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I inherited a choke of an odd design, with two windings on the same core. Separately, each winding was about 30 uH. In series, they were about 90 uH. When used in a split-bus boost converter, one winding was in the positive leg, and the other in the negative leg.

schematic

simulate this circuit – Schematic created using CircuitLab

When connected with matching polarity (fluxes additive in the core), the choke worked fine. When connected with opposite polarity (fluxes subtractive in the core), the inductance dropped, and the core got extremely hot. Running comparable currents, the core went from 130C to 250C. just with that change. The windings aren't getting that hot, it's definitely the core.

My understanding of core losses is limited, but this doesn't fit with even the little I know. If two identical windings of opposite polarity are on the same core, seeing the same currents, the flux density in the core should cancel out to zero. So where's the heating coming from?

Edit: By request, I've added a photo of the choke. enter image description here Two U-cores with a gap between. The upper part of the top U-core is blocked from view by a piece of cardboard. The two windings on the right are in series, and the two on the left are in series, so we treat the right side together as one choke (L1), and the left side together as the second choke (L2). The hottest observed part of the choke when the windings are run inverse from each other is in the middle of the exposed part of the lower U-core. I suspect the upper U-core is equally hot, I just can't measure it as easily.

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Stephen Collings
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  • Can you add a schematic? – Phil Frost Aug 19 '13 at 13:31
  • @PhilFrost Done. – Stephen Collings Aug 19 '13 at 13:37
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    I agree with your final paragraph, so the problem really is how have you managed to misinterpret the way the windings are? – Andy aka Aug 19 '13 at 16:02
  • Do you propose any possible misinterpretations? I mean, if core geometry or winding structure matters in some way, I'd love to learn how. But the windings are either in-phase or out-of-phase... – Stephen Collings Aug 19 '13 at 16:36
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    Maybe the heat isn't from core saturation, but rather resistive heating in the windings under normal operation? I'm not really familiar enough to be of much help here, but you are right that the magnetic fields should cancel, so there's a wrong assumption here somewhere. Maybe the fields actually aren't canceling, or the heat is being generated by some other mechanism... – Phil Frost Aug 19 '13 at 17:24
  • The outside of the windings was well under 100C, and the core seemed to get hotter the further I measured from the windings. How about... eddy currents in the core? I have a very vague idea of those in my head, so I have no idea if that's a sane idea or not. Perhaps someone more informed can clarify? – Stephen Collings Aug 19 '13 at 18:35
  • If the core is ferrite, eddy losses are unlikely. – Adam Lawrence Aug 19 '13 at 19:27
  • Pretty sure it's very thin laminations of amorphous steel. – Stephen Collings Aug 19 '13 at 19:44
  • I'm not familiar with this "split-bus boost converter" architecture - can it be that the currents in the inductors are not exactly out of phase (not 180 degrees)? – Vasiliy Aug 27 '13 at 16:17
  • I'm sure they're not perfect, nothing is. But I'd say they're exact within the limits of what I can measure. Would it matter much if they were, oh, 20 nS off per cycle at 4 kHz? – Stephen Collings Aug 27 '13 at 17:53
  • How are the coils and core arranged? – madrivereric Sep 09 '13 at 00:22
  • @madrivereric I've added a picture to demonstrate. – Stephen Collings Sep 09 '13 at 13:08
  • Do the transistors switch in opposition i.e. when one is on the other is off. Or do they switch together. – Andy aka Sep 09 '13 at 13:20
  • @Andyaka They switch together. – Stephen Collings Sep 09 '13 at 13:21
  • The total flux would be zero if the currents are of equal magnitude but opposite sign, but the flux density would not be zero everywhere, as the current is not run in the exact same place. – Gunnish Sep 10 '13 at 10:03
  • Can you add side shots that makes the connection details more clear? (both from the left or right and both top/bottom as viewed above) I suspect the actual connections or polarity are different than you think. If the flux adds, the core will saturate and you'll get high core losses. If the flux cancels, no saturation and a higher inductance... – madrivereric Sep 10 '13 at 14:24
  • @madrivereric I ran it both directions. Additive flux gave me higher inductance. Canceling flux, lower inductance and much much higher core losses. – Stephen Collings Sep 10 '13 at 14:46

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Your assumption that the core flux cancels out to zero is not valid because you don't have perfect coupling between L1 and L2. If you had, then the inductance for the series-additive connected inductor should measure 120 (not 90) uH, because inductance is proportional to the square of the number of turns, and you're essentially doubling the number of turns. Likewise, you would measure no inductance for the series-opposing connection. Note that even if there was no coupling you would still measure 60 uH for the series-additive pair, so it seems there is a fair amount of leakage flux.

In the opposing flux configuration, the effect on the circuit is similar to replacing the coupled inductor with two separate inductors having much lower inductance values. As a result, the current ripple is higher, and the associated core losses are higher.

user28910
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  • If I understand you correctly, you're saying that my remaining inductance isn't due to flux in the core, but flux outside the core. Correct? But if the flux is outside the core, why is the core getting hot? – Stephen Collings Sep 11 '13 at 19:10
  • The term _leakage flux_ is used to distinguish flux that links only one of the windings from the mutual flux which links both. I didn't mean to imply that it is entirely outside the core. Typically, this flux is partly inside the core (especially under the windings) and partly outside. – user28910 Sep 12 '13 at 14:20
  • Ahha! So I'm misunderstanding the concept of leakage flux! – Stephen Collings Sep 12 '13 at 14:27