Unlocking the Mysteries of Common Mode Current: A Glimpse Through Your Eyes

[@asifimran4333]

Few points, the 5V at output will be fairly stable. Hence, there won’t be high dv/dt . So the displacement current through 100pf will be very minimal. Most of the current will leak through the parasitic capacitance from switching node ( common point between two switch)to the ground and most of the CM current will be contributed by it.

[@MachOneDesignEMC]

Good point, yes, the Switch Node has the most dv/dt. A better drawing would be to draw the parasitic cap from the switch node to the test ground plane, rather than the 5V rail! Definitely agree. I guess the main point i wanted to make in the video, was, from the energy transportation point of view, the L-C filter is just some energy storage devices, taking the energy from the input capacitance and transport the energy to the load, eventually, but it takes time, therefore, all the parasitic component close to the load will also contribute to make the energy transition process, and this is the main cause of common mode noise.

[@SongVong-rm3wp]

Good experiment. Thanks you.

[@AllTheFasteners]

Very good explanation. Thanks for making the video.

[@ininjad24]

Excellent demo of the concept - thank you 🙏 It would be great to see a follow up on this same setup to demonstrate the impact of Y2 capacitors and common mode choke on common mode current.

[@MachOneDesignEMC]

Will do that for sure.

[@jluke6861]

Thank you for the Great Video. You made things so clear.

[@juliatruchsess1019]

Really excellent, thank you!

[@MikeFikes]

Thanks. Very informative!

[@SaltBoy]

Hi Min, at 7:02 could you please elaborate how does the forward current flow through the 0V line? Thank you.

[@MachOneDesignEMC]

Hi, 0V is just a name, isn't it! We all know that we name it as 0V, simply because we reference it to the same point of battery negative. But as it is demonstrated here, the battery negative is not the same as the PCB 0V, even for DC, they are not the same, because the voltage drop caused by the long wire, not to mention the RF signals. So there will be a voltage drop between battery negative and the 0V on the PCB. Same applies between the test ground plane and the PCB 0V, there will certainly be a voltage difference between the 0V on the PCB and the test ground plane, as a result, you will see current induced (via the parasitic capacitance between the 0V and the test ground plane), therefore, induced RF current will flow on the 0V line. Hope this helps

[@tubestar1979]

Very interesting that even nearby conductor will carry CM current. Does that black conductor is connected to the ground plane?

[@MachOneDesignEMC]

no need to connect anything, That is the magic of RF, if we use it wisely, we make communications, if unwisely, interference! Glad you enjoyed it/

[@abdifatahaden4761]

Very good video. I have a question Min. Every standard says reference ground plane has to be connected to earth. You mentioned in another video that causes a problem if the earth wire is long because it can act as an antenna and add noise to the reference ground plane. Lets ignore the safety aspect and lets ignore that a long earth wire may act as an antenna and add noise to the reference ground plane, will you get the exact same measurement results with radiated emissions in the two different cases where in one you have your reference ground plane connected to earth and in the other the reference ground plane is not connected to earth? Basically is the earth wire needed for standardizing measurement results or is it only for safety?

[@MachOneDesignEMC]

Hi, good question. Think about in a test house, the chamber ground is connected to the wall, so the whole Faraday shield is "earthed". Inside the chamber, the antenna is measured with reference to the chamber ground. The key to your question is to understand that the RF reference ground is not earth ground, though for safety reasons, we often earth the RF ground.

[@emc_guru]

"Earth" ground is -only- for safety or lightning protection. It has no bearing on EMC measurements.

[@CLGilbert]

What plastic is the ground plane wrapped in?