In complex equipment or racks where multiple functional modules are mounted in one unit EMC and compliance with the whole range of standards can be a major problem. And even if a complex system is assembled from equipment which is CE marked itself and fully compliant to its own standards when connected together the whole assembly is not compliant. Put simply, CE plus CE does not necessarily equal CE.

This then raises a question of tactics. Clearly the final assembly must be tested for compliance. But when a problem is identified, is it best to deal with the assembly as a whole, say by fitting a high-current filter to the complete equipment, or should one locate the individual item that is causing the non-compliance and place a filter on its individual mains supply.

The answer to this is not easy: the choice between so-called distributed filtering (where each individual module has dedicated filters or filtering elements, chokes or capacitors) and combination filtering (where the whole of the unit is filtered at the first point of entry of the supply).

In many cases the combinational approach may be the simplest. Manufacturers have extremely wide ranges of general-purpose mains filters, which combine common- and/or differential-mode chokes, X capacitors and Y capacitors mounted in a sealed casing. Indeed, this is often the approach for "last-minute" compliance, when the OEM discovers that CE plus CE does not necessarily equal CE. And in some instances, where EMC was not considered early enough in the design cycle, this is the only possible answer, to the extent of requiring a custom designed filter to overcome an unforeseen problem.

Fortunately, today designers are rather more aware of the necessity of compliance and consider such implications earlier in the cycle, allowing the consideration of a distributed filtering approach. This is particularly apt in applications where the power architecture of the equipment is also distributed, allowing problems to be addressed at source.

The biggest advantage of being able to take this approach is cost: the cost of individual capacitors, chokes, ferrites or even lower current filters etc can be considerably less than a higher current, higher performance filter for the whole system. What's more, it may be easier to locate a particular element, capacitor, choke etc close to the noise source than a much larger filter.

PCB-mounting EMI filters can be a quick-fit, simple and particularly compact solution to interference. However, their limited size and mass do create a trade-off: such devices typically offer only a single attenuation stage and have a restricted maximum current rating (maximum rating 6.5A).

PCB filters contain compact inductors (in the form of toroidal chokes) together with capacitors, all potted in a plastic or metal case. Metal cases are electrically connected to earth. Care must be taken especially with metal-cased filters to ensure that a good earth path capable of handling high frequencies is provided to avoid coupling via the stray capacitance that exists between the input and output sides.

There are two distinctly different types of applications, which require different layout strategies. In the simpler (and most common) case, the whole PCB lies within the same interference zone. Here, all the functional elements are operated from the same supply voltage, and so decoupling between the operational elements is not necessary.

The filters must be positioned at the external connections to the circuit board: and it is important to ensure that all the connections are taken into account. The position of series inductors and capacitors to earth should be the same in all the filters. A resistor can be used in place of an inductor in the case of small-signal currents.

A more complex case is where a single PCB has various interference zones. This would be typical where electronic devices such as microprocessors, op amps, ADCs etc are mounted on the same board as components such as electromechanical relays, fast solid-state power devices (inverters, ultrasonic generators, HF generators) and mains applications. Here, the filters must be positioned at the crossover points between the various zones.

As before, care must also be taken here to ensure that all the conductors running from one zone to another are filtered. In addition, careful routing of the earth connections is essential. Interference currents conducted to earth through Y-capacitors must be able to flow back to their source without influencing (electrically or inductively) any circuit where small signals are present.

The dual function of a filter must be taken into account when earthing. In the case of a mains application the filter has not only a protective conductor function but it also serves as a high-frequency earth and screen connection.

To prevent unwanted resonances and suppression drop-off at higher frequencies the earth pin must make direct contact with a solid earth surface. This can be a special position on the circuit board or a large area of copper. The width of the current path for HF currents should be equivalent to at least 33% of the length.

Also, care should be taken to ensure that no coupling can occur between the traces leading to and from any filter. Parallel traces will cause capacitive coupling; conductor loops can appear to be inductive. The area of conductor loop through which current flows must be kept to a minimum.

Mounting PCB filters is much like any standard component, in that their connection pins are soldered into the circuit board thereby fixing the filter in place.

Adequate clearance between a filter and adjacent components must be allowed aid air circulation when a filter is loaded beyond 50% of its nominal rating or if it is to run at an ambient temperature higher than 45C.

Suppression is often difficult in equipment that does not have a protective earth, and it is essential for interference currents to be returned to the source by the shortest possible route. As the current cannot be fed back to the mains via the filter housing and/or Y-capacitors, the suppression must be carried out directly on the device causing the interference.

This is standard practice with brush motors, where the brushes are connected through small HF chokes. Y-capacitors on the mains side of these chokes are then connected to the stator assembly, making the interference current path a closed circuit. The stator assembly is doubly insulated from all touchable parts.

The procedure is the same for circuit boards in plastic-housed equipment with protective insulation. Here, an earth plane is created on the circuit board, and both the plane and any parts directly or indirectly connected to it must be doubly insulated from the outside of the equipment. The earth connection on the filter can then be connected to the internal earth so that HF interference is returned to the source via the Y-capacitors in the filter and the capacitance of the earth surface to the source.

In many cases, PCB filters are best used in combination with other components. The filter takes care of the basic suppression while the complementary components can handle special requirements.

For example, symmetrical pulses up to 2kV are critical. They are barely attenuated by the capacitors used in the filters because of the low impedance. The pulse therefore endangers not only the filter capacitors but also the circuitry following the filter.

Protection can be achieved using varistor. A fuse must, however, always be inserted between the varistor and the mains. If a varistor is connected to earth then a surge suppressor (spark-gap type) should be connected in series with it to prevent constant loading of the varistor by leakage currents.

As another example, asymmetric attenuation can be optimised at low frequencies with an additional current-compensated choke stage. An X-capacitor or one wired in parallel would only be beneficial if the impedance of the symmetrical source of interference was relatively high.

A further possibility is to connect PCB filters in series. This can often result in a more compact and lightweight solution than one using larger inductors. However, care must be taken with decoupling and it may even be necessary to screen between the sections before and after the filters. Also, the permissible leakage current must not be exceeded through the supplementary Y-capacitors.

So, by combining compact lightweight PCB-mounting filters with appropriate supplementary components, and applying good EMC design practice, all the benefits of multistage filtering can be fully realised, usually at a much reduced overall cost compared with a combinational approach.

Each manufacturer must evaluate which route to compliance they will follow taking in to consideration all the above points. Once the route is defined the test plan should be established to ensure that where approval is sought repeatability is possible.

As appeared in "Electronic Product Design", December 2001.