in the system, which is suitable for the high-power LED drive system.Firstly, according to SSC integration theory, the two-stage converter has transformed to the single-stage converter. And modal analysis of the proposed circuit has been shown with theoretical waveforms. Considering the bus voltage as the intermediate variable, both SEPIC unit and LLC unit of the proposed converter have been deeply analyzed, and the analysis process has been illustrated. Secondly, the optimization design of the system is carried out, including the parameter calculation and design of the typical components, the design of the auxiliary power supply, the drive circuit and the sampling circuit. Then, the mathematical model of the LLC resonant circuit is established through the extended describing function. After that, the digital control strategy and the digital compensation method of the system are presented. Finally, the simulation of the system is carried out based on the theoretical research and analysis. The scheme of the system is designed and the experimental platform is built to verify the proposed structure. The experimental results verified that the proposed single-stage structure can correct power factor responsibly and the efficiency can reach up to 92% at the steady state, when the system works under the soft-switching condition.