Synchronous Flyback Controller for Isolated Power Supplies Needs No Optoisolator

Jan 10 2006 - MILPITAS, CA
  • Linear Technology Corporation introduces the LT3825, a synchronous flyback DC/DC controller with precision regulation and high efficiency for 10W to 60W or up to 12A output isolated power supplies. The LT3825 is a current mode controller that regulates the output voltage based on sensing secondary voltage via a transformer winding. This allows for tight output regulation without the use of an optoisolator, improving dynamic response and system reliability. Synchronous rectification increases conversion efficiency and improves output cross regulation in multiple output power supplies. For example, the LT3825 is used to convert an input supply of 36V-72V to an isolated output of 3.3V at 12A with up to 90% efficiency and mere 0.009% regulation. Applications include isolated power supplies in telecom, medical, industrial and instrumentation systems.

    The LT3825 has a unique feedback amplifier that eliminates the need for an optoisolator by sampling a transformer winding voltage during flyback period and using this voltage to precisely regulate the output voltage. This alleviates the designer from spending time trimming the power supply. Furthermore, the elimination of the optoisolater improves the output transient response. A programmable primary gate delay function eliminates multiple discrete components from the circuit and optimizes MOSFET switching time and transformer selection. To further simplify the design, the LT3825 allows its switching frequency to be adjusted from 50kHz to 250kHz with a capacitor. The IC can also be synchronized to an external oscillator.

    The LT3825 is rated for operation from –40°C to 125°C and is offered in a thermally enhanced 16-lead TSSOP. 1,000-piece price is set at $3.10 each.

    Summary of Features: LT3825

    • No optoisolator required: Senses output voltage directly from primary side winding
    • Synchronous driver for high power and high efficiency
    • Input voltage limited only by external power components
    • Accurate regulation without user trims
    • 50kHz to 250kHz switching frequency
    • Synchronizable


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