Electronics > Beginners

Magnetic Permeability

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ramonest:

--- Quote from: T3sl4co1l on June 21, 2019, 11:14:49 pm ---Simple: which one do you want?

If you take the ratio of instantaneous B and H, you have the average permeability.

If you take the ratio of a small difference in B and in H, you have the incremental permeability (or "marginal" permeability, as accountants would call it; or in calculus, for difference-->0, the slope or derivative).

You would use the incremental version for small-signal purposes (like signal filters), and the average version for large-signal purposes (like power transformers). :-+

Tim

--- End quote ---

Thanks for your answer Tim.
Consider the case where the inductor has a constant field (DC component) that brings the magnetic material to a nonlinear point of the BH curve (near the "elbow" of the curve) plus a small AC component (ripple current). That could be the case of a power inductor on a switching converter in CCM with low ripple. In such case the most accurate way to predict the inductance on that operating point would be to sue the mu (permeability) corresponding to dB/dH near the operating point set by the DC component?

schmitt trigger:
That is correct.
Many inductor manufacturers will actually provide you with a graph plotting the with inductance drop vs. DC bias.

For instance, check the graph on page 2 of the attached document

ramonest:

--- Quote from: ivaylo on June 22, 2019, 06:08:29 am ---The best 50 min you can spend on the subject - https://youtu.be/4UFKl9fULkA

--- End quote ---

I saw the video a while a ago. Thanks for bringing it tho, it would be interesting to replicate the experiment he does at the end of the video! I'll try to do so.

ramonest:

--- Quote from: schmitt trigger on June 24, 2019, 09:39:22 pm ---That is correct.
Many inductor manufacturers will actually provide you with a graph plotting the with inductance drop vs. DC bias.

--- End quote ---

The problem I had is that regardless of the definition (mu=B/H vs mu=dB/dH) inductance drops.
I attached an example with  calculations made using the BH curve of the original post, it's vary rough specially for the dB/dH case (it's done fast with excel) but still you can see the idea. Both drop, mu=dB/dH just faster than the other case.
It's true that I haven't checked any manufacturer and their L vs I curves tho... I should have done that and maybe I would be able to see the answer there too.

TimFox:
Looking at the “magnetization curve” (ignoring hysteresis) for a ferromagnetic material, the initial permeability is lower than the maximum permeability.  Usually, the material specification gives the initial mu (at B = 0), the maximum mu and the B where that value obtains, and the B value at saturation.

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