Re: squeezing a field

On 25/10/2024 3:45 am, Lasse Langwadt wrote:

On 10/24/24 12:07, Bill Sloman wrote:

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.
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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.
>
>
>

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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.
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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.

 plugging numbers pulled out of thin air into LTSpice is better that doing the actual measurement?

Obviously not. My point was that John Larkin could use LTSpice to see what his numbers actually meant.
Putting a chunk of metal next to a drum core inductor doesn't change the drum core inductor in any way - it just adds a shorted turn to the assembly, and messes up what a dumb inductance measuring device sees.
In this case, that dumb measuring device is John Larkin, who doesn't want to think about what is actually going on, as opposed to his inductance meter, which can't.
--
Bill Sloman, Sydney