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I have a doubt about the common mode noise propagation in DC-DC converters. Consider a simple Buck converter, the switching nose is where the potential difference between the varying voltage occurs with respect to the -ve rail as well as the ground. Now, how would the common mode currents travel from this point?

Would it be: (a) Travelling from the switching node then through a parasitic capacitance to reference ground, also from -ve rail through parasitic capacitance to the ref ground and then back from the parasitic cap at the i/p side? Similar to the fig below:

Noise propagation through DC-DC converter

(b) Or would it travel from the switching node towards the i/p power supply side, the same from the -ve rail towards the i/p power supply side, through the parasitic cap to the reference ground and then back through the parasitic cap at the o/p side and then towards the +ve and -ve rail? Shown in fig. below:

The CM noise path and DM noise path in a buck converter.

In either of the cases, if there is a parasitic cap between the load node and ground, then wouldn't the CM noise flow through these parasitic caps and wouldn't it occur at the load side as well? i.e. the inductor branch?

My understanding is that the CM noise flows in both the source direction as well as the load direction as in a practical scenario most of the nodes will have some parasitic cap. Then why don't the diagrams available show both these paths?

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    \$\begingroup\$ There is no significant difference between these cases as there is a large bypass capacitor at the input (V(VIN) ~= V(PGND) as seen by the LISN at high frequencies); can you explain better where your confusion lies? (Or, if this clears things up, I can add this as an answer.) \$\endgroup\$ May 24, 2023 at 10:38
  • \$\begingroup\$ The question is let's say in the first figure there are parasitic caps at the load side as well. In that case what would the CM noise path be? \$\endgroup\$
    – ORion
    May 25, 2023 at 4:22

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CM noise is a noise that appears on both of signal terminals (this can be a signal source, or Line-Neutral pair or both terminals of a DC power supply input) w.r.t. a common node (earth, chassis etc. anything that supposed to flow no current through).

Now, how would the common mode currents travel from this point?

CM noise travels along a longer path (i.e. longer than normal mode currents or power currents) and you can't just say if it takes a specific direction. What we know is that its return path is the aforementioned common node. So, any path leads to common node (via stray/parasitic capacitances) is where the CM noise flows through. If you leave it as it is then it'll find its way back to common node through wherever possible: Through the stray cap between the output terminals and the common node, or through the stray cap between the input terminals and common node, or through the stray cap between the high-voltage switching transistor and its heatsink (which is attached to common node) and its isolator, etc. For isolated converter it becomes more apparent because the CM noise at output side is almost inevitable and the path it travels along gets longer e.g. it'll find its way back even through the transformer windings.

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  • \$\begingroup\$ Thank you for your response. Then in the cases of diagrams shown, the longer path would also be from the switch node to the parasitic caps in the load side and then back to the switch node through the ground return path, is that right? \$\endgroup\$
    – ORion
    May 25, 2023 at 4:50
  • \$\begingroup\$ @ORion Just like in a circuit with a source and a few series and parallel branches with different impedances (or resistances), the current will flow through the branches based on the seen impedance there. If the stray capacitances along the path you mentioned bring low enough impedance at certain frequencies then yes, CM noise will flow through there as well. \$\endgroup\$ May 25, 2023 at 12:19
  • \$\begingroup\$ Then if we consider the image attached, is the CM noise path correct as shown by the red dotted lines?, where the dotted capacitors are the parasitic capacitors at respective nodes. \$\endgroup\$
    – ORion
    May 29, 2023 at 5:48
  • \$\begingroup\$ Then if we consider the image attached, is the CM noise path correct as shown by the red dotted lines? where the dotted capacitors are the parasitic capacitors at respective nodes: i.stack.imgur.com/1brFu.png \$\endgroup\$
    – ORion
    May 29, 2023 at 5:59
  • \$\begingroup\$ @ORion as I stated in my answer, any path that leads to common node is possible. We can't specify a direction. Common mode noise tend to be higher frequency, so depending on the impedances along the path the noise can partly flow through the output cap and input caps as well, or might be blocked by the output inductance. The statement in bold holds for every case. \$\endgroup\$ May 30, 2023 at 8:03

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