Thomas, thanks much for the info.  I don't have an Amadon
catalog, but based on your direction, I did a LOT of web surfing
and came up with lots of scattered info that has somehow got me
some answers.  Here's what I came up with, and hoping someone
wouldn't mind checking this for me before I go order some parts...

NOTE: I made a small mistake on the Input inductance.  It should
    be as follows:
    - Input = 23.38 uH, 31.35 mOhms  (Instead of 29.38 uH)
    - Output 1 = 0.75 ratio, 54.49 mOhms
    - Output 2 = 0.23 ratio, 7.64 mOhms
    - Output 3 = 0.75 ratio, 140 mOhms

NOTE 2:  I know nothing, nothing, nothing about inductors and
    transformers, so don't take anything I say below as fact...


CORE MATERIAL:
    For 100 kHz, it seems that T-xxx-3 or FT-xxx-43 is best.

WIRE DIA:
    I did get some conflicting info here, but I decided to
    use 18 AWG wire for the primary, which should be a bit
    overkill.

PRIMARY TURNS:
    Using different core sizes from xxx=50 and upwards, I
    calculated the # of turns on the primary until the
    # of turns fit in the core (as per a table).  What
    works is T106-3, which gets me 22.79 (say 23) turns of
    18AWG wire for the primary.  Table says 35 turns max.

PRIMARY DC RESISTANCE:
    Could not find any tables for this, but from tables
    showing the toroid measurements, the fact that the
    height is different the the OD-ID, tells me that the
    core cross-section is not circular.  I'll use the worst
    case for now and assume it's rectangular.  Length of
    wire for a full turn =
            2 x ((OD - ID + wire thickness)
                 + (core height + wire thickness))
          = 2 x ((1.06 - 0.57 + .0403) + (.44 + .0403))
          = 2.02 inches per turn.
    23 turns ==> 46.5 inches of 18 AWG wire ==> 25.2 mOhms
    So it seems I'm well within spec here.

REACTANCE:
    Thomas, I'm a bit confused by what you mean by:
       "the smallest ratio of reactance to resistance is
        1:4 so the driving generator must be 4.6 ohms or
        lower.  (Lower is better.) If the generator is 5
        ohms, it generates 10 volts @ 2 amps."

    I can see how you calculated the reactance to resistance
    ratios, but why?  And why should the generator be 5 ohms
    or less?  BTW, primary reactance is now 14.69 ohms due
    to the now-corrected primary inductance.

    The application is a DC-DC converter, and the generator
    is an internal hi-current switch within the converter.


I'll stop here for now, in case I've missed something
fundamental.

Thanks much,
-Neil.





Thomas C. Sefranek wrote:
>
> You need to select the material of the core to react at your frequency.
> You then pick the size of the core to handle the flux density.
> The primary reactance is 18.46 ohms, and the smallest ratio of reactance
> to resistance is 1:4 so the driving generator must be 4.6 ohms or lower.
> (Lower is better.)
> If the generator is 5 ohms, it generates 10 volts @ 2 amps.
>
> The Amadon catalog has the formulas for uH per turn per core.
> It also has how to calculate Gauss. (AC and DC)
> It also has tables for how much wire you can fit in a core,
> the wire tables tell you how much parasitic resistance in the wire used.
>
> Once you have correctly designed the primary,
> the secondary is just winding ratios with different size wire.
>

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