Series SMPS inductors interfere with each other's magnetic fields?

[treez]

We need to use three of these low profile inductors (100uH) in series in order to get the inductance that we need for our 60W offline buckboost converter LED driver. These inductors look very “open”. Will we get problems with the field from one inductor interfering with the field in the core of the adjacent one?

Inductor PA4309.104NLT (100uH) datasheet
https://www.digikey.com/product-det...ics-power/PA4309.104NLT/553-3606-2-ND/5361574

 

[dick_freebird]

There will be a solenoidal exterior field but the air gap
is likely so large that the coupling is pretty low. You
might play with spacing and orientation to drive down
what coupling there might be. Like serpentine might
aid or oppose depending on how you wire between,
etc.

If you need to space them then maybe a larger body
full value inductor is no worse (if procurable)?

 

[CataM]

These inductors look very “open”. Will we get problems with the field from one inductor interfering with the field in the core of the adjacent one?

Why don't just measure the coupling ? It takes less on measuring the coupling than in creating this thread.

From page 16 to 28 there are 3 methods. Pick one.

TO MODERATOR: I do not remember from where I got this one, but I do remember it was a free web site.

 

[KlausST]

Hi,

I assume there will be another - not that obvious - problem.
If you are talking about those smd inductors, then they have a pole at the top and a pole at the bottom.
Thinking with DC current this maybe means north at the top and south at the bottom.
But if it is assembled that both contacts are interchanged, then north is at the bottom.
And I assume during production they don't care about this polarity and they are stired in random polarity in the tape-and-reel packaging.

Then you have 8 different arrangements of the inductors.
NNN, PPP (=opposite)
NPN, PNP
NNP, PPN
NPP, PNN
Those four lines stand for four different interference results.
So, if there is interference, then asembling matters.
How much - I don't know.

Klaus

 

[andre_luis]

With Air-core unshielded inductors you should not be concerned only on mutual coupling, but also with the surrounding devices and tracks of the PCB. I cannot see usage for such components except for very rudimentary applications for which EMI requirements are not so critical or spacing between them is sufficiently far.

 

[treez]

With Air-core unshielded inductors you should not be concerned only on mutual coupling, but also with the surrounding devices and tracks of the PCB. I cannot see usage for such components except for very rudimentary applications for which EMI requirements are not so critical or spacing between them is sufficiently far.

Thanks, i remember hearing them called "dog-bone" inductors. They dont appear to have a closed magnetic circuit. Hence i see your point and believe that they will couple up to each other and as Klaus kindly says, the way they couple up will vary depending on the orientation.
It sounds like we must use shielded inductors, or at least, inductors with a closed magnetic circuit?

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Hee is the datasheet of the PA4309.104NLT (100uH ) inductor
https://products.pulseelex.com/files/product_files/P757.pdf

- - - Updated - - -

I presume these would be much beter for placing next to each other in series?
https://www.bourns.com/docs/Product-Datasheets/SRP1770TA.pdf

 

[andre_luis]

It sounds like we must use shielded inductors, or at least, inductors with a closed magnetic circuit?

This can vary from one application to another depending on project requirements, as for example if overall cost is relevant in a large scale production and if there will have no critical EMI requisites to meet I would not discard the simplest option. In general, when you do not want to deal with intermediate stages of prototyping and performance testing, let's go straight to the most reliable option. Specifically speaking of the options presented from a technical point of view: I have already used open path ferrite inductors that passed certification tests without any problem, but I admit that I have never even considered using air core inductors, but I have always done quite compact designs where magnetic coupling occurs, then you need to compute all factors for the choice, there is no general answer.

 

[FvM]

Will we get problems

Which "problems" do you particularly expect? There will be surely some degree of magnetic coupling, you may want to measure it.

Expect that the respective inductance variation is pretty below 20% component tolerance.

Also expect that a single 330 µH inductor achieves the intended effect with less coast and floor space.