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Flyback transformers, small, high power, where?

Discussion in 'Electronic Design' started by Joerg, Aug 2, 2012.

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  1. Joerg

    Joerg Guest


    Got to do a flyback design, 120VAC to 24VDC or so, at about 15-20W. Will
    run in full PFC fashion with the ouput voltage sloshing around, meaning
    peaks more like 20W but average power under 15W. 100kHz switcher
    frequency, because of a dearth of available PFC-flyback chips at higher
    and because EMC above 150kHz would be tough here. Most likely we'll use
    the LT3798.

    Here is the pickle: The transformer cannot be more than about 250mils
    high and a little over 500mils wide. Gapped ferrite gets us to less than
    5W. Soooo ... got to look at newer stuff like Kool Mu, Ni-Fe High-Flux,
    Sendust. Probably will come with some core loss penalty but the main
    issues is that I can't find small E-cores there.

    Does anyone have an idea? Does anyone know a switcher transformer
    manufacturer who'd likely be willing to tackle this transformer design?
  2. Tim Williams

    Tim Williams Guest

    Have your board guy lay out a planar transformer. Disclaimer: you didn't
    specify board area per se, only height. Though you'll still have a hard
    time with overall dimensions coming in under a parallelepiped of the
    specified dimensions...

    Don't think I've heard of Kool-Mu in planar E's.... would be cool though...

  3. Joerg

    Joerg Guest

    Believe it or not, this time cost is not the main concern. Size is
    though. If the core loss amounts to a watt or so that'll be ok.

    We have to do it because PF rules start to apply when you offer
    installations where there's dozens of these in one location. Because of
    the small space we must PFC via flyback first to get to a non-isolated
    LV section as fast as possible. That LV are would contain post
    converters and won't need so much in creepage distances. It'll be a
    small and most of all very skinny product.

    Good idea, although AFAIK Wuerth only does ferrites.
  4. Joerg

    Joerg Guest

    Can't do that because the other side is already full of parts and must
    be low voltage. It wouldn't help anyhow because there's hardly any head
    room. Plus we must remain around 100kHz.
  5. Nico Coesel

    Nico Coesel Guest

    They also sell inductors and transformers. I bet John bought quite a
    bunch of custom transformers. Last time I asked Wurth they had an MOQ
    of 20k pieces.
  6. Guest

    They also think they're wurth a lot!
  7. Joerg

    Joerg Guest

    Not for the usual ferrite that pegs at about 0.5T. Other more modern
    stuff can do 1.5T or even better. Most of that is distributed gap.

    As a kid I've redlined an iron powder core in a balun because I didn't
    have enough money to stack two of them ... *PHOOMP* ... thwack ... and
    the antenna lay on the ground. Now I was down to zero cores :-(

    But back then dash-2 from Amidon was the best money could buy for use at
    several MHz. It's been over 30 years.

    Not that way, because then you have to keep them at a distance to
    fulfill creepage path regs. The core could be longer, within reason.
    Like an inch or maybe more. But unfortunately only lengthwise, the
    contacts can't be on the sides for discharge clearance reasons.

    The reason I am asking here is that there are materials with a
    saturation flux density several times higher than ye olde ferrite. So
    I'd like to see if a smaller flyback xfmr can be build with any of
    those. From what I understand the core losses would be somewhat higher.
    If that's not a whole lot of added loss it may be an option.

    uCs are very poor switch mode controllers. Every time that was done
    (usually against my advice) they couldn't get the loop bandwidth high
    enough because the thing plain ran out of MIPS. I have to live with
    what's there in terms of ICs.
  8. Joerg

    Joerg Guest

    That MOQ is ok (after sampling), but their flyback stuff is huge, all
    regular ferrite.
  9. Joerg

    Joerg Guest

    We may have to stack. Ugly but ... man's gotta do what man's gotta do.
    Problem will be to cajole a xfmr mfg into doing that. They are usually
    very reluctant when it comes to unorthodox stuff.

    BTW, table 3 in here is why I think there may be a change to shatter the
    old size rules:

    There's also EMC. If you get above 150kHz all hell breaks loose in terms
    of conducted EMI. The filters will easily eat up all save real estate.
  10. legg

    legg Guest

    The creepage doesn't change until you've successfully isolated the
    source - it's all hazardous until you provide a reinforced isolation

    If you're doing this in two stages of basic isolation, then the
    line-input PFC section will need to be basic-isolated from the 24V
    output, as a first stage.

    Telecom stuff does this - basic isolation to battery bus, then basic
    isolation to user ground-referenced outputs.

  11. legg

    legg Guest

    This is unfortunate, because using the printed wiring is the most
    commonly used method of jumping the low profile power barrier and
    reducing or distributing surface temperature rise.

    Your LT3798 app gives a table of parts illustrating what happens using
    conventional shapes. None claim a power handling density greater than
    3w/cm^3. You're asking for 15W/cm^3.

  12. Nico Coesel

    Nico Coesel Guest

    In my experience I always had to do (most of) the transformer design
    myself when I needed a custom transformer.
  13. Joerg

    Joerg Guest

    It isn't a popular method because that way you cannot reasonably get the
    number of turns for a 180V peak primary. The only time I have seen
    planar transformers is in signal isolation or pulse transformers.

    In the core, yes.
  14. Joerg

    Joerg Guest

    Sure, that's why we want to do the flyback-PFC. Because that puts the
    iso barrier as far forward as possible. The bulk caps are then on the
    non-mains side which is a huge advantage versus boost-PFC when real
    estate is of the essence.

    This is a hi-rel app so we have to be extra careful. Longer creepage
    paths and so on. So we want to minimize the primary side circuitry.
    Ideally without having to go above 150kHz.
  15. Joerg

    Joerg Guest

    Sure looks like it here as well :-(

    But then where is the value added by domestic manufacturers? Then we
    might as well have it produced in Asia.
  16. Joerg

    Joerg Guest

    It can be done but it's expensive. 8-10 layers, board becomes brittle if
    skinny, and so on. It does look elegant though.

    And probably at a much higher frequency than 100kHz.
  17. notbob

    notbob Guest

    Don't most CRTs have flyback xfrmr? Look fer one in an itty bitty tv
    or o-scope. I've gotta tektonix 213 dmm w/ about a 2" CRT. The
    flyback hasta be pretty small. (shrug)

  18. They are used more often these days. For example, the Excelsys series
    of modular supplies use planar transformers with mains volage
    primaries. A bit pricey, but they have available good "medical"
    isolation (4kV) high-rel and low noise options.

    Here's the output module with the planar transformer: Tech Photos/POWERMOD SERIES.JPG

    I have not attempted to determine how many layers are in the board.
  19. legg

    legg Guest

    So the 13mmx13mmx6mm, 15-20W flyback transformer also requires
    reinforced insulation?

  20. Joerg

    Joerg Guest

    It doesn't follow the usual standards, this is a special hi-rel app. We
    need certain breakdwon limits and creepage paths for everythin
    line-side. Don't have them in my head right now but more than 0.150" to
    chassis, for example. This goes into a narrow tube so that presents a
    real estate issue.

    The core itself can be on the isolated (LV) side.
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