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Flyback with LAN transformers

Discussion in 'Electronic Design' started by Joerg, Jun 18, 2007.

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  1. Joerg

    Joerg Guest

    Ok, most of us have used LAN tranformers for power transfers. However,
    their datasheets are skimpy at best and usually contain no information
    about the core. Of course it won't contain any regular air gap and you
    cannot easily install one later because it's all potted up.

    So, did anyone use these for a flyback design yet? Low power, something
    well under 100mW.
     
  2. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Which voltage from which voltage?
    Do you really need flyback? I once did a funny self-oscillating one for
    that kind of power, from 50V supply. Worked pretty well with about 60%,
    maybe 70, efficiency IIRC.
    I can dig up the drawing.

    Anyway how about measuring your xformers? That's so easy...
     
  3. Joerg

    Joerg Guest

    I've used oscillators as well but then there was no DC component and
    everything was just fine. It's regular voltage levels, 12V, 24V etc.
    Flyback is very easy from a topology point of view, and regulates very
    fast. If there is no other way I'll just do the oscillator thing again.

    Sure. But you know how it goes after that. A couple of years later the
    manufacturer "improves" the design and ... POOF.
     
  4. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Then take enough safety margins. Given the power level it's probably not
    difficult.
    And then what can be improved?
    Ferrite won't change much and evolves at a very slow pace, and
    turns/geometry will stay the same or it'd be another part number.
     
  5. John Larkin

    John Larkin Guest

    We use the Tamuras as forward-mode dc/dc converters, driven p-p ct by
    a pair of 2N7002's, in the 60 KHz ballpark. They are toroids,
    ungapped, so probably don't store a lot of energy. They *are* cute and
    cheap, and come with all sorts of interesting turns ratios.

    John
     
  6. Joerg

    Joerg Guest

    I'll check those out. Forward p-p would be a bit inconvenient here
    because there is no high side drive capability. What power do you push
    through? If it's more than a watt a flyback might be possible since even
    p-p is never exactly 50.000% duty cycle.

    At Digikey the Tamura xfmrs seem kind of pricey.
     
  7. Joerg

    Joerg Guest

    Well, I've seen it a few times that a manufacturer switched to another
    source for the ferrite and problems arose. But you are right, if we had
    something like 90% margin we'd be ok.
     
  8. John Larkin

    John Larkin Guest

    Oops, it's an Alfamag/Talema ISDN part I was thinking of, SMJ-140B. It
    has 4 separate windings, 1:1:2:2, but you can get other ratios. We're
    paying $1.18 at 1K. It's a nice cased sugar-cube surface-mount thing.

    I think we're running about a watt, but it's probably good for more.
    We're driving it with two open-drain 2N7002 fets into essentially a
    center-tapped winding with the ct at +5 or +12, depending.

    I posted a pic a while back, of a VME board with 16 of these.

    It would probably work flyback, but a watt might be tricky. Just
    guessing. It does look hunkier than the typical Ethernet xfmr.

    John
     
  9. Paul Mathews

    Paul Mathews Guest

    Purpose designed miniature flyback coupled inductors ('transformers')
    are readily available a equally low prices, so why bother?
    Paul Mathews
     
  10. Joerg

    Joerg Guest

    Nice, but most of my apps can stomach >$1. I've used some Asian ISDN
    transformers for that (Murata?) but it's been a while and now it seems
    ISDN isn't exactly flourishing anymore. Just like those cute modem
    transformers that are probably going to almost vanish one day.

    That's the thing, they do not say how much DC they can take. I need less
    than 100mW here but still, if it works on the bench that doesn't mean
    it'll still work in three years because there ain't nothing about that
    in datasheets and when I ask all I get is a shrug. Because it's the
    first time someone asked.
     
  11. Joerg

    Joerg Guest


    Where?
     
  12. Joerg

    Joerg Guest

    Question: Can someone teach them how to design a web site? And how not
    to design one? It doesn't even let me get to the SMJ series and I don't
    know which distributor sells their stuff. Because they don't say.

    I hope they didn't hire the web designer from National...
     
  13. John Larkin

    John Larkin Guest

    And nice low dcr's too.

    I think you can buy less, but they're made in India, so leadtime is
    terrible. We have 1200 in stock if you need a few.

    John
     
  14. Is 1K the MOQ? Can you buy just 100? It has a nice high primary
    inductance.


    Best regards,
    Spehro Pefhany
     
  15. Paul Mathews

    Paul Mathews Guest

    Some sources with very aggressive pricing on miniature flybacks
    include Xfmrs, Inc, Premier Magnetics, Vishay/Dale, GB Int'l. Quantity
    requirements for low prices are surprisingly low....you basicallly
    have to cross their threshold for having the parts batch produced in
    Asia. Prices in lower quantities are about 6x higher. We buy UR 1W
    flybacks for about $.50 ea in 1K lots.
    Paul Mathews
     
  16. Joerg

    Joerg Guest


    Some sources with very aggressive pricing on miniature flybacks
    include Xfmrs, Inc, Premier Magnetics, Vishay/Dale, GB Int'l. Quantity
    requirements for low prices are surprisingly low....you basicallly
    have to cross their threshold for having the parts batch produced in
    Asia. Prices in lower quantities are about 6x higher. We buy UR 1W
    flybacks for about $.50 ea in 1K lots.[/QUOTE]


    $0.50/1k is expensive. Non-gapped is much less. But still witing for an
    answer from Xfmrs, let's see. Not holding my breath though ;-)
     
  17. Paul Mathews

    Paul Mathews Guest

    Expect lower pricing for non UL-Recognized parts that don't have
    inspection and creepage/clearance margins, taping, and related. What's
    your target?
    PM
     
  18. Joerg

    Joerg Guest


    Expect lower pricing for non UL-Recognized parts that don't have
    inspection and creepage/clearance margins, taping, and related. What's
    your target?[/QUOTE]


    Can't tell. But it doesn't have to be UL cert'd. However, the premium
    versus a non-gapped core cannot exceed the cost of the parts we'd save
    going from forward to flyback, maybe 10-15c.
     
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