Eliminating Switching Noise From Coupling to Secondary Ground

Posted by: Spoke on 12/05/2012

I have an application which is very sensitive to noise coupled from the primary side to the secondary ground.

The typical capacitance added from primary to secondary (C1 and C2) couples enough noise to the secondary ground to swamp the very sensitive front end of this unit.

Question is how to mitigate the coupled noise from the primary to secondary ground? I thought about a common mode choke on the secondary but not sure if this would help the situation.

Thanks,

Jerry

Files

SmartMonitor III AC-DC Power Supply S1075728_Rev4.pdf (25.03 KB)

Comments

Jerry -

Before I get into a detailed reply to your question, I was hoping you could tell me about your design choices involving C22, C1 and C2. Their intended function isn't very clear to me.

Regards,

The Traveler

The choices for C1/C2 and C22 are based on passing AC HiPot testing and some values on previous experience with power supplies.

New requirements may not include AC HiPot testing so we do have some leeway on adjustments to the circuit.

Thanks,

Jerry

Spoke -

Here are a few suggestions and observations:

1 - The center node between C13 and C14 needs to be tied to your input ground/earth connection. The purpose of these capacitors in your EMI filter is to send common mode noise to earth/ground. With your setup, you're directing this noise into the output of your supply.

2 - C1 and C2 (as well as C22) is a really unusual configuration for the use of Y-caps. To minimize noise in the output of your power supply, you want to connect the Y-cap (one not two...) across two quiet nodes of the transformer, one primary side and the other secondary side. For your schematic, this would be between pin 1 and pin 6 of your transformer.

3 - For differential mode switching noise on the output, I would suggest adding a small value 0.1uF-1.0uF ceramic capacitor at the output of your supply. This will have a much lower ESR than the electrolytic you're using for C12.

4 - For C11/C15/C22, make sure you're using low-esr or very low-esr capacitors.

Give these suggestions a try and let me know how they work out.

Regards,

The Traveler