Path: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: squeezing a field
Date: Wed, 23 Oct 2024 11:33:23 -0700
Organization: A noiseless patient Spider
Lines: 38
Message-ID: <u4gihjdgcgi1nc60s64g7pts22fm2h303m@4ax.com>
References: <kibihjlu14m022op6g2lr5hmkso5231p8n@4ax.com> <vfbef1$254rl$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
Injection-Date: Wed, 23 Oct 2024 20:31:44 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="7c9cdd419075a565072e82c6114734de";
	logging-data="2272853"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18DfdRHLmGJ4A46aTtqM2Ze"
User-Agent: ForteAgent/8.00.32.1272
Cancel-Lock: sha1:2lQ7/285uq4Mbt7HECYX7DhoVeQ=
Bytes: 2274

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.