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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: two drums
Date: Fri, 14 Mar 2025 16:46:18 +1100
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On 14/03/2025 1:41 am, john larkin wrote:
> On Thu, 13 Mar 2025 07:42:28 -0400, legg <legg@nospam.magma.ca> wrote:
> 
>> On Tue, 11 Mar 2025 19:35:38 -0700, john larkin <jlArbor.com> wrote:
>>
>>>
>>> https://www.dropbox.com/scl/fo/qpfhkvbfig7elysx78lq3/ALGqgMaRq1tx8aIiN3p1TfM?rlkey=36bcqfdb9di22ko48j89vocut&dl=0
>>>
>>> If I put two of these drum core inductors close together, and get the
>>> phasing right, I get an extra 5 mH for free, without any more ESR. And
>>> external mag fields drop too, I think.
>>
>> Check ripple and transient load response (load one - measure the
>> other).
>>
>> If chanels not synchronized at the switching frequency, odd results
>> might be seen at mixing frequency.
>>
>> RL
> 
> The two inductors will be in the two legs of, basically, a floating
> power supply, to decouple it from whatever customer impedance. So I'll
> be running the exact same current through both inductors. If I get the
> polarities right, I get the bonus inductance and the far-field
> magnetic cancellation.
> 
> I don't want signal coupling to other channels on the same board, so
> the field cancellation benefit is squared. That overcomes some
> peoples' objections to unshielded drum cores.
> 
> The physics is kinda weird. The 5 mH per inductor uses the universe
> for its energy storage, but the bonus L is energy stored in ferrite, I
> guess. I just invented the semi-shielded inductor.

It doesn't use the whole universe - just the bit in the immediate 
vicinity of the inductor. The bonus L comes from the field lines that go 
through the adjacent ferrite. A inductor wound onto a toroid can be 
completely shielded - but only if the winding is non-progressive.
Something wound around a pot core is less perfectly shielded, so the 
semi-shielded inductor was invented a long time before people realised 
that toroidal cores were a good thing.

-- 
Bill Sloman, Sydney