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    LT1249 can be used for flyback PFC stage, instead of boost?

    Can the LT1249 PFC controller IC be used as the controller in a single stage flyback PFC converter?

    LT1249 Datasheet
    http://cds.linear.com/docs/Datasheet/1249fbs.pdf


    This is for a 55W PFC with 36V isolated output.
    Last edited by eem2am; 14th February 2011 at 10:54. Reason: spelling

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    Re: LT1249 can be used for flyback PFC stage, instead of boost?

    I would think yes but the compensation of the current amplifier, CA, differs slightly. The LT data sheet does not go into any great detail about 'slope matching' but the idea of average current mode control is that maximum gain in the CA is limited to that required to match the slope at its output to the slope of the ramp during switch off time at the switching frequency. With a normal PFC circuit this current is the continuous input inductor current. In your proposed flyback based PFC circuit this 'current' is discontinuous...



    You don't end up designing to the 'transformer' current you design according to the maximum demand. The data-sheet suggests a peak current limit of 1V and that will be presented to the input of the CA when your flyback switch is off. It is that voltage, through the CA as a current that you have to integrate to produce the slope matched ramp. They say it is 320uA/V so the current will be 320uA.

    They do not give actual figures for the ramp amplitude but there is the suggestion that the peak CA swing is 5V and PWM is only active when CA rises above 1.8V so you might guess the ramp amplitude is 3.2V @ 100KHz giving a dV/dT of 320K. To achieve the same slope on the compensation capacitor at CA out you would need a 1nF capacitor and leave out both the series 10K resistor and 100pF noise filtering capacitor.

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