Conformity Magazine - July 2008 - (Page 46)

supplies, the microprocessor modules, and the analog input circuits. Selecting the Shielding Material Now that we have a handle on what constitutes a shield, we need to look at the shielding materials. For most shielding needs, a thin conductive shield will be adequate (low frequency magnetic field is the only notable exception). Most metals provide ample shielding effectiveness. But for various reasons, plastic is often the enclosure material of choice, and this means making it conductive. Conductive plastics (plastic loaded with metal filaments) have been available for many years, but have not achieved widespread use. The shielding effectiveness is usually adequate, but it is hard to get the necessary conductive contact at the boundaries. More commonly, a conductive coating is used, such as electroless plating or vacuum plating. This provides you with good shielding, but does require that the mating surfaces conductively close all around the perimeter. In order to work, you need to design the mating surfaces so that they are rigid enough to make contact without excessive flexing, and you need to bring the conductive material right up to where the surfaces mate. A simple dovetail requires bracing to keep the mating surfaces straight; tongue and groove works better. Whichever method you use, continuous mating is key, and may require EMI gasketing to finish the job. This isn’t as onerous as you might think with the gasket selection available today (all cell phones are gasketed!). If you are using a conductively coated plastic for the outer skin, the coating provides numerous discharge points for ESD. If the shield is well done and complete, this is of little consequence, but if the shield terminations are haphazard, discharge currents can be troublesome. Generally, this is a problem only where the plastic is exposed to direct touch, and this is usually only with the outer shell. Planning Your Shield Now that we have the shielding basics, we need to decide where to put the shield. If you can shield the entire enclosure with an icebox, you can effectively eliminate emissions and RF susceptibility, leaving only internal self compatibility to worry about. In practice, this concern is a good start with designing your internal shield. Lacking a good outer shield, you will need to implement one or more internal shields, along with shielded or filtered cables. You might choose to shield the power supply, the microprocessor, and the analog section, an approach shown in Figure 2. The question is what do we need to shield. Any electronics present a potential problem. You need to start by identifying the critical circuits, including internally noisy sources and circuits that are vulnerable to external interference. For emissions: • Clocks, data and address busses • Video data • Switching power devices of all types, including PWM drives • SCR and Triac power controls • Florescent lamps • RF heaters Figure 1: Entire enclosure shielded and wire penetrations shielded or filtered Frequency 1 MHz 3 MHz 10 MHz 30 MHz 100 MHz 300 MHz 1000 MHz Table 1 λ 300 m 100 m 30 m 10 m 3m 1m 30 cm λ/20 15 m 5m 1.5 m 50 cm 15 cm 5 cm 1.5 cm Figure 2: Individual modules shielded and wire penetrations shielded or filtered Conformity JULy 2008

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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