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Flyback converter - winding order

Discussion in 'Electronic Design' started by Jacques Fournier, Jan 31, 2005.

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  1. I've designed a 2-watt flyback converter, 12V to 200V, by using a
    round RM5 ferrite core with air gap.
    I wonder if it is necessary to first wind the primary and then the
    secondary on top, or does it make a difference if I do it vice versa,
    with the primary on top ?
    For example could the efficiency be higher in one way ?

    Any idea ? Thank you.
  2. I do not think it matters - often one interleaves the windings for better
    coupling thus reducing leakage inductance/losses. In your case, it probably
    does not matter - you can try it, though.
  3. legg

    legg Guest

    In the two-section configuration, there is a difference in winding
    length that has to be taken into account when determining copper loss.
    Some topologies have current stress that is more severe on one winding
    than another - flyback isn't one of these. Although HV windings
    usually have a poorer copper fill factor, LV windings in smaller parts
    can be just as bad, due to conductor dia/winding breadth ratios.

    Hazardous windings are often more simply isolated when the bobbin or
    ground insulation serves a dual purpose for the first section -
    reducing functional safety requirements for the outer wrap on
    non-hazardous layers. With a 200V secondary, this might be a

    Noisy layers in any section are also more easily screened if they are
    buried - the core and quieter windings can be employed as screening
    elements. When buried, their capacitance to other layers and the core
    are agravated, however.

  4. Ken Smith

    Ken Smith Guest

    For single output designs, I usually wind the primary first. The Start
    (Nearest the core) layer is the switch end. The second (third) layer is
    the input voltage. This gives the lowest primary capacitance, since the
    former is fairly thick.

    If you can, it is usually best to mix the windings together to reduce
    leakage inductance.
  5. Jim Thompson

    Jim Thompson Guest

    When I did that sort of thing I always preferred split bobbins, so
    that there was definite insulation between primary and secondary...
    but I was designing for a commercial application that had to pass VDE.

    ...Jim Thompson
  6. IME, flyback converters that use the feedback winding to regulate just
    won't work acceptably without the windings on top of each other-
    preferably with one winding sandwiched between two halves of the other
    or intermixed. ISTR a degradation from 5% regulation to 30 or 40%.

    Best regards,
    Spehro Pefhany
  7. Chris Carlen

    Chris Carlen Guest

    It can matter tremendously, due to proximity effect. For instance, in
    theory a 2-layer winding can have 6 times!!! the I^2R losses of a single
    layer due to proximity effect. In practice this will be less because of
    round conductors and insulation thicknesses (the initial theory
    considers rectangular foil conductors infinitesimally close together.)

    But the point is that it can matter a great deal.

    There seem to be several approaches:

    1. Ignore the proximity effect, design based on only considerations of
    AC resistance of wire not in proximity to other conductors. Then you
    can test it, and it will likely be less efficient and get hotter than
    you predicted, but if it's good enough move on. At least you'll know
    why it's the way it is.

    2. Consider the proximity effect qualitatively, but don't treat it in
    mathematical detail. Thus you will use rules of thumb to minimize it.
    For instance:

    a) use a long bobbin instead of a deep one, so that the number of layers
    stacked is minimized.
    b) interleave the windings to as much a degree as possible.

    3. Fully treat the proximity effect. (Ouch!) I have been reading how
    to do this in the following text, which covers the subject in grueling

    Robert W. Erickson and Dragen Maksinovic, "Fundamentals of Power
    Electronics, 2nd ed."

    It's quite overwhelming. I have gotten about a 33% grasp of it
    qualitatively after several skims through the sections about proximity

    I am still mired in my basic EM text trying to fully grasp the mechanics
    of the skin effect for isolated conductors.

    Note that the winding approach that may minimize proximity effect might
    not be the best one for winding capacitance, though it appears well
    interleaved windings are advantageous for both leakage inductance and
    proximity effect minimization. For a flyback with the step-up ratio and
    power level you are implementing, winding capacitance isn't a big deal.

    Good luck!

    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
  8. Rich Grise

    Rich Grise Guest

    Yes. I'd wind half the secondary, the primary, and then the other half of
    the secondary.

    Good Luck!
  9. Ken Smith

    Ken Smith Guest

    If you have to pass VDE, you have to live with the leakage inductance. If
    not, Putting the windings as close together as other considerations allow
    is the way to go on a flybacker.

    Any energy that goes into the leakage inductance must be dealt with. With
    a slightly tricky snubber, you can put the energy back onto the input
    rail. You still loss energy in the switch sloshing the power around.
  10. ..
    Hi Ken,
    Would you enlighten us on that tricky snubber?
    Sounds very interesting. Resistors don't slosh well.
  11. I know of a flyback converter that used a small 4 fet bridge input,
    full square converter to take the current from a diode snubber and put
    that energy into the secondary side, with the output voltage
    controlling the DC bus the snubber diodes dumped into, by the turns
    ratio on that converter. Very low loss for a big flyback unit.
  12. Ken Smith

    Ken Smith Guest

    Basically this is it:

    ! !
    ! )
    --- )
    ^ D1 ) L(working)
    ! )
    ! )
    ! C1 !
    ! !
    --- !
    ^ D2 !
    ! !
    ) !
    ) Switch
    ) L(Snub)

    When the switch turns on, L(snub) and C1 go sprong but not much energy is
    transfered anywhere. The end of C1 near the diodes ends up with a small
    positive voltage on it.

    When the switch turns off, the rise is caught at 2*Vin minus a bit and
    L(Working) charges C1 from there.

    D1, and D2 have to be fairly fast diodes a simple 1N400X won't do here.
  13. legg

    legg Guest

    Please be reminded that the OP is discussing a 2W flyback converter
    with a 12V primary.

  14. Sure - Still might be worth the bother if the OP is running off batteries or
  15. 12V, 2W... in % loss, yes, in w's: nada.

    Initially, I still would not care unless running off battery or in a tiny
    box with no dissiation capability.
    I think that you can wind and test a great deal of winding configurations
    before you get a decent mathematical model in place - that is my experience,
    anyway ;-)

    This is 3-D electromagnetic stuff needing either:

    A hefty understanding of Maxwell, Arcane Fortran Libraries and Numeric
    computing (and no Life),

    A hefty purse for licensing a commercial software package containing the
    Induced currents from changing magnetic field surrounding conductor adds to
    the current in the inductor - changing said field (and current distribution
    in inductor), What's so hard about that ;-))
  16. Ken Smith

    Ken Smith Guest

    Are you trying to imply that overkill is a bad thing?
  17. legg

    legg Guest

    When we vary from the specific, the likelihood of wasting time is
    increased. His inquiry was pretty specific.

    Designing for ruggedness can make sense. Overkill is just overkill.

  18. Terry Given

    Terry Given Guest

    And although its only a 2W smps, if he dumps all the leakage into a
    SOT23 "zener" it could still get pretty hot (it doesnt take many watts
    at 300K/W), so reducing leakage by interleaving is probably warranted.

    Unless I have a requirement for poor coupling I always interleave
    windings. Once you learn how to do an MMF plot the interleaving becomes
    self-evident. Winding asymmetries are easy to compensate for too.

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