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Cancellator: a magic active CM noise canceller to upgrade any brick or module SMPS
Cancellator: a magic active CM noise canceller to upgrade any brick or module SMPS
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Old 18th August 2019, 05:52 PM   #11
russc is offline russc  England
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If creating a symmetrical supply ie +ve / 0 / -ve from 2 power bricks, the +ve from one supply would connect to the -ve of the second supply. The circuit works just the same if the ground is taken to the centre 0 volt point? It does not matter if the ground is to the + or the - of V2?
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Old 18th August 2019, 07:09 PM   #12
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by russc View Post
It does not matter if the ground is to the + or the - of V2?
No, not really: all these supplies have a largish E-cap directly connected across the OP terminals, meaning that from an AC perspective they are equivalent (except for the differential-mode perturbations, but they are normally in the tens of mV range at most, whereas the CM ones are tens of volts: 60dB difference)
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Old 19th August 2019, 07:32 PM   #13
Elvee is offline Elvee  Belgium
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Two health warnings:

a) This project involves mains voltage, and has therefore to be treated with due respect.
I already mentioned it, but it's worth underlining basic precautions again, in particular for the autonomous version:

It has to be built into a robust, insulating case, the critical components need to comply with the specs (X and Y caps), and the construction should physically separate the "hot" (mains) and cold (LV supply) sides. The only things allowed to cross this virtual barrier are the Y caps.

b) The project has been actually, physically tested (no sim), and the performances displayed are real, but the effectiveness in real, problematic situations has not.

Real situations are messy and complicated, and the 37dB reduction observed in a lab setup (reasonably realistic, using an actual commercial product) might not readily translate into an equivalent reduction in noise at the speakers level.

Given the good lab performance, there will probably be a certain level of improvement in real cases, but your mileage may vary....

I could of course try to fake a "real" situation, but in general such simulations tend to be not very representative.

What is actually needed is a return from field experiments: one encounters a problematic noise, inserts the widget, and cures the problem (or not, or partly). So, if you build this, be prepared for any outcome, but please report your observations, positive or negative
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Old 9th September 2019, 05:58 AM   #14
indra1 is online now indra1  Indonesia
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Quote:
Originally Posted by Elvee View Post
Here is how I see things, for one, two or more supplies....
... A supplementary flying lead fitted with an alligator clip would connect to the output ground...
Hi Elvee, considering off line operation, how to best connect safety ground to the circuit?
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Old 9th September 2019, 08:03 AM   #15
Elvee is offline Elvee  Belgium
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The main purpose of the Cancellator is to eliminate CM disturbances in a class II environment, ie. without a safety ground present.


If one is available and the various system components are using it, then the Cancellator becomes redundant: it will try to make equipotential two ground that are already electrically connected.
It is not quite equivalent though, because "electrotechnical" equipotentiality (for safety purposes) is not the same as electronic equipotentiality, but anyway, you should not try to use the Cancellator in an exclusively class I context, because it will act concurrently with the safety ground.

If a ground is available, but the device to tidy up is class II, you can try to use the earth as a "cold and clean" reference, like here:
Click the image to open in full size..

In this configuration, if you want to use an input safety earth, you should eliminate the Cancellator completely, and connect the earth directly to the GND, with the risk of ground loops.

Click the image to open in full size.
This risk can be mitigated by inserting a 0.1µF cap (which will also eliminate the possible safety role, but it was not needed in the first place).

Regarding the Cancellator itself, it has to be treated as class II device, properly insulated, etc., and there is no point where you can connect a safety earth
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Old 9th September 2019, 10:01 AM   #16
indra1 is online now indra1  Indonesia
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Thank you Elvee.
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Old 9th September 2019, 10:25 AM   #17
anti is offline anti  Slovakia
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Aren't you missing a counterpart to C1 (150n) on the lower side of the diode bridge? As it is drawn it looks "dangerous".
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Old 9th September 2019, 01:47 PM   #18
Elvee is offline Elvee  Belgium
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No, it is not problematic, and certainly not dangerous.
I explained the simplifications here:
Quote:
Originally Posted by Elvee View Post
The circuits I provided for both versions are "ideal", in the sense that the input neutral/0V reference is extracted from the average of the two mains wires.

Thus, even if they are not equipotential (in HF), the reference will be optimum.

In practice however, the level of differential disturbance will be small compared to the CM, and anyway, 100% of SMPS have a X-cap at the mains input. This means that taking the "quiet" reference from just one side of the mains is going to be perfectly sufficient.

This allows some simplifications: here are the pruned circuits:

Click the image to open in full size.

Click the image to open in full size.
At worst, if the disturbances are heavily asymetric, it could compromise the effectiveness, but certainly not the safety, which is ensured by the Y-caps C6 and C7
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Old 12th September 2019, 09:34 AM   #19
sgrossklass is offline sgrossklass  Germany
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As usual, a neat and ingenious circuit, that I still can't quite wrap my head around (but that's par for the course for Elvee circuits).

Obviously, if you do have access to both secondary-side ground and PE at the same time, there would be a much more simple solution: Connect them with a Y class capacitor, as big as required (which might be in the tens of nF). This would reduce the mains component (stemming from mains filter caps) as well, while working into the RF range.

Now there still are several countries where having PE is considered more or less optional, so the Cancellator might be quite handy there.

I still think that Class II SMPS with Class I mains filtering are a bit of a travesty. They are never ideal for typical mains installations featuring unbalanced "phase" and "neutral". (Only a few countries use "technical" mains with balanced voltages - I think Norway does.)
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Old 12th September 2019, 08:01 PM   #20
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by sgrossklass View Post
Obviously, if you do have access to both secondary-side ground and PE at the same time, there would be a much more simple solution: Connect them with a Y class capacitor, as big as required (which might be in the tens of nF).
Yes, I mentioned a generic 0.1µF capacitor, and that is sufficient: no need for a Y or even a X type: there are no safety issues involved there
Quote:
Now there still are several countries where having PE is considered more or less optional, so the Cancellator might be quite handy there.
Even if every outlet is fitted with a PE (as it is the case in my country), it can be somewhat unpractical to use it with a class II device plugged in: for example, extension cords without a third wire are perfectly legal, provided it is impossible by construction to insert a class I device

Quote:
They are never ideal for typical mains installations featuring unbalanced "phase" and "neutral". (Only a few countries use "technical" mains with balanced voltages - I think Norway does.)
Believe it or not I "benefit" from such a balanced distribution system, but it is an exception in Belgium: only a few districts, many in the Brussels region still have this "prehistoric" system: it is a remnant of the pre-220V(now 230V) era.
Now, the norm is the 230V/400V system (Neutral-Line/LineP1-LineP2) almost everywhere.
If I needed a three-phase supply for heavy machinery or three-phase heaters or cookers, I wouldn't be able to get it.
The 230V/400V system might have some minor disadvantages for 50Hz leakage currents for instance, but regarding the HF perturbations, it is essential neutral, and it won't have any effect on the hiss caused by a SMPS.

It can have a small effect when the µA's or tens of µA's current causes a drop of some nV in the shielding of a sensitive cable, or if a poorly shielded sensitive node is exposed to mains generated electrostatic fields (but in this case, for the beneficial effect to be effective, it is necessary to have a balanced mains wiring too).

Note that I showed the Cancellator in a mains-related situation, but it can be useful for any isolated DC/DC converter too.
Useful does not mean a complete and radical cure for all situations: many factors influence both the initial and the "improved" situations.
There will normally be an objective and measurable improvement, but additional measures might still be required to make the annoyance completely unnoticeable
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