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"flux cancelling windings" ?

Discussion in 'Electronic Design' started by amdxjunk, Oct 28, 2005.

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  1. amdxjunk

    amdxjunk Guest

    Hi all,
    I noted the following (below) on a website.
    What is Nc? Number of turns compared to what?
    I want to improve a transformer with very low capacitance between the pri
    and sec, this might help improve the coupling
    while maintaning the low capacitance.
    Do you have any more information on this technique?
    Thanks
    Mike K.

    windings, each with Nc turns. Wind Nc turns first on bobbin. Then wind
    transformer. Then wind Nc turns (so one winding is "inside" the other
    "outside"). Connect the windings in parallel. Because the inner and
    outer windings have different flux linkages due to leakage, current will
    flow thereby creating a flux which counters the imbalance.
     
  2. Tony

    Tony Guest

    Nc doesn't matter, as the "Nc" windings are just shorted out anyway.
    Could be a single turn of foil for each (as long as the external links
    were of similar low resistance.

    But bear in mind, this is really a "flux shorting ring"-type of
    magnetic shielding, not a flux-canceling technique (it requires an
    imbalance to work), and of course it generates losses, and it still
    adds capacitance. If you have the space for this method, you might be
    better off putting it to more effective use by sandwiching one of your
    existing windings between a split version of the other. Maybe some
    judicious electrostatic shielding will reduce the effect of the extra
    capacitance?

    Tony (remove the "_" to reply by email)
     
  3. The Phantom

    The Phantom Guest

    Can you provide a link so we can see it for ourselves?
     
  4. amdxjunk

    amdxjunk Guest

    The website is just a repeat of threads from newsgroups.
    http://www.electronics-related.com/usenet/design/show/838-2.php
    Third letter down, I think.
    I sent a letter to the author of the info but have not received a reply.
    Mike
     
  5. The Phantom

    The Phantom Guest

  6. Genome

    Genome Guest

    Errrrrr.... I haven't come across the term "flux cancelling windings" but I
    have seen something about 'flux balancing windings' in a Unitrode
    Application note about designing transformers with fractional turns. Have a
    hunt around the TI website.

    It goes something like if you use a fractional turn to get closer to the
    right output voltage then leakage inductance goes up because more flux is
    diverted to the outer leg of the transformer not associated with the
    fractional turn. In order to compensate for it you add a 'short circuited'
    balance winding at the end of the transformer which couples the outer legs
    together and forces the flux to balance.

    DNA
     
  7. Terry Given

    Terry Given Guest

    see my other post in this thread

    Hi DNA,

    Billings has a nice section on fractional turns and flux-balancing.

    I once (OK, 15,000) stuck a 7.5 turn winding on a coupled inductor,
    without flux balancin. but thats because 14A flowed thru the 3T winding,
    25mA thru the 7.5T winding and I had an air gap :)

    Cheers
    Terry
     
  8. Terry Given

    Terry Given Guest

    ya have now :)
    probably not. He's notoriously unobservant :)

    "Rise Time Reduction in High-Voltage Pulse Transformers Using Auxiliary
    Windings, L.M. Redondo, E. Margato, J.F. Silva, IEEE Trans. Power
    Electronics Vol. 17 No. 2 March 2002

    I have not tried this myself, but the paper is a good read. They used a
    transformer with:

    Np = 50
    Ns = 506
    Nc = 25T (they call it Naux)

    The xfmr had double P-S insulation and "staircase" secondary
    construction to reduce capacitance. They play around with various
    resistances connecting the two Nc windings.

    Cheers
    Terry
     
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