From: Fred Bloggs
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Subject: Re: Magnetically Shielded Inductors
Date: Fri, 27 Sep 2002 12:36:41 GMT
NNTP-Posting-Date: Fri, 27 Sep 2002 05:36:41 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Bill Sloman wrote:
> "John Devereux" wrote in message
>>Which have lower magnetic emissions, unshielded toroidal or
>>"shielded" bobbin core inductors?
>>I am looking at inductors for a 500kHz buck SMPS and want to
>>minimize the local magnetic interference.
> Toroids should be better. A toroidal coil has no external magnetic field,
> even if you leave out the toroidal core. Real toroidal coils are rarely
> perfectly uniform toroids so there is usually some external magnetic field,
> but not a lot.
> Theoretically, a pot core is topologically equivalent to a toroid and should
> have the same zero emission, but real pot cores have slots to allow
> connections from the coils inside the pot core to the outside world, and
> these slots do allow some leakage flux - more than you get from a toroid.
> The cylindrical coil in a pot core is a lot easier to wind than a toroid,
> which is why the "shielded" bobbin core inductors are so much more popular.
> Bill Sloman, Nijmegen
I am sure that you are confusing far field from near field with this.
The toroid will have a severe near field compared to a shielded
component. He will therefore have to watch its placement relative to his
other circuit components, as well as their layout.