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Old 16th July 2013, 08:53 AM   #2061
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thanks Frank.

Symmetry's a good sign. Ooh, hyperbola. Air gap is a useful addition. First impression of hysteresis is not especially encouraging because of the need to specify a constant operating frequency. OK for approximations to mains transformers, but no good for audio transformers where frequency varies. Also, because it's a curve-fitting exercise, it's hard to estimate the error compared to the real thing. Worth ripping though. I'll look at how LTSPICE implements it and incorporate it if it's not too hard. It'll take some time though.
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Old 16th July 2013, 11:46 AM   #2062
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Originally Posted by PlasticIsGood View Post
It would be very nice if I could get a copy of Chan's paper without joining Yahoo. Otherwise I'll have to wait until I go to the library to download it. If it is just a reworking of Hymowitz, it may not be worth using.

This paper:

http://www.onsemi.com/pub_link/Collateral/AN1679-D.PDF

was intended for modelling transformers in switch-mode power supplies. Further, its main interest is leakage inductance. An SMPS transformer typically has windings of very low resistance. Leakage is much more important. They work at high frequencies, and core characteristics are very different from mains transformers.

Early in the ramble, the author puts the primary resistance in series with the primary, but later switches it to parallel with no explanation of why, AFAICS. He gets the behaviour near enough, perhaps, because the regulation due to resistance is negligible, and the relationship between Cp, Ll, and Rp is maintained, so filter characteristics may be correct or near enough.

As it stands, it is not appropriate for modelling a mains transformer, where for nearly all purposes leakage is insignificant and resistance is important. It has very limited value as a design tool even for SMPS, because flux is crucial there and it isn't included.

I have to admit I regret binning my model. It's much better and works for everything because it simply arranges the transformer equations into a computational model. If other SPICE engines can do recursion like mine can (I was surprised and amazed, just trying it for amusement) then I recommend it. It allows flux to be graphed easily, so you can see if it's too great even if you can't see the consequences because the BH curve is linear.

The tan function used in other models for the BH curve can be easily adopted, and I dare say Chan's work could be built in too. I doubt either approximation would be useful, however, outside of narrow limits. We are interested in the details of distortion, and a generalised approximation is of no value IMHO.

I binned it because simulation has very little value in designing a linear power supply. I prefer to copy best practice and tweak to suit.
I remember discussions in this thread where it was easily determined that the leakage inductance effects were much more important than the resistance, for power transformers, particularly in terms of regulation.
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