Study On Buck-Compensated DCM Flyback PFC Converter

In order to reduce the harmonic pollution problems from electronic devices, such as AC/DC converter, the Power Factor Correction (PFC) technology is wide used. For small power applications, the Flyback PFC converter under Discontinuous Conducting Mode (DCM) has been paid a lot of attention for its high efficiency, low cost, simple structure, high power factor and input-output electrical isolation. But on the other hand, the shortcomings like second-order line frequency ripple of output voltage and slow transient response of load change have also limited its extension. This paper proposes a kind of method to solve the inner second-order line frequency ripple problem by adding a Buck converter to the traditional DCM Flyback PFC converter.The operation principles, main waveforms and control strategy of Buck-compensated DCM Flyback PFC converter which has constant output voltage arc analyzed in this paper. To simplify anlysis, an equivalent circuit model is built. To the equivalent circuit model, the DC steady state and the AC small signal are analyzed. At the same time, the condition under which low voltage ripple can be realized is analyzed. The efficiency of Buck-compensated DCM Flyback PFC converter, traditional DCM Flyback PFC converter and Buck cascaded DCM Flyback PFC converter is also studied in this paper. And the values of magnetic inductance of Flyback PFC converter and Buck compensator are induced.Based on the theoretical analysis, a 48V/50W prototype is made. This paper presents the designing process, including the details of transformer design and control circuit build, and of the choice of parameters in them. Meanwhile, the other specific parameters are also shown, such as power circuit. The experiment results verify the correctness of theory analysis:the proposed converter not only realize the small output voltage ripple, but also speed up transient response of it.By changing the control strategy, the Buck-compensated DCM Flyback PFC converter can be turned into constant current output mode. This paper focuses on presenting its working principles and main waveforms. And its control strategies are under study. Simultaneously, when converter works on constant current output, the conditions that the controller should be satisfied are given. Otherwise, the controller design for the paper made a detailed study. In the end, a 0.7A/50W prototype is made. The experiment verifies the correctness of theoretical analysis.