Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Removing earth connection made no difference to common mode noise

cupoftea

Advanced Member level 5
Advanced Member level 5
Joined
Jun 13, 2021
Messages
2,991
Helped
59
Reputation
118
Reaction score
138
Trophy points
63
Activity points
15,896
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?
 
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.
 

LaTeX Commands Quick-Menu:

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top