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Core losses in dual big inductors

Discussion in 'Electronic Design' started by Joerg, Jul 25, 2013.

  1. Joerg

    Joerg Guest

    Trying to get 45-50W through a SEPIC, and unfortunatly the output
    voltage has to be up to 80V, input up to 30V. Peak primary current is
    going to be around 7A because of a wide supply voltage range. Long story
    short, a 10uH SRF1280 from Bourns will almost unsolder itself, goes 85C
    above ambient. Phsssss ... ouch.

    http://www.bourns.com/data/global/pdfs/SRF1280.pdf

    We've ordered some with larger inductance since this is all due to core
    losses. But of course there comes a point where that won't allow the
    peak current we need at the lowest supply voltage.

    So ... does anone know a brand or style of inductor that is really low
    in core losses? Or bigger SMT footprint ones without being taller?
     
  2. looks like coilcraft msd1583 is a bit bigger and only 0.6mm taller

    http://www.coilcraft.com/msd1583.cfm

    try put in some numbers: http://www.coilcraft.com/apps/loss_coupled/loss_1.cfm

    -Lasse
     
  3. Joerg

    Joerg Guest


    Thanks, got to order some of those tomorrow. If we can get them into the
    layout.
    Those simple calcs aren't useful. For example, they do not take voltage
    swing across windings into account and that is a major factor in core
    loss. I just got 130C temperature rise :)
     
  4. Bill Sloman

    Bill Sloman Guest

    Nickel/Zinc ferrites have low core losses at high frequencies. You could buy them as RM cores when I last looked, which is quite a while ago. Farnell now stocks just three toroids which I can easily identify as Ni/Zn ferrites, but more specialised suppliers may do better.

    This still leaves you with getting the cores wound by that almost extinct species, the low volume coil winder, but I'm sure that if you had to find one you could manage it.
     
  5. Isat is 6A, L goes down 30%.

    You sure its not copper losses?

    I'm a skeptic of your assumtion of core loss ;)

    Cheers
     
  6. Joerg

    Joerg Guest

    For the 10uH coil it's 11.2A.

    Yes, that is also what the Bourns engineer said when I told him the
    story and asked for core loss data.

    Tomorrow afternoon we'll know more :)
     
  7. Have you tried higher frequency? (and possibly lower inductance at the same time) Core loss increases with frequency (f^m), but if all other is kept the same, the flux density is lowered and resulting losses are reduced (B^n). n is normally higher than m

    You could try reducing the inductance at the same time to reduce RDC losses

    Cheers

    Klaus
     
  8. Download the Epcos tool and try different operating points:

    http://www.epcos.com/web/generator/Web/Sections/DesignSupport/Tools/Ferrites/Page,locale=en.html

    Cheers

    Klaus
     
  9. Joerg

    Joerg Guest

    Yes, we've tried higher frequency. Then the inductor almost began
    unsoldering itself. On the FLIR image it looked like a nuclar meltdown.
    But we did order one inductor with less inductance, just in case, to try
    running the thing faster.
    Thanks, I'll try that. Except I don't know what ferrite the various mfgs
    use and they won't tell me.
     
  10. Joerg

    Joerg Guest

    Unfortunately not. It has to work with several input voltages and the
    output must be controllable from 0V to 80V.

    Yeah, I could really use the "first bunch of volts for free" here,
    because bunch would mean 20-30V. But, can't have it.

    About 0.400" would be the limit. Area is really tough, the board looks
    like a subway in Tokyo during rush hour. Lasse suggested a bigger one
    but if I tell my layouter he is probably going to faint.

    I was really disappointed that this core didn't perform better.

    That's one reason I don't buy books much anymore. The real tricky stuff
    often isn't in there.
     
  11. Joerg

    Joerg Guest

    Copper is really not problem at all, we could easily live with 200mOhms
    here. It's the core loss that literally cooks them.

    One problem with ferrite is that it generally doesn't lend itself to be
    heatsinked.
     
  12. Joerg

    Joerg Guest

    This board is a very dense one with several power converters on it. So I
    made it have a good "heat-sucking" core and a cooling strip on the side
    which is at the same time the mounting strip. Tons of vias. And this
    inductor is located right at the thermal strip for cooling reasons.
    Epoxy is one option but I am thinking more about thermally conductive
    pliable plastics to sink heat away by pressing agains a metal surface.
    Can't remember the name, have to dig it out.

    However, first I want to push these losses down. Because burning several
    watts in the inductor isn't a good thing in the first place, it dings us
    on efficiency. Plus heat sinking has its limits if the enclosure is
    small. The whole box will eventually become a hot-plate.
     
  13. Joerg

    Joerg Guest

    Well, now we know it is core loss. We have received the shipment with
    higher inductance, which of course comes at the price of higher DC
    resistance and higher copper losses. Result -> lower temperatures. We'll
    have to see where the sweet spot is and that can only be found out
    experimentally because no reliable data is available about core loss.
     
  14. Joerg

    Joerg Guest

    Indeed, didn't know Laird had it:

    http://www.digikey.com/product-detail/en/A16104-20/926-1141-ND/2445451

    Kinda pricey though.

    Nice. Are you going to solder it lead-free? I try to avoid that wherever
    I can.

    That's just about how I did it on our board, except that we use bigger
    mounting screws because ours goes into rough environments.
     
  15. Bill Sloman

    Bill Sloman Guest

    Another argument for finding your own ferrite core and winding your own coils - or in this day and age, buying a core pair intended for use with printed windings, and getting your own windings printed, though probably not on the same circuit board as you use for the rest of the circuit, since getting a decent copper fill factor depends on putting very thick layers of copper on very thin substrates in a multilayer board.
     
  16. Tim Williams

    Tim Williams Guest

    A noble idea, but planar magnetics take up an awful lot of board area.

    I suppose if one were to crank up the board layers, components could be
    placed on the outer surfaces while routing the windings through inner
    layers between. Keep low voltages away (current sense and the like).
    Might be tolerable that way, but still, lots of area, and copper losses
    are intrinsically worse due to the poorer fill factor.

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://seventransistorlabs.com

    Another argument for finding your own ferrite core and winding your own
    coils - or in this day and age, buying a core pair intended for use with
    printed windings, and getting your own windings printed, though probably
    not on the same circuit board as you use for the rest of the circuit,
    since getting a decent copper fill factor depends on putting very thick
    layers of copper on very thin substrates in a multilayer board.
     
  17. Bill Sloman

    Bill Sloman Guest

    Which is why I explicitly rejected putting the winding on the same board asthe rest of the circuit.
    The fill factor for round copper wires isn't 100% - closer to 70% IIRR - and some board manufacturers can put down copper layers that are thicker thanthan board material, so you can probably get pretty close to round wire copper fill factors, and - in theory - might even be able to do a bit better.But it would take a specialist printed circuit board manufacturer to get you there.
     
  18. Guest


    Try the Murata A series

    http://www.digikey.com/product-detail/en/60A103C/811-2450-2-ND/3178904

    The graph shows about 15C above ambient for 7A and 10uH.
     
  19. Joerg

    Joerg Guest

    I can't, I need a dual inductor.


    They all show that kind of stuff but it is deceiving. It does not take
    into account core losses and those are the lion's share in many switcher
    designs.
     
  20. Joerg

    Joerg Guest

    Not really. There is a lot of hand-waving, inaccurate information,
    missing data. With inductors it usually already ends at the telephone,
    at the point where you ask which core material they use. Almost like
    asking Coca Cola about the secret sauces and magic potions in their
    concentrate.

    That secrecy is the reason why I never really looked into simulation
    software for core loss. If you don't know where they get the cores it
    wouldn't make much sense.
     
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