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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: squeezing a field
Date: Sat, 26 Oct 2024 19:16:45 -0700
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On Sun, 27 Oct 2024 01:14:45 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

>On 23-10-2024 19:22, john larkin 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.
>> 
>> 
>The proximity of the aluminum is probably close to the effects of having 
>ground plane or not below the inductor.
>
>Steve Sandler has tested this:
>
>https://www.signalintegrityjournal.com/blogs/17-practical-emc/post/2694-dc-dc-converters-solid-return-plane-or-cutouts-under-switch-node-and-inductor
>
>Found very little effect.
>
>Similar test:
>
>https://www.monolithicpower.com/en/learning/resources/when-is-it-beneficial-to-place-a-copper-layer-beneath-dc-dc-power-supplies?srsltid=AfmBOoq_cYcCoGN57iR4TXaq9n4hlYK1VWLt5m6yYGpVBdRF6RF7L7hz
>

They seem mostly concerned with EMI, which isn't a concern for me now.
I just wanted to be sure that the inductor would work in the switching
regulator, sandwiched close between a multilayer PCB and the aluminum
baseplate.

My 48-to-5-volt switcher should be OK. The load current is low so I
can use a lot of microhenries if needed. If I trust my AADE LC-meter,
L drops roughly 20% when the inductor is squeezed between the
conductive things.

The four half-bridge power switchers are more concerning. I think
we'll try to make room for four giant shielded inductors on the parts
side of that section... move other things to the bottom of the board.

Our policy is for PCB layer 2 to be a solid ground plane, and we very
rarely chop holes in that.