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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: squeezing a field
Date: Thu, 31 Oct 2024 01:10:48 +0100
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On 27-10-2024 03:16, john larkin wrote:
> 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.
> 
If you don't have it, I can recommend buying the Bode 100 analyzer. It's 
not terrible expensive, and the SW is great.