This article is the 3rd and final part of a three-part Filter Installation Issues series.
Read Part 1: Input and Output Conductors here.
Read Part 2: The Synergy of Filtering and Shielding here.
There are now many suppliers of PCB-mounted shielding-cans that can be used with three-terminal filters, and they have many types that can be automatically assembled like any other SMD component.
Part 2 of [1] has more details on these shield-cans, and also describes a number of different PCB layouts appropriate for filtering off-board connectors. An example of one of these layouts is shown in Figure 1.
Where filters must penetrate the shield of a product’s overall enclosure, and PCB-mounted components are not suitable, more traditional feedthrough or ‘bulkhead-mounted’ filters in metal cases are the best.
A point to watch out for is whether the metal cases of such filters are seamless – good filters are enclosed in what are actually well-shielded enclosures themselves.
Filters that have metal cases with apertures, seams or gaps in them give poor attenuation at high frequencies regardless of what their data sheet says, because they compromise the attenuation of the shielded enclosure they are assembled/installed onto.
‘Chassis mounted’ filters include types with screw terminals, spade or blade terminals, or flying leads (Figure 2 shows some examples of chassis mounted filters with spade terminals) and cost less than proper bulkhead or feedthrough types, but cannot be assembled to shields so as to reduce stray coupling between their inputs and outputs.
The result is that they are not as effective as feedthrough or bulkhead mounting types at higher frequencies, especially above about 10MHz.
Their performance can be maximised by mounting them with multiple direct metal-to-metal bonds to an RF Reference Plane that is a shielded enclosure wall, or at least a very large metal plate, plus routing their input and output cables very close to the RF Reference Plane and keeping them and any circuits or components they connect to very far apart.
However, their performance can be significantly improved by the use of what is known as the ‘dirty box’ shielding technique illustrated in Figure 3.
This figure shows a shielded enclosure, and an example of the correct installation of a traditional high-performance feedthrough filter. It also shows an example of an IEC 320 appliance mains inlet connector with an internal filter.
The important issue with such inlet filters is that they should have seamless metal bodies that make a direct metal-to-metal connection to the wall of the shielded enclosure.
Many manufacturers have fitted mains connectors with built-in filters, relying on their mounting screws and green/yellow safety earth wire to make the necessary electrical bonds, and have found the EMC performance to be almost useless.
As discussed above, the length of the green/yellow safety wire is simply too long, and a problem with most built-in filter connectors is that their mounting screws bear onto plastic mouldings, so they don’t provide any metal-to-metal connections.
The correct way to install such filters is to ensure that an area of the enclosure’s shield wall is free from paint or anodising, and has a highly conductive surface that will be pressed firmly against the filter’s metal body when it is assembled.
Sometimes it may even be necessary to bond the bodies of such filters 360° to the shield wall all around the perimeter of the filter’s metal case, requiring high surface conductivity for the metalwork on both sides of the gasket, and protection from corrosion (see on the right).
When chassis-mounted filters are applied to cables entering or exiting a shielded enclosure, the portion of the cable that enters the enclosure to connect to the filter degrades the attenuation of the filter by causing stray coupling to its other terminals.
This portion of cable also degrades the SE of the enclosure by acting as an accidental antenna (see [2]), especially at higher frequencies. To maximise the high-frequency performance of such filters and prevent degradation of the enclosure shielding, such filters should be installed using the ‘dirty-box’ method illustrated in Figure 3.
The Dirty Box is a five-sided shielded cover that fits over the filter and the external cable entry, within the overall shielded enclosure. It must have metal-to-metal bonds at multiple points between its walls and the wall of the shielded enclosure, spaced apart by much less than ƛ/10 at the highest frequency to be controlled, and covering the entire perimeter of the Dirty Box’s walls.
Conductive gaskets might help reduce assembly time by reducing the number of fixing screws, or might even be necessary to achieve sufficiently good bonding to the enclosure wall.
The filter is mounted inside the Dirty Box, with its input and output conductors kept as short and as far apart from each other as possible, to reduce their stray coupling – but even so the higher frequencies will still couple between them.
If the resulting high-frequency stray coupling is problematic and cannot be reduced by careful cable routing within the Dirty Box, soft-ferrite CM chokes and/or high-frequency feedthrough filters may be needed on either (or both) the input and output cables, fitted at the point where they enter or exit the Dirty Box.
‘Shielded room’ filters are also available, and although intended for EMC test chambers (as shown in Figure 4) they can be used for shielded equipment cabinets as well.
These are essentially screw, spade or blade terminal filters with two Dirty Boxes, one over the input terminals and their conductors, and one over the output terminals and their conductors, to minimise the stray coupling between input and output.
Conduit fittings are usually provided for the filtered side of room filters, to provide shielding for their conductors whilst they enter the shielded room or enclosure. Where the conduit enters the shielded room or enclosure it must
electrically bond 360º at the shield wall, as illustrated in Figure 5.
Shielded cables may be used instead of conduits, as long as they bond 360º at both ends, to the filter’s case and the shielded room or enclosure wall using appropriate glands or connectors.
Figure 6 shows an overview of shielding and filtering at the level of the final system or installation. Where an electrical/electronic product has an overall shielded enclosure, all of the conductors that enter or exit that enclosure must be shielded, and/or filtered, at the point where they enter/exit the enclosure.
There are no exceptions to this rule, whatever the purpose of the conductors, including safety earth wires: metal armour or draw-wires for cables, fibre-optics, or hydraulic hoses; metal pipes for gasses or liquids; metal ductwork for cables, air-conditioning, etc.
Conductors permitted to be connected directly to the shield wall should be so connected, using 360° bonding techniques just as if they were cable shields (see [2]).
Unshielded conductors that are not directly bonded to the enclosure at point of entry/exit must be filtered, taking into account all of the techniques discussed above concerning the synergy of filtering and shielding.
Designing to Prevent Corrosion
All metal-to-metal bonds associated with filters (and shielding), and all conductive gaskets, must be designed to provide low impedance for the anticipated lifecycle of the product, despite the mechanical, climatic, biological, chemical and other physical environments the product is exposed to.
This generally means choosing metals, platings and gasket materials that resist oxidation, and it also means ensuring that the materials in contact are sufficiently close in the galvanic series so that they don’t suffer unduly from galvanic corrosion.
IEC 60950 is a safety standard but provides some useful guidance on these issues, and there is also a lot of information available freely on the Internet.
Effective ‘vapour-phase corrosion inhibition techniques’ are claimed to have been developed in recent years, by Cortec Corporation, and should be investigated, especially where corrosion is a significant problem.
Filters Connected in Series
It sometimes happens that a product is supplied with mains filtering, but its RF emissions are too high (or its immunity too low) for the equipment, system or installation it is used in.
This is often a problem where a large number of identical or similar devices are used in one product or system, for example a number of low-power
inverter motor drives in one industrial cabinet.
Each product may meet the relevant emissions limits individually, but when a number are all operating at once the aggregate of their emissions might exceed the permitted limits.
In such situations it is tempting to simply add another mains filter, which would then appear in series with the mains filters already fitted in the products. Often a single-stage filter is chosen because the filtering requirements are only modest.
The gain problems that can occur with filters with ‘mismatched’ source/load impedances, especially single-stage types, were discussed earlier – but sometimes connecting filters in series can result in resonances that are not present in any of the filters when they are tested individually. So adding the extra filter can sometimes create worse emissions or immunity than before.
Solutions include replacing the original filters in the products with ones that achieve higher performance, or experimenting with different types of additional filters to find ones that work well when connected in series with the filters in the products.
If the circuits of the filters involved (product and additional) are known, circuit simulators such as Spice should be able to predict resonance problems in advance, and guide the choice of appropriate devices.
REFERENCES
[1] Keith Armstrong, “EMC for Printed Circuit Boards – Basic and Advanced Design and Layout Techniques”, February 2007, www.emcacademy.org/books.asp, £47 plus p&p, perfect bound: ISBN 978-0-9555118-1-3, spiral bound (lays flat): ISBN 978-0-9555118-0-6.
[2] Keith Armstrong, “Design Techniques for EMC, Part 2 – Cables and Connectors”, The EMC Journal, May and July 2006, available from www.compliance-club.com.
