Safety Restrictions on the Total Y-capacitance / Equivalent DM and CM Circuits / Some Notable Industry Experiences in EMI

Safety Restrictions on the Total Y-capacitance

Y-caps don't just bypass high-frequency noise, but also conduct some of the low-frequency line current. That is what the X-caps do too, the difference being that the Y-caps carry this current into the protective earth/chassis. To prevent a fatal electric shock from occurring, international safety agencies limit the total RMS current introduced into the Earth by the equipment to a maximum of typically 0.25 mA, 0.5 mA, 0.75 mA, or 3.5 mA (depending on the type of equipment and its 'installation category'- i.e. its enclosure, its earthing and its internal isolation scheme). But note that somehow, 0.5 mA seems to have become the industry default design value, even in cases where 0.75 mA or 3.5 mA may have been allowed by safety agencies. It’s important to know how high one can actually go in terms of ground leakage current, as this dramatically impacts the size and cost of the line filter, in particular the choke.

Keeping the discussion here at a theoretical level, we can easily calculate that we get 79 µA per nF at 250 VAC/50 Hz. This gives us a maximum paralleled capacitance of 6.4 nF for 0.5 mA, or 44.6 nF for 3.5 mA, and so on. So, a typical configuration in off-line power supplies consists of four Y-caps, each being 1nF or 1.2 nF or 1.5 nF. Or only two Y-capacitors, each of value 2.2 nF. Note that there may be other parasitic capacitances and/or filter capacitances present, which should be accounted for in computing the total ground leakage current, and thereby correctly selecting the Y-caps of the line filter. However we must keep in mind that if for better EMI performance/CM noise rejection, a Y-cap is connected from the rectified dc rails to earth (or from the output rails to earth), there is no ground leakage current through these capacitors in principle. Therefore there is no limit on their capacitance either.

Equivalent DM and CM Circuits

The filter in the upper half of Fgr. 1 reduces to the CM and DM equivalent schematics shown in the lower half of the same figure. The equivalent schematics are from the viewpoint of the noise coming out via the bridge, heading toward the mains wiring (LISN).

Some observations are:

++ We see that the DM choke acts as a CM filter element too.

++ The leakage inductance of the CM choke appears as a DM filter element too.

++ Both the Y-caps also appear in the DM equivalent circuit (though arguably they won’t add much to the heftier X-capacitance).

++ Considering that a very small value for Ldm is usually enough (because of the much larger X-capacitance possible), no "intentional" DM choke may be required. The leakage inductance of a common mode choke is roughly 1 to 3% of Lcm, depending on its construction. That is usually enough to serve as an unintentional, but effective DM choke.

++ Though CM chokes usually have a high inductance (and that is certainly needed - particularly for complying with CISPR 22 limits below 500 kHz), a good part of the CM noise is usually found in the frequency range of 10 to 30 MHz. So we must consider the fact that not all ferrites have sufficient bandwidth to be able to maintain their inductance ( AL) at such high frequencies. In fact, materials with a high permeability tend to have a lower bandwidth, and vice versa ("Snoek's law").

Therefore a "high-inductance" CM filter may look good on paper, but may not be as effective as we had thought, at high frequencies. See Table 10-1 for typical values of initial permeability vs. bandwidth (bandwidth being defined here asa6dB fall in permeability).

++ A DM noise generator is more like a voltage source. So putting in an LC filter works well for a DM source, as it simply presents a "wall" of impedance that serves to block the DM emissions from entering the mains lines. But this strategy by itself is not going to be very effective for CM noise, because a CM noise source behaves more like a current source. And we know that current sources demand to keep current flowing, and can therefore surmount any "wall" of impedance we may place in their path (by increasing the corresponding voltage). However, if, besides placing a "wall" of impedance, we also present an alternative route for the current to keep flowing, we would be successful in preventing the CM noise from entering the mains wiring of the building. Thereafter we could "kill" the noise by dissipating the associated energy. This places a rather unusual-sounding demand on the CM choke - not only do we need high bandwidth, but we should actually lower its quality factor 'Q', especially at high frequencies. One way to achieve that is to increase the DCR. But that will impede the line current too, and thereby lower the efficiency of the entire power supply. A better solution is to use a "lossy" ferrite material for the CM choke. The usual ferrite used for power transformers and inductors is predominantly of manganese-zinc composition. But lossy ferrites of nickel-zinc composition are actually more helpful in "killing" high-frequency CM noise components. Unfortunately, they also have such low initial permeabilities that it’s impossible to get the desired high inductance (at lower frequencies). Therefore the lossy CM choke is usually an add-on to the normal CM filter stage. It could be just a small bead/toroid/sleeve made of similar lossy material, with both the L and N wires passing through its aperture.

++ Engineers are often mystified to find that making the DM choke out of (low permeability) powdered iron or lossy ferrite helps too, when all else has failed - despite all the talk about DM noise being essentially a "low-frequency emission." The reason seems to be as follows - the CM noise in a power supply is actually a non-symmetric mode, at its point of creation. Though ultimately, by cross-coupling, it does tend to spread into both the lines equally. It has been shown that non symmetric noise can be considered as a mix of CM and DM components. Therefore in practice, we do get a fair amount of high-frequency DM noise too - arising out of the non-symmetric CM noise. That is why high bandwidth/low permeability/lossy materials can help in DM noise suppression too.

++ The DM and CM filters are usually laid out in the order shown in Fgr. 1. The basic idea seems to be that the last stage the noise encounters (as it travels from the power supply into the mains) should be a common mode filter. Because, if the last stage was a DM stage For example, it may not be very well balanced from the viewpoint of the noise emerging from the CM filter. And so the CM noise could get converted into DM noise, as previously explained. However, we do have a DM stage now, to hopefully take care of these additional DM noise components! Therefore, many successful commercial designs have reversed the order as drawn in Fgr. 1, with the DM stage being placed closer to the power inlet. In brief, there seems to be no hard and fast rule for which stage should come before which other stage.

++ A possible location for an additional X-cap is directly on the prongs of the inlet socket (at the entrance to the power supply). We remember that in this position any line-to-line capacitor will be exposed to a huge current surge at power-up, and could degrade, if not fail immediately. So if this X-cap position seems to be the last resort, it should at least be made as small as possible (typically 0.047 µFto0.1 µF). Or we can try ceramic capacitors in this position (approved ceramic X-caps or Y-caps should be tried here).

++ Similarly, the two front-end Y-caps ("C4" in Fgr. 1), or two additional Y-caps, can also be connected directly on to the prongs of the ac inlet socket, rather than on the PCB. This can help a great deal if the wires going from the PCB to the mains inlet socket are themselves picking up stray fields (and are therefore beyond assistance from the main filter stage, which unfortunately lies on the PCB - before the point of noise injection).

++ Sealed chassis mountable line filters (sometimes with integrated standard IEC 320 line inlets) are available from several companies like Corcom (now part of Tyco Electronics) and Schaffner, Germany (at www.schaffner.com). Such filters perform excellently but are less flexible to subsequent tweaking, and also far more expensive than board-mounted solutions.

Note: Incidentally, Schaffner also makes some of the most widely used, standard test equipment for immunity surge-testing.

++ Note that the performance of most commercially available line filters is specified with 50 Ohm at both ends of the filter. Therefore its actual performance in a real power supply may be quite different from what its datasheet says.

++ In general, the traces on the PCB corresponding to the filter section should be thick and wide for low inductance. CM noise suppression also usually requires a very good high-frequency connection to the enclosure. So, the relevant traces of the PCB should be connected to the chassis through several metal standoffs if possible.

However, if standoffs are not feasible, the connection (to the enclosure) should be made via thick braids of fine insulated wire. A "good" connection is usually also helpful between the enclosure and the earth wire (middle prong of the IEC inlet).

In that case braided wire can also be used. In the past, major power supply manufacturers had their own special custom-made metal brackets to connect the earth prong of the IEC inlet socket to the enclosure. But nowadays, standard IEC 320 inlets with built-in metal brackets are directly available, such as that from Methode Electronics Inc. at www.methode.com.

Some Notable Industry Experiences in EMI

One of the most stubborn cases of conducted EMI failure encountered by the author was ultimately (and rather mysteriously) solved by simply reversing the orientation of the CM choke (turning it by 180-degree on the PCB). It was later deduced that the leakage from the core was being picked up by a nearby trace or component, and so the phase of the coupling had somehow become an issue (interference pattern). But since most inductors/chokes are symmetrically built, and also don’t carry any marking to distinguish one side from the other, implementing such a fix was not easy in production. However nowadays, with so many similar "orientation-sensitive" cases being reported (even relating to the main inductor of the converter itself), some key inductor manufacturers have taken the step of placing a 'polarity mark' on their inductors/chokes.

In another well-documented EMI problem at a leading power supply manufacturing house, it was discovered that the CM choke had to be rotated by 90-degree (not 180-degree) to comply. That clearly spells "bad news" if the unit is already in production, because it means the PCB layout has to be redesigned (and perhaps the power supply needs to be re-qualified too).

DM and CM Noise in Switching Power Supplies