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Flyback controller duty cycle clamp 45-50%

Discussion in 'Electronic Design' started by Hammy, May 18, 2009.

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  1. Hammy

    Hammy Guest

    Anybody know of a current mode controller for flybacks with duty cycle
    limitations of 45-50%,with direct opt coupler feedback?

    I've been searching Digikey and Newark but all I'm finding are UC3844
    types I can use this but if there is a similar one like the NCP1217A
    I'd prefer this. The duty max is 50%,and can directly take an opto
    feedback ( NCP1217A) unlike the UC384X series which the opto feedback
    is through the EA comp pin. The NCP1217A isn't stocked at Newark.

    Or is there a way to limit the duty of controllers externally? Under
    normal operation my max duty won't exceed 40% but at start up and
    shutdown, short cct this would be exceeded. I have several
    UC3842,NCP120x CONTROLLERS on hand, so this would be ideal.

    This is for a 60W 90-140VAC two switch flyback (60kHz).
     
  2. MooseFET

    MooseFET Guest

    You are after something like the LT1247 that doesn't allow duty cycle
    to go above 50%.

    A fairly simple comparator circuit using a SOT23-5 packaged comparator
    may be the best way to control the duty cycle if you don't want
    anything else effected. A simple RC on the "gate" output of the chip
    can measure the on time. The output of the comparator can pull the
    "Control" voltage down. Since this goes in at the point where the
    integration is done integrator wind up is prevented.
     
  3. Guest

    Though Duty cycle hits 78% at startup the Power integrations part
    TOP256 may be of interest.
    Internal power switch etc.
    Stocked by Newark.


    regards

    Louiiv
    www.Ozure.co.uk
     
  4. Though Duty cycle hits 78% at startup the Power integrations part
    TOP256 may be of interest.
    Internal power switch etc.
    Stocked by Newark.


    regards

    Louiiv
    www.Ozure.co.uk

    I seem to remember that TI (?) produced what was apparently a 3844 with a
    TL431 and an opto-isolator all in a special long DIL package with the
    creepage limits built in.

    Graham H
     
  5. Hammy

    Hammy Guest

    Thanks for the responses everyone.

    What I had in mind is to use a MIC4427 (fet driver dual) between the
    controller gate drive out and use this to drive the low side FET Q1
    and to drive a small PULSE transformer that will drive the high side
    FET Q2. See image.

    http://i39.tinypic.com/117z3og.png

    I was planning on using a small RC to sync the drivers adding a small
    delay to the low side input so that both FETS would turn on/off
    simultaneously. The only problem is I only have 3842's and NCP120X's
    these have no duty cycle clamps. I know the 3842's can be sync with an
    external OSC but I would prefer to use the NCP120X if at all possible.

    MooseFet Could you elaborate on your comparator circuit please.
     
  6. MooseFET

    MooseFET Guest

    Switcher
    Chip
    0-Vcc R1 V1 --------
    Gate-----/\/\--+-----!-\ R2 !
    Drive ! ! >---/\/--+--!Control
    C1 --- -!+/ ! ! Voltage
    --- ! ! Pin
    ! ---K1 * Vcc
    GND

    The time constant R1*C1 is made so that V1 goes above K1*Vcc when the
    on time exceeds the desired. It is about equal to the cycle time of
    the switcher. R2 may not be needed. It just serves to limit how much
    current the comparator yanks from the Control Voltage of the switcher
    chip. Any time the gate drive trys to stay on too long the comparator
    discharges the control voltage a little thus regulating the maximum on
    time.
     
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