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From: Lasse Langwadt <llc@fonz.dk>
Newsgroups: sci.electronics.design
Subject: Re: squeezing a field
Date: Thu, 24 Oct 2024 18:45:20 +0200
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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.
>>>>
>>>> Something like this just fits
>>>>
>>>> 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. 

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