| Compared to conventional hard-switched converters with fixed switching frequencies, the quasi-resonant converter (QRC)topology is a very attractive alternative for power supply designers. The increasing popularity of the QRC approach is based on its ability to reduce electromagnetic interference (EMI) while increasing power conversion efficiency. In the high-power situations, the switching technology for example LLC resonant, etc, could achieve high efficiency power conversion, however, if it is less than 100w QRC has small application circuit size and can give superior performance. Therefore, people will pay more attention to QRC while in low-power situations. (Such as laptop, small-screen flat-panel TVs, portable equipment, etc.) This paper is focus on the methodology of designing low-cost QRC controller.This paper expresses the basic quasi-resonant converters operational principle by analyzing the typical flyback converter principle. Based on the basic principles and market needs, A QRC chip——FT8401 was designed. The paper gave the circuit implementation process details. In order to avoid human hearing the noise during the chip practical application, FT8401 uses frequency constraints; According to the standby power consumption restrictions, I added a manual adjustable Soft Burst Mode function to reduce the consumption; Within the chip, the integration of the Over-voltage & Over-temperature protection circuit can also handle the over voltage or over temperature condition safely and timely. This chip can automatically detect the valley voltage of the power MOS drain without any external signal, for example the transformer auxiliary winding voltage, which leads the circuit design much simple. Furthermore, all the blocks have been designed & simulated by Cadence software with CMOS models (5V, 40V), the simulation applied to the whole application circuit too. This simulation results indicated that all the guide lines of this chip can meet the requirements of the design. |
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