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Offline PFC controller eliminates opto-isolators

Linear Technology has introduced an isolated flyback controller with single-stage active power factor correction (PFC) the LT3798. It achieves, Linear says, power factor of better than 0.97 by actively modulating the input current, eliminating the need for an extra switching power stage and associated components. You also do not need to add isolation in the form of an optical device or a transformer for feedback: the device uses the primary-side flyback signal to gauge the output voltage level.

Linear says that a design using the chip will meet the the IEC61000-3-2 Harmonic Current Emissions specification, and that you can realise efficiency of over 86% at output power levels up to 100W, with universal AC-mains input. You can also use the LT3798 in high input voltage DC applications: it will operate with input voltage levels up to 800V. Re gualtion of output voltage and current is typically within ±5%, and no-load power levels are within regulatory limits at less than 500 mW. Your design will also not require a large bulk input capacitor. The controller employs critical conduction mode operation, enabling the use of a smaller transformer compared to continuous conduction mode designs. Linear packages the chip in a thermally enhanced 16-pin MSOP outline, in standard and extended temperature ranges, priced from $2.25 (1000).

—by Graham Prophet

Linear Technology, www.linear.com/product/LT3798