Re: SMPS mains circuit

From: R.Legg (legg_at_magma.ca)
Date: 07/10/04 

Date: 10 Jul 2004 07:37:02 -0700

> >Has retesting ever produced unexplainable non-compliant results in a
> >fully-functional product?
>
> Well, not non-compliant, as far as I know, if the standard is understood
> correctly (which in some cases it wasn't), but variations in measured
> value of 4:1 have been observed, which is not exactly reassuring unless
> the reason is fully understood.

If this 4:1 variation occurs at harmonics higher than n=11, then I
don't find it very surprising. The occasional higher harmonic can
unexpectedly null in a unit, for purely serendipidous combinations of
components and load.

There isn't a corresponding unexpected peaking, as the stray is
undocumented series leakage inductance elements in CMFilter. This
depends on winding method, orientation, turns accuracy and just
sometimes just plain sloppiness. There is an upper limit achievable
without turning the choke into a recieving antenna at other
frequencies.

This stray leakage begins to look like 100mOhms or larger at the
+decade harmonic, and appears twice in every common-mode position (not
neccessarily with identical measurable values). Even low impedances
strongly influence the sharpness of the impulse input current
waveform, and the higher harmonics that this represents.

Your comments that an LISN is not required in the test procedure is a
little ingenuous. The networks were developed to allow predictable and
reproducible high frequency EMI performance. The fact that a similar
line source impedance is not specified in the testing of low frequency
harmonics, when 100mOhm impedances are capable of producing errors
(that may be of concern), is laughable. At the very least, the test
method should indicate that any LISNs present for any other test
purposes should be removed, if their LF impedance is likely to affect
producable harmonic levels.

RL

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