Re: squeezing a field

On 24/10/2024 5:33 am, john larkin wrote:

On Wed, 23 Oct 2024 18:14:57 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

john larkin <jl@glen--canyon.com> wrote:

I'm designing a small PCB with essentially 5 sync buck switching
regulators. Board space is tight so I want to put the inductors on the
bottom of the multilayer board. There's a 0.2" gap between the bottom
of the board and a big aluminum flange.
>
Unshielded drum cores have the most energy storage per volume or
dollars. They store energy in the universe instead of in ferrite. Good
cooling too.
>
Something like this just fits
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https://www.digikey.com/en/products/detail/bourns-inc/SRN8040TA-470M/6155133
>
Its mag field lines will bounce off the PCB planes and the flange,
change from the classic bar magnet pattern into a pancake . I wonder
what that will do to its electrical behavior.
>
>
>

>
Not a whole lot.  In the near-field region, B obeys Laplace’s equation,
which means among other things that the field falls off on the length scale
of the gap, not of the whole inductor.
>
Cheers
>
Phil Hobbs

 You are right. A similar part is 47 uH in free air, 44.6 mounted on a
multilayer board, and 42.1 squeezed between the board and a big chunk
of aluminum.
 So it will work.

Somebody who knew what they were doing could model it in LTSpice. The adjacent metal-work is a poorly coupled shorted turn. Model your inductor as 47uH coil with 0.135 series resistance and 4pF of parallel capacitance, and model the metal as a coupled - perhaps 1nH single turn - with perhaps 1% coupling and maybe a milliohm of resistance. The difference between your two test situations is going to be mainly the coupling you get - though the loop resistance of the shorted turn is going to be lower in the second case.
Assuming that it will work might be a touch optimistic.
--
Bill Sloman, Sydney