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Subject: Re: Core gapping techniques and general SMPS magnetics stuff
References: <6hNK9.982$qA3.email@example.com> <3DFC2439.5F2A4EC7@ieee.org> <3DFCE76E.EE24B0A7@ieee.org>
NNTP-Posting-Date: Sun, 15 Dec 2002 23:36:36 GMT
Organization: AT&T Broadband
Date: Sun, 15 Dec 2002 23:37:09 GMT
Mike Engelhardt, analog wrote:
>> Sounds like you had a very interesting problem. Just of curiosity,
>> why and how are electron microscopes so sensitive to low level, high
>> frequency magnetic fields? Wouldn't this apparent Achilles heal
>> preclude the use of lots of similarly emissive everyday electronic
>> equipment within the near vicinity of the microscope? Is this a
> The SMPS's fringe field coupled into the scan drive. It blurred
> the image. With care, you can get pretty much as much electronics
> near an SEM as one wishes. We had all sorts of instrumentation
> electronics(this SEM was an e-beam probe for IC diagnostics using
> voltage contrast to make a non-contact oscilloscope for IC's) and
> a Sun Workstation in a desk-sized unit which included the microscope
> column and vacuum system. The core with the outer legs gapped was
> *much* more offensive than typical electronics and the experience
> reinforced my opinion that there isn't much excuse in using ecores
> gapped in the outer legs in non-prototype situations.
Going with a single, larger center leg gap in a flyback transformer
can increase fringing and proximity losses within the windings so that
this may be the best choice if the external field is not a problem.
>> And just to clarify, what was the core size, geometry and operating
>> frequency of the transformer in the offending power supply (and its
>> replacement)? Also, was it a flyback, what was the power level, and
>> how far removed was it from the sensitive parts (which are?) of the
>> microscope? And lastly, what was the physical nature of the added
>> shielding (both the copper and the mu-metal)? I'm curious about
>> thickness, width, etc.
> The core was about and inch square E-core running at 100kHz. It
> was mfg. by Lambda. I never looked at it's schematic and can't
> remember the input/output power(this was in '92). The replacement
> I found was a Jap unit. Same outside dimensions and the circuit
> looked similar, but the similarly sized e-core was gapped in the
> center, not the outer legs, and that completely eliminated the
> problem with the fringe fields. No further shielding was required.
Perhaps the external shorted turn you tried on the transformer with
the gaps in the outer legs was made of relatively thin copper tape
with adhesive backing. Since copper is not inherently a magnetic
material it relies upon eddy currents to generate a opposing (and,
thus, canceling) field. The copper must be four or so skin depths
thick in order for this to work really effectively (>40dB reduction),
so it's probably a good idea to use cut copper sheet material for the
outer wrap instead of the more convenient copper tape.
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