Path: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Lasse Langwadt Newsgroups: sci.electronics.design Subject: Re: squeezing a field Date: Thu, 24 Oct 2024 18:45:20 +0200 Organization: A noiseless patient Spider Lines: 54 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Injection-Date: Thu, 24 Oct 2024 18:45:21 +0200 (CEST) Injection-Info: dont-email.me; posting-host="005e913f7bc19b0a537a4691282cb823"; logging-data="2847877"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+HnUHGnZD1aJDtAoDDW8WHLk3gyd2gFeU=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:V3fO58w4gITrGaUUSWyntIZxDq4= In-Reply-To: Content-Language: en-US Bytes: 3067 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 >> wrote: >> >>> 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. >>>> >>>> >>>> >>> >>> 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?