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Applying SG5841 to control a flyback power supply

Posted: 30 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:application note 5841? SG5841 PWM controller? design strategy? guidelines for PCB layout?

Fairchild Semiconductor Corp.'s application note 5841 describes a detailed design strategy for a high-efficiency, compact flyback converter. Design considerations, mathematical equations, and guidelines for a PCB layout are provided.

The SG5841 is a highly integrated pulse-width modulation (PWM) controller IC. It provides features to satisfy the need for low standby power consumption. With low startup current and low operating current, high-efficiency power conversion is achieved. Typical startup current is only l4?A and operating current is around 4mA. In nominal loading conditions, the SG5841 operates at fixed PWM frequency. As the load decreases, its proprietary green-mode circuit gradually reduces the PWM frequency. This green-mode function dramatically cuts the power loss in no-load and light-load conditions, enabling the power supply to meet power conservation requirements.

Additionally, the controller incorporates many protection functions. Once the power supply is overloaded, the controller forces the power supply into "hiccup" mode to limit output power. The built-in line-voltage compensation circuit maintains constant maximum output power for a wide input line voltage range. An external negative-temperature-coefficient (NTC) thermistor can be connected to the RT pin for over-temperature protection.

View the PDF document for more information.

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