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another strange idea

Discussion in 'Electronic Design' started by John Larkin, Apr 28, 2006.

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  1. John Larkin

    John Larkin Guest

    Has anybody made RF transmission-line transformers out of multilayer
    PC boards? I've seen switching magnetics integrated into a board, with
    traces for the windings and a few strategic holes for mounting an
    e-core.

    John
     
  2. Joerg

    Joerg Guest

    Hello John,

    The UHF section of all those VHF/UHF 4:1 splitters had "phenolic
    transformers". Not exactly transmission line but close. At least all the
    ones I mounted in my teenage days were that way. Lower end or higher
    frequency reflectometer bridges (SWR meters) often contain directional
    coupler strips made from FR-4.

    The E-core thing, well, I am not a fan of that. Seen to many come apart
    on those desert truck rides. I do it with double-holers.

    I see no reason why multi-layer couldn't do it. Except that FR-4's RF
    properties are, as Archie Bunker would put it, lousay.

    Regards, Joerg
     
  3. I have designed one on an integrated chip, which is more or
    less the same thing, just in a smaller scale

    Regards,
    Mikko
     
  4. Joerg

    Joerg Guest

    Hello Klaus,
    Ferrite is a wonderful material. Remember the old VHF/UHF splitters? The
    VHF section usually consisted of a double-hole ferrite balun and is
    useful up to 300MHz.

    Personally I am just not a great believer in split cores or anything
    that relies on adhesives to hold core halves together. At least not for
    stuff that ships into fields where handling can be a bit rough.

    Regards, Joerg
     
  5. Terry Given

    Terry Given Guest

    Hi Joerg,

    I've built quite a few planar converters, using a combination of clips
    and glue. My last dc-dc converter had a planar xfmr and a planar coupled
    inductor, and we've made about 30,000 so far. Some of these get carted
    about on the back of large trucks )in theory with air riders, in
    practice without), and while there have been problems, the cores have
    been OK for 5 years now. Some of the thermosetting epoxies I tested were
    far stronger than the core, and the operating temperature rise hardens them.

    I prefer thermosetting epoxy to glue the cores together, and some form
    of CTE-, tolerance- and vibration-absorbing structure to hold it to the
    PCB. its usually a good idea to space the core slightly away from both
    sides of the PCB, because the permittivity of MnZn ferrite is so high


    Of course the production guys then changed to a rather crappy glue that
    softens when it gets hot, because it was easier to apply. It remains to
    be seen if this was a wise idea (they no longer do it), but I suspect not :)

    Hi John,

    I've seen some papers out of South Africe detailing integrated magnetic
    structures, where the ferrite material is formed around the windings. J
    D Van Wyk springs to mind, but IMBW.

    There have also been a bunch from IIRC Korea about coreless transformers
    (spiral windings on FR4) and various uses as gatedrives etc. And some
    spinoff papers looking at the effect of placing Cu and/or ferrite plates
    on the outside. air core gatedrive transformers sound like a great way
    to create mysterious blowups, along with maximising EMI emissions and
    susceptibility.

    any transformer you can make with an E-core, you can make with a planar
    core (E-E or E-I). All they have done is reduce the height and increase
    the thickness of the cores. Glued cores can have a lot less permeability
    than a single piece of magnetic material (eg toroid, n-holer), unless
    they are squeezed very close together. several tens of microns can have
    a noticeable effect. very high creepage and clearance can be done, but
    is a pain (buried windings, coatings of gunk, fancy bobbins etc)


    Cheers
    Terry
     
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