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FEA modelling of Litz wire

Discussion in 'Electronic Design' started by Spehro Pefhany, Feb 3, 2011.

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  1. Anyone done FEA modelling of power magnetics incorporating litz wire?

    Creating a physical model of the individual strands in a 3D winding
    looks a little challenging..
     
  2. legg

    legg Guest

    You might look here first:

    http://engineering.dartmouth.edu/inductor/papers.shtml

    http://engineering.dartmouth.edu/inductor/programs.shtml

    They use piece-wise linear analysis and develop physical models for
    litz wire. There's a matlab app as well, that might be useful.

    FEA modeling of mixed materials has always looked like a head-ache to
    me. Takes a while to figure out that the CAD package can't do it, too.

    RL
     
  3. The cores are custom ferrite shapes- iterations are a bit slow and a
    bit expensive.
     
  4. legg

    legg Guest

    They already exist, or are you forced to specify a new shape?

    Never had much trouble finding core suitable core shapes, but never
    had much luck convincing mfrs to produce them in 'outside of the
    application' material. Complex shapes can also be prototyped from
    simpler structures.

    FEA modeling with sheet/strands should tell you a lot about non-litz
    window fills - at least enough to know whether litz is potentially
    justified. Is this just a layering issue or a predictably troublesome
    core shape?

    RL
     
  5. It's all special/custom, but at least standard materials.
    It's amazing (to me) what can be done when unit cost is not very
    important.
    I'm hoping we'll be close enough, then just do an a/b comparison with
    the litz in a prototype.
     
  6. CNC
     
  7. legg

    legg Guest

    But they know what they want........
    Meaning the sky's the limit, or that they can make anything needed,
    for peanuts, in volume?
     
  8. Sure, it should work reliably. ;-)
    Meaning, if I don't care too much about unit cost (unfamiliar
    concept), they can make whatever I can imagine (almost).
     
  9. legg

    legg Guest

    Well, then. I would advise you to do a simple optimum volts per turn
    at frequency sweep, based on the power loss budget, before fiddling
    with winding structures. For any isolated topology with leakage
    limitations to power transfer duty cycle, there will be an optimum
    frequency range for the core volume, or an optimum volume range for a
    spot frequency.

    Ferrite core designers seem to have forgotten that most parts,
    nowadays, are fixed with a close-to-unity turns, resulting in a
    situation that does not permit physical interpolation of core loss
    data in increments of less than 1/2 order of magnitude in a regulated
    system.

    RL
     
  10. What's the best way to deal with a non-negotiable large amount of
    leakage inductance?
     
  11. legg

    legg Guest

    Either resonant, or clamped recovery, preferably with recovery
    directly into the intended load.

    It's not non-negotiable if you have control over the magnetic
    structure.

    RL
     

  12. What FEA are you trying to resolve? Thermals? Mechanical stresses?

    Or are you talking about electrical characterization of such items?
    THAT is NOT FEA.
     
  13. Truly, the only way to properly "select" an inductor or transformer is
    to experiment and characterize in the lab. I spent two years going over
    a power supply for the med industry where a single turn more OR a single
    turn less than the final number made a huge difference.. THEN simply
    using Litz improved it further.

    Rather than trying to model Litz, one should use standard models for
    ordinary transformation. THEN use different Litz configurations and
    FIND the best one for that form factor xformer and operating frequency.

    There will ALWAYS be an improvement. THAT is all one needs to count
    on. Put numbers to the standard, and KNOW that you will get better
    performance from the change to Litz (even simple bi or tri filar yields
    results). The inductor was the same way. The result is/was not merely
    better performance, but the unit ran down to a lower drop out point on
    the line side as a result as well. So it would work all the way down to
    3.5 Volts when it used to drop out at 6 Volts.
     
  14. Once you have had one batch made, subsequent batches cost far less.
     
  15. legg

    legg Guest

    You mean a BIGGER box ?

    Pure heresy.

    RL
     
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