On Wed, 6 Aug 2008 12:44:05 +0100, "Arfa Daily"
put finger to keyboard and composed:

The caps are located on the primary side of a resonant mode smps. It is a
sort of half bridge circuit with two FETs. The drain of the upper one is
connected straight to the 400v rail, and the upper connection of the
transformer primary, is returned to the same point via one of these caps.
The upper transformer connection is also connected to the second capacitor,
the remaining end of which is returned to deck via a very low value R, which
is used to sense primary current. The lower FET has its drain connected to
the upper one's source, and it is this 'mid point' which hooks to the lower
connection of the primary winding. The source of the lower FET is returned
to the same deck as the low value R detailed above. So I guess that the caps
are for coupling and resonating the primary. Looks like they would be
working hard ...

Arfa

That explains a lot. For example, you measured an ESR of only 38 ohms
instead of 80, which is what you would expect if the two caps were
connected in parallel. In fact your circuit does effectively parallel
the two caps via the low ESRs of the current sense resistor and 400V
bulk capacitor.

If one cap is OC, then you will see an ESR of 80 ohms, which is the
ESR of the other good cap. If both are good, then you will measure 40
ohms.

- Franc Zabkar
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