Cost containment: EMC design for universal compatibility delivers savings
Neil details an enclosure design that makes selecting a lower-cost frame possible while at the same time bumping up the degree of EMC protection over traditional designs.
Effective electromagnetic shielding of enclosures has always been a design challenge, reflected in the added cost to enclosure customers.
In contrast to simple construction for conventional standards, an enclosure designed for universal compatibility must meet more stringent regulations, while at the same time offering cost advantages. A focus of this universal design concept is cost-effective electromagnetic shielding.
Creating the quiet environment
Enclosures are a means for creating a quiet environment in which to operate electronics, and the shielding that enclosures offer allows components to communicate at the lowest possible power. Using the lowest possible power quiets the overall environment and makes the full radio transmission spectrum available for more precise and compact data transmissions.
The enclosure has to offer complete protection against mechanical interference. EMC shielding against high-frequency radiation must also be present. Depending on the application, designers choose to shield either the individual plug-in units, chassis, or cabinets. With especially high demands on the shielding, several areas are usually shielded at the same time in order to achieve a higher degree of shielding protection. EMC shielding always means additional design considerations, and the key is to achieve maximum shielding at the lowest possible cost.
The most visible and obvious component of the EMI enclosure system is an electrically conductive skin connected or grounded to earth. Electromagnetic radiation (EMR), which is an oscillating electromagnetic field, induces a current in conductive materials, which creates a balancing and canceling electromagnetic field. By grounding the cabinet, all leftover currents travel through the enclosure and dissipate harmlessly to ground.
EMI enclosures are most commonly built from solid metal sheets, perforated metal sheets, expanded metal, metal wire mesh, or some combination of these. The two most common enclosure materials are aluminum and steel. Both steel and aluminum absorb high-frequency RF very efficiently. In low-frequency applications, steel has a higher permeability and lower conductivity and performs better than aluminum. For indoor environmentally controlled installations, steel is the obvious choice. It has high shielding effectiveness at all frequencies, even in thin sheet thicknesses. For outdoor or harsh environments, aluminum’s corrosion resistance make it a better choice. The small sacrifice in EMI effectiveness and higher cost is outweighed by aluminum’s ability to withstand weather extremes. To a much lesser extent there are specialty metals like Monel, or materials like conductive plastics and conductive paint that can be used in special design situations. These materials are costly.
Conductivity is a component of skin effect, the tendency of alternating current to distribute itself so that it is more dense near the surface of a conductor than at its base. Skin effect is controlled by skin depth, the minimum material thickness required to most fully absorb a particular EMR. Permeability, conductivity, and radio frequency together can be used to calculate a skin depth. In the 10 GHz range, skin depth approaches the micrometer scale for most conductive materials. The effects of permeability and conductivity play an ever smaller role as frequency goes up, and most metals will almost fully absorb high-frequency radiation with a foil thickness.
Filling the gaps
Gaps in the enclosure’s conductive surface will allow EMR to leak out. The larger the gap, the larger the range of frequencies that leak. The perfect EMI enclosure would be a thick steel ball, perfectly round and completely smooth without seams. There is no way for an EM signal to get in or out. This, of course, is an impractical solution. Users want hinged enclosure doors, removable panels on all sides of a free standing structurally stable frame, conductive cables running in and out of the enclosure, plus unimpeded airflow for thermal management. Whatever the design needs, they must be accommodated without greatly reducing the EMI performance of the enclosure.
How does an engineer design a structurally freestanding enclosure with easy access and free flowing air, without creating EMR gaps? Conventional cabinet shielding concepts require conductive contact between frame, inner assembly, and covers. This approach demands costly pretreatment of the frame with special conductive finishes.
Contrary to conventional methods where the EMC gaskets are put on the enclosure covers and contact one another over a conductive coating, the Schroff VARISTAR cabinet design conducts interference directly into the covers without entering the frame. This design allows the use of a lower-cost frame and provides a higher level of EMC protection than traditional designs.
The VARISTAR cabinet incorporates a shock- and vibration-resistant tubular rolled steel section frame. Its mounting grid has a pitch of 25 mm and a symmetric 45° angle. This 45° angle is the basis of the new sealing concept for the EMC and Ingress Protection (IP). A conductive textile sealing material is mounted directly on the angle of the frame, and interference is carried from the cover via the seal directly to the next cover.
The shielding system creates a Faraday cage, and the frame itself is not part of the system. This shielding concept allows the use of a cost-effective standard frame, without the need for pretreatment. As an added benefit the cabinet’s shielding effectiveness is rated at 60 dB at 1 GHz and 40 dB at 3 GHz in tests conducted according to IEC 61587-3.
Figures 1 and 2 depict the test results. The cabinet exceeded the sensitivity of the test apparatus at the frequency range from 0-300 MHz. An additional test was required for this range of frequency using equipment with increased sensitivity. Both tests have been superimposed on the same plot to illustrate the true shielding effectiveness of the VARISTAR platform. The solid-panel configuration test is shown in blue.
The structural frame of the VARISTAR design is common to all versions of the cabinet. The frame requires no special finishing and remains the same, with or without gasketing and with or without HF shielding.
Neil Petraca is Sr. Mechanical Engineer, Electronic Cabinetry at Pentair Electronic Packaging.
Schroff a Brand of Pentair Electronic Packaging