I have a question about putting two power inductors in parallel. It seems like they won’t share the current very well. The reason I say this is that inductors lose inductance as the current increases. So if I put two of them in parallel, then the one with slightly less inductance will get more current, and move to a lower inductance and get still more current and move lower still. Seems like one of the two coils will tend to hog the current due to this fact. Any thoughts? I can't find any articles saying don't do it, but it seems intuitive that they won't share the current well because of this behavior.
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\$\begingroup\$ Not all components "parallelize" well. \$\endgroup\$– Ignacio Vazquez-AbramsCommented Oct 28, 2013 at 19:03
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\$\begingroup\$ I think what you're discounting is there is some sort of steady state which would prevent this "hogging" of the current. \$\endgroup\$– ShababCommented Oct 28, 2013 at 19:51
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\$\begingroup\$ Just checking. Are both of your inductors same model? \$\endgroup\$– Nick AlexeevCommented Oct 28, 2013 at 20:28
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\$\begingroup\$ All components have tolerance associated with them and you could easily get part at different end of the tolerance spectrum. Is it AC or DC ? \$\endgroup\$– user16222Commented Oct 28, 2013 at 21:53
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\$\begingroup\$ "Inductors in parallel won't share current well" but they'll share changes in current well. \$\endgroup\$– Alfred CentauriCommented Oct 29, 2013 at 2:15
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2 Answers
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It depends totally on the core material. Generally, ferrite core materials, as they get warmer, increase their permeability which increases the inductance. Compare 3C90 and 3F4 from ferroxcube: -
Both increase their permeability (and hence inductance) as they warm from -50ºC to about +100ºC. 3C90 then drops a bit before resuming its upward journey until it reaches its curies point at about +200ºC. 3F4 never reduces its inductance until the curie point is reached.
Iron-powder cores may behave differently and there are certainly ferrite cores that exhibit a stronger negative trend in permeability w.r.t. temperature in some areas of their characteristic.
What inductor are you using and what is the circuit in which it is used in?
answered Oct 28, 2013 at 19:22
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\$\begingroup\$ I am worried about inductance decrease with current. As one inductor gets more current, then it decreases it's inductance and gets still more current. Its positive feedback and the imbalance tends to want to increase. \$\endgroup\$ Commented Oct 28, 2013 at 19:38
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\$\begingroup\$ I can see this applying in cases where maybe the self-rise current for the inductor is in excess. But how would this apply when you are applying well-below the self-rise current? \$\endgroup\$– ShababCommented Oct 28, 2013 at 19:42
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\$\begingroup\$ I do see your point that the inductor getting more current will warm up faster. That pushes us back toward balance. \$\endgroup\$ Commented Oct 28, 2013 at 19:42
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\$\begingroup\$ I see I got a downvote on this answer. Let me be clearer; for most ferrites, inductance increases as temperature rises. If the temperature rise is due to an increase in current then if the current is AC then we have negative feedback and the current will stabilize. \$\endgroup\$– Andy akaCommented Oct 28, 2013 at 20:43
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\$\begingroup\$ @user31000 what precisely is the inductor you are using? \$\endgroup\$– Andy akaCommented Oct 28, 2013 at 20:44
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They have to have same characteristics. If they don't, the system will be unbalanced. You can imagine them as two paralel resistors.
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\$\begingroup\$ Resistors don't become less ressitive as current is increased. My inductors do become less inductive however. Seems like a runaway situation where one inductor will hog the current. \$\endgroup\$ Commented Oct 28, 2013 at 19:36
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\$\begingroup\$ @user31000 Your first statement is correct for resistors in their Ohmic region. However, this is not always the case. For example, at large currents filament lamps exceed their Ohmic region to where the V-I curve plateaus and is no longer Ohmic. Also, gas discharge tubes don't behave this way for small currents. Refer to a-levelphysicstutor.com/images/electricity/I-V-graphs.jpg. Main article at a-levelphysicstutor.com/elect-concepts-units.php \$\endgroup\$– ShababCommented Oct 28, 2013 at 20:03
