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LTspice question...Inductifiers

Discussion in 'Electronic Design' started by Martin Griffith, Oct 13, 2007.

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  1. Just mucking around with a LT1533 low noise smps simulation, and I
    realised how little I know (what's new?) about spice, and it's too
    damn windy to walk up the hill for a beer (spanish, yuck)

    Each inductor has a value called Parallel Resistance, and I really
    don't have a clue how to give it a nominal value/guess

    Any rules of thumb?
    Nominal L's are from 470u to 8mH


    Martin
     
  2. D from BC

    D from BC Guest

    I've been wondering about this too.

    I did some reading on...
    http://www.intusoft.com/articles/inductor.pdf

    "
    The magnetic loss can be modelled reasonably well as a
    parallel resistor (Rp) across the existing model. The value
    can again be calculated from data sheet parameters, using
    the quality factor (Q). In a parallel RLC3 circuit the relationship
    between the quality factor and inductance is given
    by; Q = Rp/2piFoLo "

    So...I suppose.. Since V^2/Rp = power then
    Hot core from losses ..low Rp
    Cool core from gentle use...high Rp

    I think Rp generally drops in value with increasing B swing or
    increasing frequency.
    It's gonna be core material dependent too..

    Come to think of it...I think Rp is fake in spice.. It's not really
    there.. I think it's just included to explain where the power is
    disappearing.

    But I'm just learning and thinking about this now..


    D from BC
     
  3. Ha
    I'm just going to send the ltspice file to my tranny maker/guru and
    ask him to guestimate.


    Martin
     
  4. James Arthur

    James Arthur Guest

    Martin, if you're just poking around you don't have to enter any value
    at all--the simulation will just run with a 'perfect' inductor.

    In real life, a decent guess is to omit the parallel value, and assume
    a series resistance for the inductor such that (iL.average)^2 * R =
    2-3% of total power. With equal copper and core losses, that's about
    5% of efficiency loss budgeted for the inductor. Adjust per
    application.

    HTH,
    James Arthur
     
  5. great stuff, LTspice is excellent, but when doing a transformer,there
    are too many thingummies to make sense of, unfortuantely LTspice on
    yahoo groups is a bit like snowboarding through mud


    Martin
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    Won't this cause a problem in some applications? A series resistor will have
    a DC drop, whereas a parallel will not. If you know the Q, you can make the
    parallel resistance 2*PI*f*L*Q, or a series resistor of 2*PI*f*L/Q; or,
    split the R between series and parallel. For an air core, you can make the
    Rp=10E10. A stab in the dark guess for inductors wound with decent size wire
    might be Q=100 for the 470u and Q=30 for the 8mH.

    Tam
     
  7. Winfield

    Winfield Guest

    I've had good results simulating my transformers in spice.
    Most of of time I design them to avoid core saturation, or
    even excessive core losses, if I can. I also like to see
    the important transformer parameters right on my circuit,
    so I use a perfect transformer with its turns ratio, add a
    parallel input inductor for the core magnetizing inductance
    Lm = AL N^2, where you get AL from the core's datasheet.
    Getting the right value for Lm usually isn't very important.

    Then, very important, I add input and output resistors for
    the two copper winding resistances (these do a better job of
    dealing with the high peak currents from rectifiers driving
    storage caps, etc., than a general parallel loss parameter),
    an ohmmeter is handy for this.

    Series leakage inductance is important. This can be measured
    (while shorting the other side), or it can be calculated (see
    posts by Tony Williams) or you can try Lell = 1% or 2% of Lm
    as a starting estimate. You only add the measured leakage
    inductance once, on either the primary or secondary side - its
    value includes both sides. If you have multiple secondaries,
    measure Lell between them and place half that on each one,
    then measure the primary and subtract the turns-ratio-squared-
    adjusted value from the secondary side you had shorted.

    This overall scheme gives spice results that are pretty close
    to the bench results, unless you're at high frequencies and
    high ac voltages, where core loss is an issue. You can try
    estimating the power loss from core-material curves, and add
    a parallel resistor to implement it. But generally I'm less
    concerned with an accurate primary power consumption estimate
    than I am with knowing an accurate secondary waveform result.
     
  8. Winfield

    Winfield Guest

    Let me hasten to add, my statement was for forward converters,
    or for push-pull, etc, as used with the LT1533. OTOH, flyback
    converters, etc., depend rigorously upon the Lm value, with the
    core's air gap closely determining AL.
     
  9. JosephKK

    JosephKK Guest

    D from BC posted to sci.electronics.design:
    That is not too bad D. It shows a willingness to do a little work to
    learn some more.
     
  10. Fred Bloggs

    Fred Bloggs Guest

    For someone who makes sweeping generalizations like RC-timers and
    discrete logic is for newbies, you sure don't know a heck of whole lot
    about electronics...
     
  11. JosephKK

    JosephKK Guest

    Winfield posted to sci.electronics.design:
    If you do your measurements correctly, you can derive the parallel
    resistance like components of core losses this way. And by extension
    for any specific core with similar windings by scaling as an
    approximation.
     
  12. Can you add me to that list :)


    Martin
     
  13. Pleased tell us more. The core loss goes as something
    like f^1.8 and B^2.5, etc., so you'd have to test at
    the right frequency and amplitude, or be confidant of
    the scaling factors. Is there an easy trick?
     
  14. JosephKK

    JosephKK Guest

    Winfield Hill posted to sci.electronics.design:
    Not in the least, i specified the same core with similar windings. I
    clarify, not more than 50 percent difference in turns, peak and
    average ampere turns, and volt-seconds, and maximum MMF (assuming
    that the difference does not approach core saturation and fairly
    linear cores). The idea is that it is measurable and that the
    measurements can be used in SPICE simulations.

    Almost sorry to have to back off so much, but it is useful for single
    frequency of operation designs.

    Alas, this poor butterfly is pinned.
     
  15. James Arthur

    James Arthur Guest

    I don't see how it would cause problems. Real inductors really do
    have series resistances, and they have an honest d.c. drop if there's
    a d.c. flow. Putting in a realistic series resistance, then, makes
    the simulation more realistic (and grounded more closely on actual
    physical parameters).

    Real inductors do not have parallel resistances carrying any
    meaningful current. Rp, as used in Q and filter calculations, is a
    mathematical convenience to model certain losses. It *is* convenient,
    especially for low-level signals where the core isn't being pushed,
    but it's not a real resistor.
    That's handy stuff for RF filter work. For my occasional switch-mode
    power supply work, cores are loss so variable with load and frequency--
    and small to begin with, by careful choice of core and windings--that
    they aren't that critical. I usually just estimate, and don't bother
    simulating them. If I'm really interested, I measure (in real life).

    Cheers,
    James Arthur
     
  16. colin

    colin Guest

    tbh I usually just use series resistance.
    typically taken from available data sheets of suitable sized inductors.

    so might be anything from say 0.1 ohms to 20 ohms.

    theres usually a defualt series resistance, of some milliohms,
    it often goes crazy if this is zero.

    Colin =^.^=
     
  17. Hmm, it should be pointed out that the actual
    ac resistance is higher than the dc resistance,
    sometimes considerably, due to proximity effect.
     
  18. Phil Hobbs

    Phil Hobbs Guest

    This is pretty well quantified in a series of old NBS Circulars from the
    1920s...I have C22, which is sort of a summary of the more detailed
    studies.

    Cheers,

    Phil Hobbs
     
  19. I don't have any proximity-effect papers from that era,
    although I know a lot of work was done on inductance
    and it's subtleties at the NBS, as they developed the
    theory for and created various inductance standards,
    etc.,. and I have copies of some mid-30's stuff.

    I wonder if you could scan that gentle beast for us?
     
  20. Phil Hobbs

    Phil Hobbs Guest

    It's about 300 pages long, and now quite fragile because it's printed on
    toilet paper and bound with staples like raffle tickets--from the front
    cover through the whole thickness to the back cover. They decently
    covered this offence with tape. I'll see if I can make a couple of
    readable digital photos of the curves. IIRC they talk in terms of
    frequency dependent resistance for various winding styles, but don't
    specify how it arises.

    Cheers,

    Phil Hobbs
     
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