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multiple output flyback converter

Discussion in 'Electronic Design' started by tech_savvy, Jan 3, 2005.

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  1. tech_savvy

    tech_savvy Guest

    I wanted to design flyback converter.
    How to determine transformer specifications...IC to be used...etc.
    Vpri=5 V,Vsec=5V(8 secondaries),Isec=200mA each.
  2. John Larkin

    John Larkin Guest

    You might as well ask how to build your own helicopter.

  3. Leon Heller

    Leon Heller Guest

  4. Ken Smith

    Ken Smith Guest

    5V * 200mA * 8 = 8W this is quite posible with many chips

    The input side current will be a few amps, lets say 5. Look for a chip
    that can handle that much.

    Pretend that you have just one secondary with 1.6A of load and follow any
    of the many app-notes for doing that design.

    You need a low leakage inductance in the transformer. This should work
    for you:

    Your inductor will be a 1:1 so take 16 wires, form a wire rope, wind the
    inductor and then use 8 in parallel as the primary. Try to spread the
    primary windings evenly around in the wire rope and the 8 others as the
  5. Rich Grise

    Rich Grise Guest

    I think that if they guy thinks flyback is appropriate for 1:1, he needs
    to re-take the course.

    Or maybe he thinks he needs a flyback because in the diagram, the
    stack of secondaries is so tall. ;-p

  6. Ken Smith

    Ken Smith Guest

    Flyback designs work just fine at a 1:1 ratio. With a 1:1 transformer,
    the switching waveform has equal transistor and diode conduction times.

    It is about the only way that one inductive element can give you many
    semi-regulated outputs. Current-fed, psudo-resonant and true resonant
    designs generally need two inductors. Voltage fed forward converters need
    N inductors for N outputs.
  7. Hey Ken,
    Since it is a 1:1 Flyback, might as well add the cap from drain to output
    diode anode and gets the benefits of a SEPIC. Leakage inductance causes no
    ringing, cleaner switching. Eight outputs, eight coupling caps, unless he
    wants isolation which was not stated.
    cheers, harry
  8. Ken Smith

    Ken Smith Guest

    I take 8 outputs to imply 8 isolated outputs. If they are not isolated,
    there is little reason to make 8 instead of one. For that matter, his
    input and output voltages were equal so he could use 8 wires if isolation
    was not needed. The 8 wires solution produces a lot less ringing than
    even the SEPIC design would.
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