# Pi or not Pi input filter for AC/DC switching PS?

1. ## Pi or not Pi input filter for AC/DC switching PS?

What are the Benefits or Disadvantages of Using Pi Aragemnt as on Picture B
versus the classic concept as on picture A?

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2. ## Re: Pi or not Pi input filter for AC/DC switching PS?

The chief difference appears to be position of the coupled inductors (transformer icon). In schematic A it is placed in wires which carry alternating current.

In schematic B it is in a DC circuit, placed after a smoothing capacitor.

So it's a question where the inductor has greatest effect on waveforms applied to it.

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3. ## Re: Pi or not Pi input filter for AC/DC switching PS?

As I can see, the solution in B is slightly better. L1 is a common-mode choke, which is use to stop the common-mode noise in both ways. In theory L1 doesn't affect the input current since the current is differential. But in reality there always exists DC resistance for the coils inside common-mode choke. So the two 22uF capacitors and this small resistor will compose a PI RC filter. It is better than just use a single 47uF capacitor.

I also would like to say that both solutions have a shortage. There should be four small capacitors (from 100pF to 10nF, depend on the noise in the system) between the L1 terminals to the chassis ground. This will filter-out the common-mode noise more efficiently. Because Pi filter (C-L-C) works much better than just an inductor.

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4. ## Re: Pi or not Pi input filter for AC/DC switching PS?

Thank you for the information. The scheme has been simplified. In real it it my attempt on the power supply 230V / +12V, +5V, -12V max 20W with high efficacy.

I use two Y2 capacitor between PE and +- C14 L2.
I think it will be enough for a 20W power source.
Actually using 2x 22uf 400V is more because I have 100pcs in the drawer.

Full schematics

5. ## Re: Pi or not Pi input filter for AC/DC switching PS?

In the above sketch (post #1) the cap C1 will add nothing to common mode filtering, but will improve DM filtering of the fundamental sw freq, due to the leakage L in the CM choke.

In the immed above it would be better to put your Ycaps by C13 and your CM choke immed after C13 for best CM performance, and make the Y caps 2n2 with a bead or 4E7 ( 1206) res in series with each for damping ...

Also C1 needs experimenting with size to get best CM noise reduction, again a damping resistor, say 10E may very well help with VHF ringing in this cap...

The tracks around the fuse are very long and will pick up noise, defeating any CM filtering ...

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6. ## Re: Pi or not Pi input filter for AC/DC switching PS?

somehow like that?

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7. ## Re: Pi or not Pi input filter for AC/DC switching PS?

... looking better ...

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