Cancellator: a magic active CM noise canceller to upgrade any brick or module SMPS

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It's Christmas again!!!!

Well, not quite yet, but it looks like it: the Cancellator is a simple, active circuit that cures one of the most unpleasant flaw of class II SMPS's, the common mode noise.

Class II supplies are attractive because the absence of earthing makes them floating, thereby eliminating a major source of ground-loop problems.

Unfortunately, there are no free lunches in engineering, and this advantage comes at a cost: they spit a significant amount of HF trash on the output circuit, and unlike differential-mode disturbances, no amount of filtering can eliminate CM noise. CM chokes should in principle be able to do it, but their effect is rather imperfect, due to relative impedances, and anyway, all SMPS's are already fitted with one or more.

Recently, Daniel sent me a supply example to evaluate its suitability for his pet amplifiers: EZmos and OldFashioned.

Cancel36V5A.jpg

The supply delivered the goods, and would be sufficient for two amplifier channels, but I also noticed its horrendous CM noise. Here is the CM voltage picked up at the output GND:

CancelNoise.jpg CancelNoise2.jpg

The oscilloscope is earthed, thus the waveform is measured wrt. earth. There is a large 50Hz component, which is normal, but regions of the waveform also show a significant HF hash, between 20 and 30Vpp. Such a residue can cause various symptoms in audio equipments: hissing, buzzing, whistling, etc.

The 100Hz modulation is caused by the conduction of the input rectifying diodes: when they conduct, the primary circuit is connected to the mains, and the level is heavily reduced. In-between, the converter circuit is fed from the filter cap and is left floating, causing a maximum of disturbances. It is not always the case, and sometimes an opposite situation is observed.

This converter module is particularly poor, which means it is an ideal test-bed for a correction circuit, the Cancellator:

Cancellator0.png

The principle of operation is extremely simple: the output GND is taken as a reference, and an amplifier actively forces the input 0V (or neutral, or whatever) into a virtual ground by injecting a suitable counteracting signal. Note that in fact the opposite might be true, but action=reaction, and the amplifier does what is needed to make the two grounds equipotential.

As always, the actual details are a bit more gory: in particular, it is essential to do the above whilst respecting the integrity of the safety isolation barrier. To do that, two Y-rated capacitors are used: one for the input, one for the output. The output one is a 4.7nF, the maximum reasonable value for such an application, to keep the correction path as low impedance as practical. It is also necessary to provide a supply for the amplifier, something the 36V output does perfectly.

How does it work? Quite well: here are the before/after oscillograms, you barely notice a thickening of the trace on the after pic:

CancelBA.jpg

Here is the breadboarded prototype:

CancelBread.jpg
 
The base switching frequency is ~50kHz, but most of the feedthrough is from the edges, because it is a squarewave, meaning significant harmonics at 500kHz and beyond.

This is why I used a good old LM318: it has a good bandwidth, and accepts the 36V supply.

Many modern alternatives have a much wider bandwidth, but they are often more limited in their supply range. One could think of reducing the supply voltage, and it could be sometimes possible, but with this example, a really large correction amplitude is required: significantly more than 16Vpp.

This pic shows the correction signal (measured with a differential probe) along with the corrected signal:
CancelCorr.jpg

The bandwidth only depends on the opamp used: with something better than a 318, it would be possible to go in the tens of MHz range, but it does not look particularly necessary: the cancellation level reaches almost 40dB, because the converter module includes basic measures like a CMC and snubber
 
Yet again - super scheme Elvee.

Early days, but I can see a strong interest related to commercial plug pack supplies such as laptop supplies, but also 12VDC supplies that I have used that don't have mains protective earth connecting through to 12VDC output. Hassle with commercial plugpacks is that there is no mains side electrical connection available, unless some form of interface socket/plug is prepared for easy access to the local protective earth.
 
Yes, practical connection details are going to be an obstacle.

However, since the DC output voltage is not always going to be sufficient to properly power the cancellator, an alternative needs to be found anyway.

With this example (OK, rather extreme), the DC supply couldn't be smaller than 20V, otherwise the correction amplifier would begin to saturate and clip.

Thus, I devised the reverse alternative, where the amplifier is referenced and supplied from the mains. The mode of operation remains exactly the same, but the supply and correction amplifier could now be housed in a box inserted between the mains and the brick (or adapter). It brings some additional complication, but it is workable:

Cancellator1.png

The ~8mA required by the opamp is supplied by a special capacitive supply: it has a symetrical configuration to ensure a continuous low impedance path to the mains for HF signals. The rest of the circuit is essentially the same, except that now the output ground is servoed to the input, but the end result remains exactly the same. Here is the prototype:

CancelBreadV.jpg

Note that an actual implementation needs to be built properly, not improvised, and housed in an insulating case, to ensure an adequate insulation from the mains. It is also important to respect the Y and X ratings of the capacitors where indicated
 
I have also tested other opamps, beginning with a 741.

You probably guess the result, but I had to do it and begin somewhere. It managed to improve the situation compared to nothing, but it was clearly lacking: a significant uncorrected residue remained visible.

Next one was a LF411. This was a surprise: it did barely better than the 741. By contrast, a LF356 did almost as well as the LM318. It is probably due to a tighter "grip" of the OP stage, compared to the LF411. Finally, a NE5534 did as well, or even slightly better than the 318, confirming that the robustness of the OP is paramount for such an application.

In very difficult situations, an additional buffer might become necessary.
 
Some details and context about the measurements:

I made them in my lab environment, without attempting to change anything: the scope has a 6 feet mains cord, and is connected to the master lab power switch via an extension, probably 5 feet or so.
The probe is also 5 feet long, and the DUT is connected via a "suicide cord" 5 feet long, and another extension 5 feet long.

This is in fact very similar to a typical audio configuration (except for the suicide part, hopefully), meaning inductances and parasitic effects are reasonably realistic and should reflect a real life situation quite closely.
 
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:

776003d1566119722-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancellator0s-png


776004d1566119722-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancellator1s-png
 

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Here is how I see things, for one, two or more supplies.

You would build the cancellator into a case of this style (or a multiple outlet plug or extension):

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A supplementary flying lead fitted with an alligator clip would connect to the output ground.

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In the case of multiple supplies, they would of course have their output grounds connected together, and that's where the alligator clip (or other connector) would be attached.

A single cancellator can correct multiple supplies sharing the same input and the same ground, it will generate whatever composite signal is required to correct all the supplies at once provided its dynamic range is not exceeded.

The scheme is applicable to bricks, wall-warts, etc

BTW, you could earn thousands of $$$ selling these gadgets to audiophiles (its only flaw is that it actually works, unlike snake oil supposed to do the same)
 

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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?
 
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)
 
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
 
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:
775496d1565899795-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancellator0-png
.

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.

776128d1566146496-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancelpract-png

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
 
No, it is not problematic, and certainly not dangerous.
I explained the simplifications here:
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:

776003d1566119722-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancellator0s-png


776004d1566119722-cancellator-magic-active-cm-noise-canceller-upgrade-brick-module-smps-cancellator1s-png
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
 
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.)
 
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
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

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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