Re: one more try at my input filter question


"David Collier" <from_usenet_sci_elec_des@xxxxxxxxxxx> wrote in message
news:memo.20060505141516.3084C@xxxxxxxxxxxxxxxxxxxx
Sorry to get repetitive...
I asked a question above, but I'm not sure I got the specificity of
answer I was hoping for. Could I try again in more concrete terms please

I have a source of DC, 12 to 24V say. I have a switching PSU running off
the DC. The sink takes current in bursts at 2us intervals. The 500KHz is
approximate, but reasonably stable and accurate.

see http://www.Micrel.com/_PDF/mic2198.pdf

I'd like to be able to draw 15W in bursts, so assume average input
current of 1 to 2A on full load.

That causes noise on the wire going back to the supply. That stops me
getting CE approval for the box ;-(

I need to kill the noise.

All the PSU chips which run at fixed frequencies say "this makes the
noise easier to filter" - Hmmm. how exactly?

I appreciate that there will be noise at 2us and harmonics at higher
frequencies. We have already fitted a proprietary input filter, which
sadly seems to provide little attenuation as far down as 500KHz.

BNX002-01 http://www.Farnell.com/datasheets/6821.pdf page 14

It doesn't appear that [ after the filter get to them ] the harmonics
are large enough to cause a problem.

Yes I know, shoot the designer who matched that filter with that
switcher. The bullet is on it's way.

So I have just the one problem - noise at 500KHz.

I COULD treat it by just building/fitting a 'bigger/better' filter with
a cut-off frequency well below 500KHz. But I have a PCB artwork already,
and PCBS built, and that would require making new PCBs. I'd like a
retrofit mod if I can work one out.

Now, back when I did exams, I seem to remember circuits which would
exhibit a high impedance at a single frequency. usually consisting of a
cap and inductor.

something like
http://www.allaboutcircuits.com/vol_2/chpt_8/6.html

Could I "fix" the problem by fitting such a cct? Would the components
required fit inside a small building if I did? Should I stop trying to
be clever?

I can buy a 2A-rated inductor at about 33uH
http://www.Coilcraft.com/pdfs/dc1012.pdf

to get the resonant frequency to be 500KHz I'd need a capacitor of

C= 1/( 4 * PI^2 * L * F^2 ) ???

3nF???

Sounds a bit small... maybe I could use a smaller inductor and a larger
cap?

Maybe I should just fit a 1R/inductor in series and change the input cap
to 220uF low ESR instead of 22uF.

Is there a better way, maybe by building a suitable notch-filter circuit
and fitting it into the DC supply lead outside the box for now?

All help appreciated.

David

Hello David,

Have you tried putting the filter the other way round - it's designed to
protect your ciruit from the PSU noise. This may not help with your pcb.
Try a serious ceramic capacitor across the input terminals so the inductors
in the filter do more good - you can get 10uF in 0805 out of the Farnell
catalogue. I think that might go on the back of your board.
You haven't said if the noise is common mode or not (ie is the same on both
input terminals, in which case the capacitor won't help).
Good luck - let us know what happens.

Michael Kellett

www.mkesc.co.uk




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