Removing earth connection made no difference to common mode noise

[cupoftea]

Hi,
I was just diff probing a signal on the primary side of a 100W , 3 phase delta offline SMPS. It has a 3_coil common mode choke. It also has y capacitors to earth, upstream of the common mode choke. The signal was deluged by common mode noise, and not actually visible.

Then I added a y capacitor across the smps isolation transformer. And also a y cap downstream of each common mode choke coil to earth…so 4 extra y capacitors.
I could then see the signal really well, since adding the y capacitors cleaned up the noise.
But then I removed the earth connection to the SMPS….and still found the noise was cleaned up.

Would you expect this?

 

[D.A.(Tony)Stewart]

Hi,
I was just diff probing a signal on the primary side of a 100W , 3 phase delta offline SMPS. It has a 3_coil common mode choke. It also has y capacitors to earth, upstream of the common mode choke. The signal was deluged by common mode noise, and not actually visible.

Then I added a y capacitor across the smps isolation transformer. And also a y cap downstream of each common mode choke coil to earth…so 4 extra y capacitors.
I could then see the signal really well, since adding the y capacitors cleaned up the noise.
But then I removed the earth connection to the SMPS….and still found the noise was cleaned up.

Would you expect this?

Yes.

With a Delta output, each phase is connected to the next such that one would expect crosstalk from adjacent phases from load currents and go unstable into oscillations with a SMPS. But not with neutral return on Y configured outputs.

Defeating the transformer high impedance isolation with 4 additional Y caps across them shunts noise between phases and thus removes the internally generated common mode noise current to ground. This is more like 3-X caps in differential mode & 1 Y-cap for leakage to ground. ( If I understand it)

But now makes it prone to external comm mode noise without the isolation or large CM chokes.



FWIW , that's my guess.