Timeline for Why does this metallic magnetic core have so little core losses up until 100s of kHz?
Current License: CC BY-SA 4.0
9 events
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| Jan 6, 2023 at 13:35 | comment | added | John Birckhead | I agree that the permeability is unaffected. As the tape becomes thinner, the frequency increases but at a cost of maximum flux density as the insulation becomes a larger percentage of the total cross section. | |
| Jan 6, 2023 at 11:29 | comment | added | Tim Williams | @tobalt Consider permeability as a function of frequency (as the mu', mu'' plot above): eddy currents shift the curve leftwards. Hence, acting as a shorted turn. Note that mu ~ 1/f is equivalent to Z ~ constant, and flat impedance has a phase of 0 degrees i.e. is all resistance. At signal frequencies where we'd like to use it, the impedance is reduced and made more resistive, hence, "shorted turn". Mind, it might be a very "thin" shorted turn -- but that's just a matter of how much resistance it presents! | |
| Jan 6, 2023 at 7:16 | comment | added | tobalt | @TimWilliams could you elaborate on that shorted turn argument ? I dont get it. the DC (LF) permeance of the core is unaffected if the type is insulated or not. What is affected is the AC losses because of eddy currents. IMO the real permeability (µ') shouldn't be affected either. | |
| Jan 6, 2023 at 2:34 | comment | added | Tim Williams | As I understand it, coatings include surface oxide (more so with traditional transformer iron), varnish or resin, or maybe not much at all. Note that contact resistance between layers would reduce the overall permeability (it manifests as a shorted turn in parallel with the magnetizing current) -- so CMCs are also best with insulation here. | |
| Jan 5, 2023 at 18:17 | comment | added | hobbs | @tobalt given the heading "Common-mode chokes and tape-wound cores" in that PDF you found, I'm guessing not :) | |
| Jan 5, 2023 at 15:43 | vote | accept | tobalt | ||
| Jan 5, 2023 at 14:53 | comment | added | John Birckhead | I sure don't! Tape-wound cores were more prevalent in the days when switched-mode power supplies all operated below 100 kHz, and circuit design has progressed to where ferrites have supplanted them in many applications. The entire idea of the tape-wound approach is to reduce losses, so I don't think you would use this approach for an intentionally lossy core. | |
| Jan 5, 2023 at 14:43 | comment | added | tobalt | Aha! I found no mention anywhere that these cores used an insulating layer. Would surely make sense for a signal transformer core. On the other hand an attenuating choke, such as a CMC, would probably do better without the insulating tape. Do you know if the insulating tape layer is always used ? Seems a bit hard to tell from the outside. | |
| Jan 5, 2023 at 14:33 | history | answered | John Birckhead | CC BY-SA 4.0 |