And for the same reason you read it from the datasheet of the core, not measure. One can also resort to FEM sims, but I believe it is pretty rare outside of academia.
Best regards, Piotr
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3 years ago
Flux density
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3 years ago
In my experience, l_e and A_e are very close to the expected mechanical dimensions -- i.e., cross section of the wound limb(s), mean circumference of expected path. I don't think that's necessary, and is in part a consequence of conventional shapes being well behaved -- compact, symmetrical, optimized for cost and performance.
Also, v_e ~= l_e * A_e, which I'm not sure has to necessarily be true. (There could be vestigial core features that don't magnetize, so the core volume is greater than the active volume; but then, it's _effective_ volume, so that wouldn't be counted anyway?).
And when you bring nonlinearity into things... As magnetization rises: mu_eff falls, A_e rises some (fringing fields), l_e rises some (because the inside track saturates first, especially inside corners, pushing the active volume outwards).
The changes in mu_eff and A_e partially oppose, so it's not immediately obvious how to separate them; since they're both effective parameters, we might just assume one or the other remains constant instead, and measure the other as the combination.
These are hopefully effects we can ignore... which for power application, yep, no problem. For signals, well obviously you want to keep the magnetization low to avoid distortion, frequency shift, etc. Some airgap helps ballast changes in core mu, which would otherwise be rather sensitive (not to mention, to temperature as well as signal level).
Tim
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