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Choosing and Installing Mains FiltersKeith Armstrong and Tim Williams
To reduce RF emissions and improve immunity, designers must specify the right filter for the job.
System designers often may specify an off-the-shelf mains filter for a piece of equipment, expecting it to perform exactly per the manufacturer's specification. Then these designers are surprised and disappointed when the unit makes little apparent difference to the emissions or immunity they wanted to control or even makes matters worse at some critical frequency. This article looks at two factors that can contribute to this state of affairs: the effect of the impedances on the filter and the effect of the filter's method of installation. Source and Load Impedances The performance of any filter depends heavily on the impedance at its terminals. The four relevant impedances for a simple single-phase mains filter are
Although all of these impedances will be complex and frequency-dependent in real life, most filters have their performance specified by tests done with 50-W source and load impedances, which brings up a very important point—filter specifications are optimistic when compared with their actual performance. Consider a typical supply filter installed between an ac power supply and an ac/dc converter typical of the dc power supply of electronic apparatus. The impedance of the ac supply varies from 2 to 2000 W with both time and frequency, depending on the loads that are connected to the supply, the nature of the supply transformer, and the wiring to the point of connection. When the rectifiers are turned on near the peaks of the supply waveform, the impedance of the ac/dc converter circuitry looks like a low impedance. At all other times it looks like a high impedance. The situation is far from being the matched 50/50-W setup used to measure filter attenuation. Filter specifications employ 50-W source and load impedances because most RF test equipment uses 50-W sources, loads, and cables and because the main specification standard, CISPR 17, requires this usage. For most practical uses of filters, the specifications obtained by this method are at best optimistic and at worst misleading. Filters made from inductors and capacitors are resonant circuits, and their performance and resonance can depend critically on their source and load impedances. An expensive filter with excellent 50/50-W performance may actually give worse results in practice than a less expensive one with a mediocre 50/50-W specification. The Problem of Resonant Gain The most sensitive filters to source and load impedances are supply filters with a single stage. They can easily provide gain rather than attenuation when operated with source and load impedances other than 50-W. This gain usually appears in the range of 150 kHz to 1 MHz and can be as much as 10 or 20 dB. Therefore, it is possible that fitting an unsuitable mains filter can increase emissions, worsen susceptibility, or both. Filters with two or more stages are able to maintain an internal circuit node at an impedance that does not depend very much on the source and load impedances, so they are better able to provide a performance at least somewhat in accordance with their 50/50-W specification. Of course, they are larger and cost more. The best way to deal with the source/load impedance problem is to use only filters whose manufacturers specify differential-mode (symmetrical) performance for both matched 50/50-W and mismatched sources and loads. CISPR 17 requires that mismatched figures be taken with 0.1-W source and 100-W load, and vice versa. Drawing an attenuation-versus-frequency curve consisting of the worst-case figures from each of these various curves yields graphical data for use as the filter's specification. Figure 1 shows an example of this procedure.
When filters are chosen using this technique to try to meet the predicted or actual requirements, their performance can be as good as or better than expected. When the 50/50-W figures alone are used to predict filter performance, the result is often disappointing. The worst-case method will often end up with the specification of a higher-performance filter, especially if performance below 1 MHz is the main concern. But any alternative method would require knowledge of the RF impedances at either port, which is not normally available. Layout and Installation Incorrect filter construction or mounting technique can easily compromise radiated emissions and immunity. Poor shielding can easily compromise conducted emissions and immunity. The correct way to view filtering and shielding is as a synergy where each task complements the other. Locating the Filter A filter is normally located at a zone boundary for two reasons:
In addition to this general rule, filters should be located as near as possible to the apparatus that is expected to be the source or victim of disturbances to minimize the impedance of the connection. If the filter needs to be positioned outside the protected area or apparatus, the wiring between the filter and the protected area should be twisted and positioned close to the earthing structure.
Filter Construction and Mounting The higher the frequency, the more a filter is compromised by RF leakage from its unfiltered side to its filtered side. Many engineers have been surprised by the ease with which RF will leak around a filter. At the point where an external cable to be filtered enters a shielded enclosure or room, a filter should be fixed into the metal wall and RF bonded to the metalwork around the aperture. Through-bulkhead filters are the best since they maintain the integrity of the shield, but they are often expensive to purchase and install. For mains supply filters, the IEC-320 inlet is the most common commercial style of bulkhead filter for up to 10 A, single phase, 230 V rms. Because of commercial pressures, most mains filter manufacturers only specify their parts over the frequency range of the conducted emissions tests (up to 30 MHz). The filter becomes progressively less effective above 30 MHz and can compromise the shielding integrity of shielded enclosures, possibly causing problems with radiated electromagnetic disturbances. Segregating the layout inside the enclosure will minimize high-frequency coupling onto the internal (filtered) supply and hence control the potential for radiated electromagnetic disturbances. For high currents most commercially available filters use screw-terminal block or Faston connections, making bulkhead mounting impossible. Figure 2 shows how to mount a screw-terminal filter using the "dirty box" method. This procedure encloses the filter in an individually shielded, segregated box within the main shielded enclosure. The filter input and output cables in the dirty box must be very short and faraway from each other. Unfortunately even this may not prevent high frequencies from leaking across the cables, so ferrite sleeves may be needed on either one or both of them.
Filters sold as "room filters" generally combat the problem of leakage caused by cables with a filtered side, which is enclosed in a metal filter box. The cables pass through their mounting base via a standard circular conduit fitting. Such filters are intended for mounting directly onto the external metal wall of a shielded enclosure (any size) with only their filtered output appearing on the inside of the enclosure. This construction effectively shields the unfiltered cables from the filtered cables, thus allowing the filter to function effectively up to the highest frequencies. This is the method used by most of the supply filters intended for EMC test chamber applications. The Earth Connection All commercial filters are housed in metal bodies of some sort, with the body forming the filter's earth connection. An IEC inlet filter with a metal body installed within a shielded enclosure can only provide good attenuation at frequencies above a few tens of MHz if its body has a seamless construction and is RF bonded to the shielding metalwork. The same is true for any other metal-bodied filter, because any inductance due to a wired earth connection will turn the filter into a high-pass configuration. The greater the inductance—that is, the longer the wire—the lower the frequency at which this effect becomes significant. Figure 3 shows the measured effect—about 25 dB difference at 15 MHz—of two different lengths of earth wire to chassis on the attenuation of a simple single-stage mains filter.
Bonding the case directly to the chassis earth is the only sure way to realize the attenuation performance specified by a filter's manufacturer. The often-provided separate tag to the case is for safety purposes, not for use as an EMC connection. Wiring to Filters Filtered and unfiltered cables must be strictly segregated as they are always at least one class of cable apart (see IEC 61000-5-2 on cable classification). The rules on segregation generally call for between 150 and 300 mm separation distance between adjacent classes and 600 mm or more between the most sensitive and the noisiest classes. Of course, this may not be possible at the filter terminals themselves, because the filter body may be smaller than this separation distance. But in situations where there is no screen across the filter, input and output cables should be carefully dressed away from each other as they leave the filter terminals until the required separation distance is reached. Note that conduit may be needed close to the filter. If a filter must be installed in-line in a cable tray or conduit acting as a parallel earthing conductor, then all the cables in that conduit must be filtered. Otherwise, coupling across the cables will compromise the high-frequency attenuation. Under no circumstances should the installation technician be allowed or encouraged to intertwine the input and output wiring. This desire for neatness (Figure 4) is going too far and will render an expensive filter almost worthless.
Conclusion Off-the-shelf filters are a convenient and cost-effective way to reduce RF emissions and improve immunity. Proper specification and installation are critical to ensuring these filters meet expectations for performance. Keith Armstrong is a founding partner of Cherry Clough Consultants (Denshaw, Oldham, UK), an independent firm specializing in EMC and safety. He may be contacted via e-mail at karmstrong@iee.org. Tim Williams is employed with Elmac Services (Chichester, UK), and he may be e-mailed at elmactimw@cix.co.uk. This article is based on an excerpt from the authors' book, EMC for Systems and Installations, soon to be published by Butterworth Heinemann. |
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