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This paper presents an application-specific integrated circuit (ASIC) controller that is suitable for double-stage AC-DC power converters, which employ power-factor correction (PFC) and flyback topologies. The PFC cell is used as a preregulator to comply with line-current harmonic standards, while the flyback converter operates in quasi-resonant (QR)/discontinuous conduction mode to deliver to the load a maximum output power of 120 W. The control functions of the PFC and flyback converters are combined in a unique system-on-chip-ASIC solution to reduce system complexity. The ASIC implements valley-switching control of both the PFC and flyback cells to achieve QR operation. The concept of ldquovalley skippingrdquo is also recalled by the proposed ASIC to determine the operating point in the power-frequency characteristic of the power converter. Depending on the output-power requirement, the ASIC drives the flyback stage into three different states, which result from the combination of the QR and valley-skipping modes. This allows the flyback converter to operate at a fixed on-time, while its off-time is changed according to the output power. The PFC stage is also driven in the QR mode with valley skipping to provide the DC-DC stage with a fixed input voltage of 400 V. To evaluate the performances of the presented ASIC device, a demoboard of a 120-W PFC-flyback system has been realized and tested. Several experimental results have been carried out to confer the validity of the approach that is discussed throughout this paper and to evaluate the performances of the AC-DC rectifier.
Date of Publication: Oct. 2007