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From: "Mike Engelhardt"
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: 15 Dec 2002 21:25:00 GMT
Organization: Concentric Internet Services
References: <6hNK9.982$qA3.email@example.com> <3DFC2439.5F2A4EC7@ieee.org> <3DFCE76E.EE24B0A7@ieee.org>
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> 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.
> 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.
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