Re: SMPS mains circuit

From: legg (legg_at_nospam.magma.ca)
Date: 07/10/04 

Date: Sat, 10 Jul 2004 22:51:51 GMT

On Sat, 10 Jul 2004 17:38:56 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

>I read in sci.electronics.design that R.Legg <legg@magma.ca> wrote (in
><e715b5cc.0407100637.2e5de5c8@posting.google.com>) about 'SMPS mains
>circuit', on Sat, 10 Jul 2004:
>>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.
>
>For an SMPS without PFC, there are nulls or quasi-nulls all through the
>spectrum, above a specific odd-order. I thought that they could occur in
>practice as low as the 9th order, but some recent data indicates that
>they can occur at lower orders.

For straight square waves, the harmonic absolute content vs duty is
plotted for us by Wenzel Associates.

http://www.wenzel.com/pdffiles/choose.pdf

What isn't obvious from this chart is that the sign of the amplitude
has been discarded. The minima actually represent crossings of the
zero axis, going into increasingly inverted ampitudes.

But, like I said, I'd thought that trapezoids, triangles and
haversines showed less evidence of minima, reversals and (signifigant)
higher harmonics generally.

RL