Re: EMI and harmonic filter


legg wrote:

> On Sun, 04 Sep 2005 07:26:29 +0100, Pooh Bear
> <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:
>
> >"krishmaniac@xxxxxxxxxxx" wrote:
> >
> >> hi,
> >> this discussion was not to annoy anyone. It was merely for learning
> >> purpose. The person who himslef do not know the dobut
> >> runs to beat the other back. It has been clearly seen from the
> >> discussion.
> >> The harmonics i was refering is to switching frequency harmonics is
> >> SMPS.
> >
> >I thought as much initially. To suppress that type of interference (
> >switching noise ) you use a *common-mode* filter.
> >
> >But you should be aware that 'harmonics' are another issue entirely. See
> >IEC 61000-3-2.
> >
> >I'm not aware of the use of any differential mode filter in typical EMI
> >applications btw.
> >
>
> If you are refering to commodity power supply AC line filters, then
> perhaps your impression is due to the fact that common mode choke
> leakage and loss are also effective on differential mode components,
> if the appropriate differential mode capacitors are present.

They are bound to be. That's not their primary function however.


> At the actual conversion frequency, however, these small parasitic
> inductive and resistive terms are seldom sufficiently to allow
> reasonably sized and priced capacitors to work, without additional
> discrete differential mode inductors. I'm suprised you've not noticed
> their presence.
>
> Differential filters are everywhere, in general EMI control of signal
> lines, on input and output, even if only comprised of a single series
> resistor, or feedthrough capacitor.

Yeah I do those too ! I don't see a feedthru cap helping PSU emisions much
though.

Pls don't 'muddy the waters'. The methods for dealing with classic SMPS noise
are well established.

Graham

.