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I am designing an ultralow noise 5V/3A SMPS.

On the DC side, I see the ground (DC) is earthed (chassis ground) using a direct connection 0 ohm or using a Y capacitor. What are the differences between them and whats best for low noise on DC output?

On the positive side (DC) I also see that sometimes it's earthed (chassis ground) using a Y capacitor. Is it recommended to make this connection (if our goal is low noise on DC output)? Is 330 pF enough as a value for the Y cap?

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  • \$\begingroup\$ Common mode or differential low noise? \$\endgroup\$ – Andy aka May 22 '18 at 9:25
  • \$\begingroup\$ Differential noise. \$\endgroup\$ – Johan B. May 22 '18 at 9:50
  • \$\begingroup\$ The capacitors you mention in your question only reduce common-mode noise. \$\endgroup\$ – Andy aka May 22 '18 at 9:58
  • \$\begingroup\$ Thats fine . Our differential noise is under 1mV. Questions still stand , whats best for as DC ground connection and should we use the Y cap on the positive of DC ? (to reduce common mode noise) \$\endgroup\$ – Johan B. May 22 '18 at 10:03
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    \$\begingroup\$ It can cause a slight increase in DM noise if the inductors are not perfectly balanced and the output capacitors are not perfectly matched. Personally I'd get rid of the CM choke. You can easily simulate the effects of course in micro-cap or LTSpice (both free) and this would become clear when you imbalanced the values. \$\endgroup\$ – Andy aka May 22 '18 at 11:29
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Initial thoughts

Your target load and whether it is earthed or has imbalanced impedances to ground will dictate common-mode noise seen on the line and, given that we don't know this, it's hard to guess. Be aware that just putting CM capacitors on the output of your power supply may sometimes be NOT the best way - it might be that your target load will perform better with capacitors to ground on some of its nodes. There is no general answer to this.

Not knowing the target load...

Not knowing the target load is a problem and on some applications I have had to get a supplier to remove common-mode capacitors on their SMPS offering so that I could effectively move them to the output lines of my target circuit. This was done to enable our target circuit to pass EMC tests without EMI currents passing through our circuit and causing problems.

Adding CM chokes to the output can be a mixed blessing

It can cause a slight increase in DM noise if the inductors are not perfectly balanced and the output capacitors are not perfectly matched. Personally I'd get rid of the CM choke. You can easily simulate the effects of course in micro-cap or LTSpice (both free) and this would become clear when you imbalanced the values.

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