...Thanks, I agree with this and always have agreed with it, i am not sure why you assume i wouldnt agree with it.It won't cause one mode to transform into the other.
the thing is, noise that couples out of the product can couple back through a y capacitor and go to neutral and go through the second half of the common nmode choke.....and then it would go back via neutral, and it would be deemed diff mode noise.
Because you said this:...Thanks, I agree with this and always have agreed with it, i am not sure why you assume i wouldnt agree with it.
Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
That post doesn't even make sense. Y capacitors force conducted EMI from the load side back into earth, away from the mains. If the filter is balanced, then common mode signals do not see hot or neutral as being different.I think this is the crux of the matter, from my above post
..i think this is the key point, (from my post #18 above) unless this one is ironed out i think we are talking at crossed purposes. do you agree that this can occur?...or are you saying that this cannot occur?..i think this is the defining point.
I disagree.Thanks, the whole process of common mode emissions is somewhat steeped in the “magic” of RF engineering
100pF is very substantial. 10K of impedance sounds high for an interference source, but it also has an amplitude in the hundreds of volts. You still need to attenuate it quite a bit to meed EMC specs.As you know, 100pF has an impedance of 10600 Ohms at 150kHz.
Above
1 MHz, current emissions which exceed the desired specification
are usually common mode emissions caused by either ringing
waveforms identified earlier or resonances caused by parasitic
components themselves.
I don't really like to generalize these things too much. The amount of emissions you see will depend on a lot of factors such as converter topology, operating frequency, transformer design, heatsink design, etc.OK thanks,
Do you believe that page 17 of this….
**broken link removed**
..which states that..
is at best “off the mark”?
Do you agree….it should say that “above 1MHz, conducted emissions are predominantly common mode, and below 1MHz, they can be either differential mode or common mode”
No, this is expected, since common mode is by definition equal on both lines.Also, isn’t it odd that when you have a common mode emissions problem, the live and Neutral EMC scans can be exactly the same as each other in amplitude, all along the frequency range?
I mean, due to the nature of common mode emissions, you would expect the live and neutral EMC scan plots of offline power supplies with common mode emissions problems to be different?
Thanks, so in that case, if you have only a differential mode conducted EMC problem, then you expect your EMC scan plots of live and neutral to be different?No, this is expected, since common mode is by definition equal on both lines.
That depends on how the data is acquired. If you've measured the current one each line individually, then converting those measurements to differential/common mode components requires that you add/subtract them, which means the phase of the measurements must be preserved. If you measure each line with a spectrum analyzer which only returns magnitude plots, then you can't really determine whether the interference is common mode or diff mode. Refer to your relevant regulations for details.Thanks, so in that case, if you have only a differential mode conducted EMC problem, then you expect your EMC scan plots of live and neutral to be different?
Basically not. Presuming a standard LISN setup with separate L and N decoupling and pure DM versus CM interferer, you get equal magnitudes in both lines. Superimposing CM and DM interferer or asymmetrical ground impedance results in different magnitudes.if you have only a differential mode conducted EMC problem, then you expect your EMC scan plots of live and neutral to be different?
Thanks, so if its pure DM interference, and no CM interference, then Live and neutral scans would be the same?Basically not. Presuming a standard LISN setup with separate L and N decoupling and pure DM versus CM interferer, you get equal magnitudes in both lines.
The practical reason for having L1 and L5 is
- better interference attenuation by multi stage filtering
- increasing attenuation at higher frequencies by supplementing inductors with higher SRF.
Thanks i agree, but for costs sake, i think you would agree that a single bigger inductor, in either live or neutral only , would be preferable?The practical reason for having L1 and L5 is
- better interference attenuation by multi stage filtering.........
Purely CM interference and purely DM interference would show up the exact same results in magnitude measurements on each line. When both CM and DM interference sources are present, then you will see different spectrums on L and N, however it's not possible to determine the relative contribution of CM and DM interference. In some tests, a measurement with a combiner can be used to extract just the common mode component, as discussed here.Basically not. Presuming a standard LISN setup with separate L and N decoupling and pure DM versus CM interferer, you get equal magnitudes in both lines. Superimposing CM and DM interferer or asymmetrical ground impedance results in different magnitudes.
I believe, the simple equivalent circuit illustrates why.
View attachment 139163
They will have a common mode impedance (equivalent to L1||L5) which will act to filter CM signals, but usually their CM impedance will be much less than that of the CM choke, so their impact is much less.Thanks, i am wondering why the top Europe apps engineer for xxxxx company (a very respected company) told us that the L1 and L5 inductors help reduce common mode noise?
Having asymmetric filter design would bring up the possibility of differential interference being transformed into CM interference, possibly making some EMC regulations harder to pass.Thanks i agree, but for costs sake, i think you would agree that a single bigger inductor, in either live or neutral only , would be preferable?
Thanks, yes, the thing is, it still surprises me that we get such a high common mode peak at around 150kHz. This is because as you know, capacitive current flow (in eg the stray capacitance) goes as i = C.dv/dt100pF is very substantial. 10K of impedance sounds high for an interference source, but it also has an amplitude in the hundreds of volts. You still need to attenuate it quite a bit to meed EMC specs.
..(from post #34 above.)Having asymmetric filter design would bring up the possibility of differential interference being transformed into CM interference, possibly making some EMC regulations harder to pass.
(from post #26 above)No, this is expected, since common mode is by definition equal on both lines.
(from post #14 above)Y caps filter common mode noise because they are connected to earth ground.
again from post #14 above.Y caps filter common mode noise because they are connected to earth ground.
Review FvM's figures in post #30, they describe things as simply as possible...(from post #34 above.)
Thanks, so that is a declaration that DM noise can be transformed into CM noise....do you think that the reverse can happen?...ie CM noise getting transformed into DM noise?
(from post #26 above)
Thanks, do you mean that CM noise is equal in phase and equal in magnitude in each line......?
Also, is DM noise opposite in phase and equal in magnitude on each line ?(L and N)
(from post #14 above)
What is "the circuit?"Thanks, but isn’t it strange that it is the stray capacitive coupling between the circuit and earth ground that causes common mode noise in the first place?
In other words, capacitance_to_earth (the stray capacitance between the circuit and the earthed heatsink) has caused common mode noise…..and is then being used (as in Y capacitors) to solve the common mode noise problem. Isn’t this a kind of slight contradiction?
:-|
This is a false dichotomy. Any filter has both shunt and series elements used in combination, and we just call that a "filter."-This "diversion" is a purpose for which Y capacitors are often not given credit....do you agree?
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