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shielded inductor -> low frequency?

Discussion in 'Electronic Design' started by Michael, Sep 20, 2007.

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  1. Michael

    Michael Guest

    Greetings gents of the SED variety - I have a couple quick questions
    for you all:

    I am working on a DC/DC switcher that will use the Linear LT1930A. It
    switches at 2.2MHz. I need to find a ~8-10uH inductor that can handle
    about .5-1A for this. I identified the Sumida CR43 series as a good
    candidate. For example, there is a 8.2uH (at 7.96MHz) CR43 series with
    166.2mohms DCR speced at 0.84A. It also has a very small 4.3x4.8mm
    footprint. It would be perfect, except that it isn't shielded. I would
    really prefer it to be shielded. I started looking at other Sumida
    inductors, and it seems that all the shielded ones are speced in the
    low KHz region. Am I missing something here?

    Also, what is the relationship between inductance and frequency? I
    googled around about that for a bit but couldn't find anything.

    Thanks,

    -Michael
     
  2. LVMarc

    LVMarc Guest


    Electrostatic (conductive) Shielding does not effect the magnetic lines
    of flux you need high u material for that. and high u magnetic shielding
    material only works at low < 100Kc frequency.


    so placing a metal can around a coil is only partially effective in
    reducing radiated emissions.

    Marc

    marc


    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=300142128658&ssPageName=STRK:MESE:IT&ih=020

    link to actual mixer site

    www.fwt.niat.net amazing antennas

    http://www.youtube.com/profile?user=LVMarc
    video instructions antenna VOIP adapter

    BS benevolent Stuff for engineers
     
  3. Joerg

    Joerg Guest

    I believe what Michael meant is a closed versus open magnetic path.
    Closed path is what inductor company marketeers call "shielded". For
    example, a ferrite toroid would be quite good in that respect, that's
    what I usually use. I found that custom fab in Taiwan/China is actually
    less expensive than a fancy catalog inductor. Provided the quantities
    are at least a few thousand a month.
     

  4. http://www.cooperet.com/3/Coiltronics.html
    http://www.coilcraft.com/prod_pwr.cfm
     
  5. Rich Grise

    Rich Grise Guest

    Xl = 2 * pi * f * L

    Inductive reactince is equal to two pi times the frequency times
    the inductance.

    http://www.google.com/search?q=inductor+tutorial

    Hope This Helps!
    Rich
     
  6. Chris Jones

    Chris Jones Guest

    You might want to look at toroidal inductors - these tend to have less
    external magnetic field that solenoidal inductors.

    Chris
     
  7. Phil Hobbs

    Phil Hobbs Guest


    One of these times I'm going to spend a week doing some measurements and
    demonstrate how effective copper shielding can really be against
    magnetic pickup. People are always focusing on the skin depth argument
    (how the surface field penetrates the bulk) and not the shorted-turn
    argument (how the presence of the metal reduces the surface field).

    An enormous amount of misinformation gets repeated over and over on this
    topic.

    I have 4 weeks holiday left this year--maybe I'll do it over Christmas.

    Cheers,

    Phil Hobbs
     
  8. Nobody

    Nobody Guest

    Inductance is proportional to the relative permeability of the core, which
    is relatively constant at low frequencies then starts to decrease at
    higher frequencies. The exact relationship depends upon the particular
    core material.

    As they cite an inductance at 7.96MHz, it's likely that the inductance is
    relatively constant from DC up to that frequency, including your 2.2MHz.
    There wouldn't be much point in specifying the value at a frequency where
    the graph is no longer relatively flat.
     
  9. Terry Given

    Terry Given Guest

    these "shielded" bobbin cores have a ring of ferrite material placed
    around them, spaced out a mm or two from the OD of the bobbin flanges.

    it reduces external field leakage from an awful lot to a fair bit. that
    might suffice.

    Joerg's approach (toroidal cores eg koolmu, -52 etc) is by far the best.

    Cheers
    Terry
     
  10. They seem to be nearly pot-core in design, as I see them.
    What do you think of page 52 of Linear's AN70?
     
  11. Terry Given

    Terry Given Guest

    there are some pot-core SMT inductors. I've seen some nice ones, with
    spiral strip windings, ludicrously low Rdc (eg 1mOhm), for VRMs etc.

    the Sumida style shielded inductors are bobbin cores with a ring, as are
    the ABC ones. sometimes the "rings" are qctually square. And the gap
    between the bobbin inductor and the ring is invariably filled with goop.
    it kinda shows how not to wind a toroid. bifilar, thats the go.

    very interesting though. and of course low-perm cores (koolmu etc) have
    plenty of flux leakage too.

    Bruce Carsten has done some nice papers showing the sorts of field
    leakage you can get from a toroid.



    Cheers
    Terry
     
  12. Robert Baer

    Robert Baer Guest

    FAR OUT!
    Most of us will be delighted to hear the results (aka facts).
     
  13. Joerg

    Joerg Guest

    But it can also be the most expensive, especially if quantities are low
    and you can't find a catalog part.

    Man, that prototype on page 37 sure looks ugly. The generator on page 46
    is worthwhile peeking at. I wonder how they kept it so fresh. I've got
    one from the 50's but it looks way more tired.

    Page 52: H-field EMI can be dealt with (to some extent) with crossover
    turns but it is an art to get these just right for best cancellation.
    Almost like adjusting microwave stuff. It is even tougher to then
    explain all that in a fabrication procedure. T'is where even us
    electrical guys need to whip up a mechanical CAD drawing.

    BTW since it's often said that shielding doesn't work I have to
    disagree. Even a Faraday type shield has saved the bacon many, many
    times in my EMI work. And EMI is >30% of my bread and butter.
     
  14. LVMarc

    LVMarc Guest

    ARE YO MENAING THAT WINFDINGING THE COIL ON A TORRUS, CNTAINS THE STRAY
    MAGNETCI FEILD FLINES? IF SO .yes THIS WOULD BE A GREAT WAY T GO AND
    THEY HAVE STEP UP AND DOWN TRANSFORMERS TTHAT ARE WOUND ON A CLOSED
    FORM, IE TORRUS.. BIT EXPENSIVE AS T IS HARDER TO WIND!

    mhp
     
  15. I have some Coiltronics DRQ coils. I was going to break one to see
    how well the gap is closed. It looks pretty tight, but I'm not sure.

    The DRQ's have filler in some parts. I've also wondered if the "goop"
    is impregnated with ferrite particles.
    I have to build that probe he described in AN70. I must!
     
  16. Joerg

    Joerg Guest

    Well, yes, that's why those are usually outsourced. At large qties you
    can get that done for well under 50c for a half-incher with a few dozen
    turns including materials and shipping.
     
  17. Right. Actually, it takes no more then a scholar math to estimate the
    attenuation of a static field by a conductive shield if the geometry is
    enough simple.
    :))) As always. SED is not a source of information - it is a place for
    leisure.
    Good luck.
    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
    www.abvolt.com
     
  18. Robert Baer

    Robert Baer Guest

    I have a few dumb questions about that probe described in AN70.
    Refer to page 67; what are the wires "to signal generator" for?
    There does not seem to be any reference to such foolishments any
    where else.
    In other words, doesn't the coil connect *directly* to the resistor /
    coax?
    On another note, it seems that the ID of the plastic tube may be
    non-critical.
    Perhaps ceramic, glass, or even bakelite could be used.
    If there is an optimum capacitive coupling, then wall thickness VS
    material should be specified, and if dielectric loss is also an issue,
    then types of plastics should be mentioned ("optimim", "useable",
    "poor", "too lossy", etc).
    Not a lot of detail...
     
  19. Robert, that dwg on page 67 is not the Sniffer Probe,
    it is a test coil to test the Sniffer Probe.
    Put the SP nose into the centre of that 20-turn coil,
    turn up the signal generator, the scope across the 12.4
    ohm resistor measures the current injected into the
    test coil, etc.......

    A rough sketch of the Sniffer Probe construction is
    given on page 55 and the address to buy it from on
    page 54.
     
  20. Robert Baer

    Robert Baer Guest

    Thanks!!
     
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