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I have seen in many SMPS there is a Y cap placed between AC ground and DC ground. What is the need of this Y cap?

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The high frequency switching used in the SMPS can inflict some quite considerable high frequency common-mode noise on the DC output. This is due to the interwinding capacitance of the internal isolation transformer (aka flyback transformer).

To reduce this noise (and obtain EMC approval) requires that the DC output is either grounded or capacitively connected to ground. In most cases, connecting a Y cap between the DC output and AC side of the SMPS can achieve this.

The downside is that you can see quite high AC power frequencies on the DC output but these "arrive" via the Y capacitor so are easily "extinguished" when there is a load on the DC output that has reasonable capacitance to ground.

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  • \$\begingroup\$ Isn't capacitive coupling between the switched node and GND or PE also a source of CM noise? \$\endgroup\$ – Simon Aug 20 '19 at 12:49
  • \$\begingroup\$ @Simon If you mean that capacitively coupling between the switching node and GND or PE might increase the DC output CM node noise then this is only slightly true if the GND or PE node is not adequately grounded. Bear also in mind that inter-winding capacitance is going to be much higher than spurious primary to GND or PE capacitance. \$\endgroup\$ – Andy aka Aug 20 '19 at 14:27
  • \$\begingroup\$ I thought I heard this with respect to cooling components and such. Your answer refers to CM on the secondary, right? But what about CM on the primary where you're required to use mains filters and stuff. \$\endgroup\$ – Simon Aug 20 '19 at 14:34
  • \$\begingroup\$ @Simon I'm sorry, but I'm sticking to the remit of the question and not getting hauled off-track! And no, I'm referring to CM on the DC output (not strictly the secondary). \$\endgroup\$ – Andy aka Aug 20 '19 at 14:39

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