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RM12 core - high temperature.

Discussion in 'Electronic Design' started by Syd Rumpo, Mar 15, 2013.

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  1. Syd Rumpo

    Syd Rumpo Guest

    Anyone know if it's possible to get RM12 cores - say Al = 160nH or
    thereabouts - which will work at maybe 250'C ? Those I've seen have a
    curie temp of 210'C which is a bit too close to my ambient 180'C for

  2. Have you looked at the materials available to you?

    N49 from Epcos has Tc > 240°C

    Your bobbin will probably not withstand that.
  3. Syd Rumpo

    Syd Rumpo Guest

    Thanks, I'll look. The bobbin can be PEEK.
    Just did - 'Optimum frequency range' is 300kHz - 1000kHz and I want
    10kHz - 80kHz. I don't know enough about this to know how the optimum
    is determined and how it behaves outside this.

    Any recommendations for an idiot's guide to magnetics? So many variables.

  4. Joerg

    Joerg Guest

    If you get stuck talk to Solvay.

    You are almost on the turf of tape cores. I'd talk to these guys:

    As for literature, Amidon is pretty good but there's also lots of
    others. Lots of variables to deal with and if you've never done a
    transformer design like this I suggest to get in contact with a good vendor.
  5. Google is a good place to start.
    I don't think there is any problem going to a lower frequency.
    (But I know little about magnetic materials.)

    What are you going to solder it with? (I always start to worry if the
    temp gets above 150C...) Hey, what about wire insulation?

    What the bleep are you doing at 180C?

    George H.
  6. Since you already have a core size/shape in mind, then you can look at
    vendors (eg. Epcos, Ferroxcube) that make that core and decide which
    available material (if any) fits your application from the material
    datasheets. The 160nH Al requirement will determine your air gap.

  7. Tim Williams

    Tim Williams Guest

    The minimum for a material is more like, you wouldn't want to use it for
    less because other materials are cheaper, with higher Bmax and mu.

    Actually, Bmax is quite good for N49, typical of MnZn. It looks
    comparable to Ferroxcube 3F35 (or 3F3 or 3F4).

    Mu of course doesn't matter here as you'll be gapping it. As long as it's
    enough to begin with.

    Just looking at the Ferroxcube listing, while I'm there,
    # Tc
    3C92 280C
    3F45 300C
    3S5 255C
    3B46 255C

    3B46 has unusually high mu and Bsat; with a crossover frequency around
    700kHz, it wouldn't be very lossy at 10-80k.

    NiZn cores appear to have Tc and mu inversely related, so you could pick a
    4E1 (mu = 15) with Tc > 500C, but you'd never get your AL either.

    AL seems kind of high for a choke, you might also consider using more
    copper and less ferrite. Not sure why really, but I suppose the tempco
    could be better -- less reliance on a teeny gap. Tempcos suck all over,
    of course.. copper ain't as pretty at 200C (about double R).

    It's probably also worth mentioning average transformer varnish softens
    below 200C. In fact, cooking parts at ~200C is a good way to salvage the

  8. Nico Coesel

    Nico Coesel Guest

    IIRC I have used N49 for a transformer prototype at 80kHz. If its for
    a switching regulator you'll get more ringing at lower frequencies
    because the material isn't so lossy.
  9. Bill Sloman

    Bill Sloman Guest

    That could actually be an advantage in his application - lossy ferrite
    gets hot because of the current circulating in the ferrite core
    itself, and if the ambient is already 180C you want to generate as
    little heat as possible inside the core. The windings are at least
    made of copper, and can conduct the heat they generate without too
    much of a temperature rise - though he probably needs to use forced
    convection (fan cooling) to minimise their running temperature.
  10. Phil Allison

    Phil Allison Guest

    "Syd Rumpo"
    ** For use in a 180C ambient, you will need Teflon bobbins and wire.

    Use an oversize core to allow for loss of mu as the Curie temp is

    And of course, practically no electronic components made will stand it for
    long - with the exception of all glass valves.

    .... Phil
  11. Syd Rumpo

    Syd Rumpo Guest

    Hi Phil

    Usually custom made PEEK bobbins are used as it's easy to machine, and
    these are very small quantity instruments, but Teflon would work too.
    If someone already makes such, that would be very useful.

    High temperature wire is usually 'enamelled' with polyimide (Kapton) and
    is good for 250'C.

    Electronics at 180'C needs careful selection and testing, and doesn't
    last too long. That's not always a bad thing :)

  12. Nico Coesel

    Nico Coesel Guest

    Using lesser windings may also be an option (increase flux swing
    IIRC). The core will run hotter but it has more outer surface than the
    copper inside the transformer.
  13. There are a lot of materials that are still okay well over 200°C. We
    use polyimide insulated wire, as well as PTFE. If you had to go really
    high, for the bobbin, Macor is good to something like 1000°C. More
    reasonably, the OP mentioned PEEK.. there are also things like Vespel.

    Depending on the atmosphere, I'd worry about fine wire lasting very
    long. I've seen failures of fine Cu wire occurring at pinholes in the
    insulation. The resistance will increase quite a bit at the high
    temperatures too- more than +50% compared to room temperature.
    On N49 anyway (Tc> 240°C), the mu goes _up_ almost monotonically from
    room temperature to about 225°C, after which it plummets. There's a
    little dip between 80°C and 140°C, but nothing significant. A bit
    There are some semis that are claimed to last in excess of 10 years at
    180°C (5 years at 225°C!). Expensive, but required in some
    applications, many of which are okay with limited lifespans.
    Honeywell, for example.

    Changing the subject more than a bit, but I wasn't aware that
    so-called "all glass" valves require stopcock grease.

    Best regards,
    Spehro Pefhany
  14. I've seen open open air coil forms made of teflon, where the teflon
    'flowed' before the wire insulation failed. I have no idea the
    temperature, perhaps it's better when enclosed in the transfomer.

    George H.
  15. Tim Williams

    Tim Williams Guest

    Actually, mu always goes up with temperature. Stochastic resonance --
    think of it as white noise dithering the magnetic domains' hysteresis.

    At low temperature, the domains just stick around. The B-H loop is wider
    (I think... I'd like to see some data though), and around B=0, it's much
    flatter (low initial permeability). When looking at a B-H curve, you have
    to remember it's not actually a nice sigmoidal loop: real materials often
    have pronounced flat spots around the axis ("butterfly curve"), which
    manufacturers typically ignore.

    At higher temperatures, thermal energy scatters the magnetic domains,
    raising the ground state energy level, reducing hysteresis but leaving
    less room for magnetic energy. You can still force the domains into
    alignment, but you don't get as much flux from doing it -- so, saturation
    Bmax drops linearly as Tc is approached, hitting zero at Tc.

    Presumably, materials with a gradual, progressive curve are a blend of
    many Tc's, and materials with two spikes are mainly two Tc's. And
    accordingly, Bmax would drop off in a sum-of-parameters slope. I don't
    know if this is a correct interpretation or if there's more subtle physics
    going on.

  16. Fred Abse

    Fred Abse Guest

    At 120 Celsius, the highest characterized temperature for dissipation, at
    100kHz, and 100mT, N49 dissipates 115kW per cubic meter.

    Initial permeability at 180C, the OP's operating temperature, is 1820.

    Figures from Epcos ferrite magnetic design tool.
  17. Bill Sloman

    Bill Sloman Guest

  18. josephkk

    josephkk Guest

    Various people here, but not all the time have made high temperature "down
    hole" devices that operate up to 250 C for a short time. Sometimes it is
    cheaper to make them expendable. They have their methods. I might
    recognize one if posting to this thread, or i might not.

  19. whit3rd

    whit3rd Guest

    You may have to do some math: by a generally applicable susceptibility law,
    the Al value will drop as ( T-Tc) **gamma in the vicinity of the Curie point (with
    some minor correction because the Al value isn't exactly zero with
    only an empty space for a core). Gamma, for iron, is 1.33; I don't know
    about ferrites.
  20. Syd Rumpo

    Syd Rumpo Guest

    Thanks for all the suggestions, that's very helpful. I have a fair bit
    of experience with high temperature work, but not with magnetics.

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