Practical EMI Line Filters: Line Filters

We now look at a typical power supply line filter, as shown in Fgr. 1. Its ultimate purpose is to control conducted emissions in general, and therefore it has two stages (as highlighted) - one for differential mode and one for common mode. Let us make some relevant observations.

++ Both the CM and DM stages are symmetrical (balanced). From the viewpoint of the noise emerging from the bridge rectifier and flowing toward the LISN, there are in effect two LC filters in cascade (both for DM and CM noise). This filter configuration can provide good high-frequency attenuation (roll-off).

++ Occasionally, unbalanced filters may be tolerable - For example a single DM choke (i.e. on one line only). Or sometimes, in very low-power applications, just a plain decoupling capacitor (e.g. C1) may suffice. Sometimes tuned filter stages are seen in commercial off-line power supplies (e.g. from Weir Lambda, UK). But there are some anecdotal industry experiences that suggest that under severe line transients or under input surge waveforms, as those typically used for immunity testing, tuned filters can display unexpected oscillations (resonances), ultimately provoking failure of the power supply itself. Therefore, tuned filters are generally avoided in most commercial designs.

++ Note that the filter is usually placed before the input bridge (i.e. toward the incoming ac line input) - especially because in that position it also suppresses the noise originating from the bridge diodes. Diodes are known to produce a significant amount of medium- to high-frequency noise, especially at the moment they are just turning OFF. Small RC snubbers (or sometimes just a "C") are therefore often placed across each diode of the input bridge. Though sometimes, we can get away simply by choosing diodes with softer recovery characteristics.

++ Note that input bridge packs using ultrafast diodes are often peddled as offering a significant reduction in EMI. In practice they don't really make much difference - at least not enough to justify their steep cost. In fact typically, the faster a diode, the greater are the reverse current and forward voltage spikes that it produces at turn-off and turn-on. So very fast bridges may in fact produce worse EMI scans.

++ Typical practical values for the inductance of a CM choke in medium-power converters range from 10 to 50 mH (per leg). The DM choke is always much smaller (in inductance, but not in size as we will see). Typical values for the DM choke are 500 µHto1mH.

++ In Fgr. 1, we have shown both the CM and DM filter stages as being symmetrical (balanced). So For example, we have placed identical DM chokes on each of the L and N lines. In Fig. 10-1 we see that in fact the DM choke is also a part of the CM equivalent circuit (and vice versa). And since line impedance imbalance can cause CM noise to get converted into DM noise, it’s always advisable to keep both the CM and DM stages symmetrical (balanced).

++ One obvious way to maintain equal CM inductances in both lines is to wind them on the same core (e.g. a toroid). That automatically assures a good inductance match (assuming of course that there are an equal number of windings per leg).

Note that if we are winding the CM choke ourselves (as during prototyping), we must note the relative direction of the windings, as indicated in Fgr. 1 (see third sample choke picture). With such a winding arrangement, the magnetic field inside the core will cancel out completely (in principle) for DM noise. Similarly, the flux due to the operating ac line current will also cancel out (that too being differential in nature). Therefore the choke will present an impedance only to the CM noise component.

Note: The reader is cautioned that there are several widely used but confusing symbols for the CM choke found in schematics in related literature. But whatever the symbol, as long as it’s meant to serve as a common mode choke, the direction of the windings must be as shown for the toroid in Fgr. 1.

++ If we reverse the current direction in one of the windings of a CM choke, then it becomes a DM choke (for both lines). However, now it’s also subject to the flux produced by the ac line input current (no cancellation occurs). DM chokes, in general, should always be put through a "saturation check" - because of the impedance they present to the line current.

++ We see that DM chokes may need to be quite large, just to avoid core saturation - despite the fact that their inductance is usually much less than that of CM chokes.

But in fact, a CM choke can also be very large. That, however, is primarily necessitated not by the typically higher inductance required, but more so by the desire to provide the required inductance with the minimum amount of copper losses.

So, a core with a high AL value is sought, and that usually spells "bigger core." We should also keep in mind that we don't want the core to "topple over" and saturate, on account of small imbalances in symmetry of the windings. So we may ultimately need to oversize the CM choke for various such reasons.

++ Theoretically, there is no need for any air gap in a common mode choke, because the flux due to the line current is expected to cancel out completely. In practice, it doesn't fully, mainly due to slight differences in the individual winding arrangement (despite the equal number of turns). At a minimum, this causes the core to get dc-biased in one direction, and thereby cause an imbalance in the inductance it presents to the two lines. This would expectedly degrade the EMI performance, but in extreme cases, the core may even saturate. Note that core saturation in the filter is clearly not a catastrophic event (like the saturation of the main inductor/transformer of the converter can be), but since it’s accompanied by severely worsening EMI-suppression efficacy, we need to prevent that too. Therefore, as in a forward converter transformer, a small air gap is usually present, even in a CM choke.

This may be an actual air gap (between split core halves), or it may be a distributed gap, as in powdered iron cores. Though this lowers the inductance index ( AL) somewhat, the resulting solution is much more immune to production variations, and is also more stable over time. In general, whenever we introduce an air gap, the core starts partially acquiring the properties of the interposing air - and since air never saturates, the air-gapped core too has a much softer saturation characteristic.

++ We can consider spending some more money and avail ourselves of magnetic materials like "amorphous" cores or 'Kool Mu' if we want to achieve higher inductance (with higher saturation flux densities), in a smaller size.

++ Toroidal CM chokes in particular, when used in off-line applications (i.e. with both windings on the same core) must meet safety requirements relating to the separation distances between the windings ('clearance' and 'creepage,' as discussed later). So For example, we cannot simply wind the two windings carelessly overlapping each other - we need to maintain a specified physical separation. Nor can we just use a bare toroid core to wind them on - we need an approved coating and/or a suitable bobbin.

++ A bare ferrite can be a rather good electrical conductor, especially if it’s the more commonly used manganese-zinc ferrite (as opposed to nickel-zinc formulations).

This can be confirmed by simply pressing the tips of an ohmmeter at two points on the surface of any bare ferrite lying around in the lab. Further, if we are trying to rely on the enamel coating of a typical copper magnet wire to protect from shorts, we should know that the coating is considered to be just operational/functional insulation, and is not considered to be even basic insulation.

++ Note that Lcm in Fgr. 1 is the inductance of each leg of the CM choke.

Therefore, it’s the inductance measured across either winding, with the other winding open. Now, if we repeat this measurement, but instead of keeping the other winding open, we short its ends together, what we measure is the leakage inductance Lk. By definition, the two leakage inductances in each leg are uncoupled, and therefore they cannot be sharing any magnetic path. Therefore the leakage inductance of a CM choke behaves differently from the rest of the choke - differential currents no longer cancel out for this inductance. In effect, Lk presents an inductive impedance to DM noise. This "hidden" inductance of a CM choke has been successfully exploited by filter designers, to serve as an "unintentional" DM choke. Therefore, in low-power converters, we usually won't see any separate DM chokes - just CM chokes. The good news here is that the leakage inductance is effectively an air-cored inductor, so it never saturates - even if for some reason, its "parent" CM choke saturates completely. Thus the efficacy of a leakage-based DM choke is maintained at any supply current level.

Note: In any transformer, if we measure its leakage inductance (by shorting the secondary winding), the reading remains virtually unchanged even if we remove the core completely from the bobbin. That is because leakage, by definition, is uncoupled and does not pass through the magnetic core - if it did, it would be "coupled."

++ The inter-winding capacitance of a choke affects its characteristics significantly at high frequencies. This can be intuitively visualized as providing an easy detour for noise to simply flow past the windings. To minimize the end-to-end capacitance of a toroidal winding, it’s recommended that the winding be single layer. Also, in Fgr. 1, the sample CM choke picture in the middle is better than the one to its left, in terms of minimizing the end-to-end capacitance. That is because of the split introduced in each winding section by the special bobbin used. The split also helps increase the leakage inductance (which helps reduce DM noise). Bobbins with several such splits are also available at a price.

++ Line to line capacitors are called 'X-capacitors' ("X-caps"). X-caps when used in off-line applications before the input bridge must be safety approved. But after the bridge (on the rectified side), it's basically a 'don't care' situation from the safety point of view. Note that since it’s essentially a front-end component, approved X-caps are typically impulse-tested up to 2.5 kV peak.

++ Line-to-earth capacitors are called 'Y-capacitors.' Since Y-caps are critical in terms of having the potential to cause electrocution if they fail, approved Y-caps are typically impulse-tested up to 5 kV peak. Note that Y-caps used anywhere on the primary side (in off-line applications) must always be safety approved. Depending on the location in the power supply, we may even need two Y-caps in series (basically corresponding to double insulation). However, sometimes we can also find Y-caps placed between the secondary ground and earth/enclosure (for EMI suppression purposes). In this position, it’s usually acceptable to use any ordinary 500 V ac rated capacitor (unapproved).

++ Traditionally, off-line X-caps were of special metallized film + paper construction whereas Y-caps were a specially constructed disc ceramic type. However we can also find X-caps that are ceramic, as we can find Y-caps that are film type. It's a choice dictated by cost, performance, and stability concerns. Film capacitors are known to always provide much better stability over temperature, voltage, time, and so on - than most ceramics. In addition, if they are of 'metallized' construction, they also possess self-healing properties. Note that ceramic capacitors don’t have any inherent self-healing property. However, ceramic Y-caps are specifically designed never to fail shorted under any condition, as this would pose a serious safety hazard.

++ If for any reason (e.g. filter bandwidth or cost) ceramic is preferred for the Y-cap positions, then we need to carefully account for its basic tolerance, its variation with respect to temperature and applied voltage, and all other long-term variations and drifts. That is because we need a certain filtering efficacy, but at the same time we can't increase the leakage current into the chassis. In this regard, we should keep in mind that the capacitance stated in the datasheet is not just a nominal (or typical) value, but in fact it happens to be a fairly misleading value. For example, the fine print may reveal that the test voltage at which the capacitance is stated is close to, or equal to zero Volts! So the actual capacitance it presents in a working circuit may be very different from its declared value. This is, in general, especially true for ceramic capacitors that use a high dielectric constant ("high-k") material (e.g. Z5U, Y5V, and so on). We should also know that ceramic capacitors age, except for COG/NPO types. A typical X7R capacitor ages 1% for every decade of time (in hours). So its capacitance after 1000 hours will be 1% less than what it was after 100 hours, and so on. Higher dielectric constant ceramics like Z5U can age 4 to 6% for every decade of time. So in effect our filter stage, too, gets less efficacious with time. And we need to account for this in the initial design.

++ Theoretical filter performance is based on the assumption that we are using "ideal" components. However, real-life inductors are always accompanied by some winding resistance (DCR) and some inter-winding capacitance. Similarly, real capacitors have an equivalent series resistance (ESR) and an equivalent series inductance (ESL).

At high frequencies, the inductance will start to dominate, and so a capacitor will basically no longer be functioning as one (from the signal point of view). However, capacitors with smaller capacitances generally remain capacitive up to much higher frequencies than do larger capacitances. See Table 10-1 for some typical self-resonant frequencies (the point above which, capacitors start becoming inductive). Therefore, quite often, a smaller Y-cap may help, where a large Y-cap is not yielding results.

We can also consider paralleling a larger value Y-cap with a small Y-cap.

Table 10-1: Practical limitations in selecting components and materials for EMI filters X-Capacitors Y-Capacitors Capacitance (pF) Resonant Frequency (MHz) Capacitance (µF) Resonant Frequency (MHz) Magnetic Materials for EMI Chokes Powdered Iron Ferrite

++ Surface mount ("SMD") versions of off-line safety capacitors are also now appearing - For example from Wima, Germany (http://www.wima.com) and Syfer, UK (http://www.syfer.com). But we must realize that it’s not enough that the capacitor merely 'complies' with a certain safety standard - the capacitor should actually be approved (tested by various safety agencies, and carrying their respective certification marks). From the electrical point of view, one of the great advantages of SMD components is their virtually non-existent ESL. This improves their high-frequency performance in any filter application. On the flipside, some ESR or dc winding resistance ("DCR") is often useful in helping damp out oscillations.

Without any resistance altogether, oscillations would last forever. That is one of the reasons why engineers sometimes pass one or both of the leads of a standard through-hole Y-cap through a small ferrite bead (preferably of a material with lossy characteristics, like Ni-Zn). This can often help suppress a particular high-frequency resonance involving the Y-cap, which is showing up in the EMI scan. But we must be careful that in doing so, we are not ending up with a radiation problem instead.

++ Designers of low-voltage, low-power dc-dc converters may find the "X2Y" patented product range available from Syfer (and from the company X2Y itself - at http://www.x2y.com) very useful if they need to miniaturize and lower the component count. This is a three-terminal integrated SMD capacitor-based EMI filter that simultaneously provides line-to-line and also line-to-ground decoupling. Picor (a subsidiary of Vicor) at http://www.picorpower.com is also now selling what is billed as the industry's first active input EMI filter stage for standard 48 V bricks.

It may be a viable choice if board space is at a premium, despite its roughly $20 cost.

++ We note that a Y-cap is always tested to higher safety standards than an X-cap.

So we can always use a Y-cap at an X-cap position, but not vice versa. For example, we can consider placing a ceramic Y-cap in parallel with a film X-cap, so as to improve the DM filter bandwidth.

++ Generally, we try to maximize filter performance by increasing its 'LC' product as much as practically possible (thus lowering its resonant frequency). Further, given a choice, we would prefer to harness that improvement by using larger capacitances, instead of impractically-sized inductors. But as we know, the maximum Y-capacitance we can use is limited by safety considerations. X-caps too were limited for many years to a maximum value of 0.22 µF (though occasionally

0.47 µF was also seen). But that was simply availability and component technology limitations. Nowadays, we can get X-caps up to 10 µF. We should be conscious, however, that large input capacitances can cause undesirably high inrush surge currents at power-up. This may also cause eventual failure of the X-cap, especially if it’s the very first component after the ac input inlet. Film caps can self-heal from such an event every time it occurs, but eventually the capacitance gets degraded slowly over time with each successive event. Therefore, despite EMI concerns, we should try and place X-caps after any input surge protection element - For example, the NTC (negative temperature coefficient) thermistor, or wirewound resistor, and perhaps even after a front-end choke.

Note: What were traditionally called X and Y capacitors are now more accurately called X2 and Y2 capacitors respectively. From the viewpoint of safety regulations (like impulse voltage rating etc.), the X1 and Y1 are considered virtually equivalent to two X2 and Y2 capacitors in series, respectively. For example Y1 caps are impulse tested to 8 kV. Also, the original terms 'X-caps' and

'Y-caps' have recently started getting defaulted to refer to the more uncommonly used (higher voltage) X1 and Y1 capacitors instead.

Note: In off-line power supplies, for better EMI suppression, we may decide to place Y-caps from the rectified dc rails (either one or both) to earth. So sometimes we place Y-caps from primary ground to secondary ground (usually connected to earth ground), or from the HVDC (high voltage dc) rail to secondary ground. In either of these positions, a Y1 capacitor (or two Y2 capacitors in series) may be required.

Note: Safety regulations for Nordic regions (and Switzerland) may require each Y-cap shown in Fgr. 1 to be actually two Y2 capacitors in series (or a single Y1 capacitor). Historically, this has been necessitated by the fact that earthing is poor in those geographical regions. In fact, it used to be pointed out that even the main conference room of the Norwegian safety agency NEMKO (literally Norwegian Electric Material Control) did not have any earth connection available in the wall outlets. Therefore, practically speaking, a lack of earth is not considered a fault condition in many parts of the world, but is just a normal condition (this actually also includes about one-third of homes in the United Sates). Therefore, very often, whether the equipment is supposed to be earthed or not, it’s expected to have reinforced insulation anyway. Earthing, if present, is then just for helping out with EMI. We see that Y1 caps will often find use even in single-phase equipment.

However, X1 caps are basically meant only for 3-phase equipment, since there is no pressing safety need for such a high voltage rating between the L and N wires in single-phase equipment.

Safety Restrictions on the Total Y-capacitance