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Gapping Pot Core

Discussion in 'Electronic Design' started by WJLServo, Jan 23, 2008.

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  1. WJLServo

    WJLServo Guest

    Working on eval design for power inverter; will use a pot core for
    transformer. Have picked a core size, based on allowable hysteresis
    loss. Also have a pretty good idea what magnetization inductance
    needed. Requires an AL value a good bit smaller than that of largest
    gap core in vendor's catalog...

    Thought of buying a few core sets with largest available gap, then
    modifying them to test for proper indutance, and get vendor to make a
    special order core, same gap, for production. But, damned if I can
    figure out how to remove material from core center posts. Could just
    use gap shims, I guess, but that would open up gap at periphery as
    well as center of core, and I would not then be able to put core in a
    shield cover without incurring eddy current losses in metal cover.

    An E core would be easier, I think: just come in from the side with a
    grinding wheel and shave down the center post, using a bit of flood
    coolant to make a nice, smooth ground surface. Getting to the center
    post of a pot core would almost seem to require a jig grinder, which I
    don't have. Any suggestions?

    W Letendre
  2. Joerg

    Joerg Guest

    Well, I have used a drill press with a very good grinding bit in there.
    But be very careful. It can easily start to vibrate and then you have
    stuff flying about at high speed. If you use a Dremel in a drill press
    stand don't run that at full speed. Use eye protection and make sure
    nobody stands close. Of course the liquid carries it's own risks. So be
    careful, proceed at your own risk ...

    Oh, BTW, if the drill press is one of those el-cheapo deals and you can
    feel the bearing slack forget it, then you need a better one.
  3. John Smith

    John Smith Guest

    Dremmel tool with diamond wheel?

    How steady is your hand? How good your eye?

    How many cores you got to waste? :-|

  4. Phil Allison

    Phil Allison Guest


    ** Consider using push pull drive to the transformer - obviates to need
    to create gaps.

    ........ Phil
  5. Joerg

    Joerg Guest

    Yep, that's what I always do. And don't forget the inductor after the
    diodes, especially if it's a big one.
  6. Winfield

    Winfield Guest

    My advice: During the prototype phase, add the spacer
    and don't worry about the flux leakage and eddy-current
    losses with the shield -- they will be small. Go ahead
    and measure the inverter's performance. Take heart in
    the thought it might do better with a proper ground core.
    Second, add the shield to get a handle on magnetic-field
    leakage, etc., which at any rate will be less with the
    production pot core. What's the problem? If you're
    happy with the prototype design, tested this way, the
    production one will be a bit better! So... what's the
    big problem?

    Oh, more advice: just forget grinding your ferrite core.
  7. Jim Thompson

    Jim Thompson Guest

    Yep, You can buy them already gapped.

    ...Jim Thompson
  8. WJLServo

    WJLServo Guest

    Push pull drive would work, and work well. But, believe design I've
    got in mind will be ~ $1.50 lower part cost. Target production volume
    definitely high enough for part cost savings to pay off R&D labor.

    In fact, had rather hoped that target volumes would be high enough so
    that we could sweet talk vendor into making up an eval lot of cores
    with assorted gaps "bracketing" intended AL value. Will still pursue
    this, but, vendor is in Far East; they make good cores, and they make
    them cheap, but they have not generally given us very good lead time
    for prototype lots. So, thought I would try "fiddling" gap in our lab,
    then giving gap spec to vendor for production order. May try Dremel
    tool; do worry about cracking core!

    W Letendre
  9. WJLServo

    WJLServo Guest

    Heh! Think your advice may be best! Only reservation is wondering how
    easily the gap generated with spacer shim can be translated into a
    production value. Had thought that a modified core with center post
    ground to match desired AL could then be measured using dial indicator
    to give an exact gap spec for vendor to use for production cores. At
    the end of the day, though, may just use spacer, measure AL rather
    than gap, and ask vendor to take AL value as their production spec...

    W Letendre
  10. Terry Given

    Terry Given Guest

    firstly, if you gap across the whole core face then the total gap is 2x
    the gap thickness (this is apparent if you think about the path the flux
    traverses, it crosses the gap in the centre leg, and again in the outer
    limbs). so centre-leg-gap = 2 x shim-thickness

    Secondly, you can always calculate it, if you know the material
    permeability. its pretty easy to do.

  11. WJLServo

    WJLServo Guest

    Tried calculation, first up. Values I came up with were close to
    vendor's published values, AL versus gap, for the half dozen stock
    gaps they offer. Close... but not close enough for design use,
    especially as I need to go ~ 2X out from largest stock gap. Think I do
    need to test design with actual gap.

    2X rule of thumb (center post versus dore face gap) may also be close
    enough; would like to think that it is. Think, though, that I will
    trust the measurements over the math!

    W Letendre
  12. Eeyore

    Eeyore Guest

    Get yourself a copy of Epcos's 'Ferrite Magnetic Designer' program. It makes
    life so easy.

    Why aren't you shimming rather than gapping ? Even low volume manufacturing
    (and some not so low volume) uses shims rather than gaps.

  13. Terry Given

    Terry Given Guest

    he already explained that - gapping across the core allows fringing flux
    to escape, which defeats the purpose of using pot cores, which is to
    keep the fields contained.
  14. Terry Given

    Terry Given Guest

    for "large" gaps (> 0.1mm or so) then assuming all the energy is
    concentrated in the gap is a very good approximation. for small gaps,
    this is not very accurate, but again (assuming you have access to
    Snellings Ferrites book) this is fairly easy to calculate.

    because you want a big gap, you may also want to take fringing into
    account, which increases the effective pole area.

    even without the finer considerations, the all-energy-in-gap approach
    will get you very close to the desired gap.

    its not a rule of thumb. follow the flux path, and you see where the 2x
    comes from. the maths is based on some pretty sound (and well
    understood) physics.

    for power converter work, IME the exact value of L isnt *that* critical,
    provided you allow for sufficient margin in the overall design to suck
    up tolerances. Heck, just look at a graph of relative permeability vs
    temperature! or look at the spec'd tolerances in pre-gapped magnetics.....

  15. Winfield

    Winfield Guest

    That's right, except I suggest you begin comparing
    your A_L measurements to the limited standard A_L
    values available for that ferrite, and regap/rewind
    your core until you match one, then test your setup.
    Continue this process until you like your design,
    then order that AL ferrite core.

    Measure A_L, not gaps, and compare to the catalog.

    A_L is easy to measure, wind 5 - 10 turns and measure
    the inductance. A_L = L / N^2, where L is in nH.
  16. Joerg

    Joerg Guest

    Then just give it to a really good machine shop. They'll do it
    professionally and precise.
  17. WJLServo

    WJLServo Guest

    True enough. Does require a shop with at least a little experience
    with brittle, high Rockwell materials, though. Ferrite, as a ceramic,
    almost certainly requires grinding rather than machining. Would hate
    to give a ferrite core to a machinist who might try to cut it with an
    end mill!

    W Letendre
  18. I doubt that would happen- most machinists are pretty canny. If it's
    an RM type pot core your machinist may be able to handle it on an
    ordinary surface grinder with a narrow wheel and a magnetic chuck.

    Best regards,
    Spehro Pefhany
  19. Joerg

    Joerg Guest

    Use a shop that is experienced in handling ceramics. They should be able
    to grind it down precisely to within a few micrometers.
  20. qrk

    qrk Guest

    Win's method is what I have used in the past. Beware that the junk
    spitting out from the core can get into other electronics and cause
    problems if your gap is large enough. I've used business cards as
    shims. Seems to be the best use for them. For proto purposes, you can
    use black electrical tape to hold the core halves together. Since you
    are gapping your cores, the offical hardware isn't necessary for your
    experimental stage.

    You need diamond tools to gap your own ferrite. Best to use the shim

    Once you have a gap you like, simply calculate the AL of your core set
    by measuring the inductance of a coil with a known number of turns. Be
    sure your bobbin is nearly full as the AL value is more sensitive to
    winding height with lower AL vaules (a 0.2 relative winding height
    will give an AL value 6% lower at and AL of 160, 2% lower at an AL of
    600 for many pot cores). When ordering custom gapped cores, all you
    need to give is the AL value you want and they do the rest. However,
    see if you can use a standard gap as that makes life much easier and
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