Conformity Magazine - November 2007 - (Page 48)

This article proposes a new approach that involves using a step-by-step simulation procedure, similar to that developed for thermal analysis, to evaluate EMC at the early design stages. The approach begins with rough models that are used to identify problem areas and progresses to increasingly more detailed models that are used to make design decisions. Need to Address EMC Issues Earlier Most electronics and OEMs and component suppliers have long ago accepted the need to identify and resolve thermal issues in the early stages of the design process. They have in large numbers adopted software that performs component- and system-level analysis to address thermal management prior to physical testing, with the goal of avoiding additional design iterations. On the other hand, the main approach used to address EMC issues prior to the prototype stage has been the use of rules of thumb that are becoming inappropriate as higher frequencies become commonplace. At high frequencies the wavelength becomes comparable to aperture dimensions, causing increased leakage of electromagnetic fields in and out of the enclosure. Cavity resonances occur inside the enclosure, which further increases the emissions. Metal components such as heat sinks placed directly on switching components (ICs) can turn into very efficient antennas. The trend towards integrating wireless capabilities presents further challenges as intentional radiators are designed into systems. The result is an increasingly high failure rate during EMC compliance testing. The cost of design changes typically increases by an order of magnitude as the design advances each stage in the process, from conceptual to detailed to test and validation. Often, expensive fixes on the existing design are the only alternative at the late stage when problems are discovered. The increasing emphasis placed on thermal design at the early design stages has inadvertently increased EMC problems. Thermal design often conflicts with EMC design, so fixes that are implemented to address thermal concerns often exacerbate or create EMC problems. The most obvious example is that thermal design requires large holes to enable adequate airflow while EMC design requires small holes to reduce emissions. Another example is that thermal connections (metal studs) are often added to conduct heat away from a hot component. For example in a small module where forced cooling is not available, it may be necessary to conduct the heat to the enclosure. Switching currents can couple to the stud and cause it to radiate like an antenna. In thermal design, a large surface area is often used to increase convective heat flow. The same large surface areas may also increase capacitive coupling to the enclosure, which enables displacement current to flow from components such as heat sinks to the chassis and then onto cables connected to the chassis. Figure 1: The design of 2U servers shows how thermal and electromagnetic compatibility simulations can use the same model and progress together. 48 Conformity noVEmBEr 2007 http://www.ddbunlimited.com http://www.ddbunlimited.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - November 2007

Conformity - November 2007 Contents Editor’s Note Commission Sets Date, Terms for 700 MHz Auction FCC Reaffirms Roaming Obligations Commission Moves to Improve USF Management FCC Proposes Fines for Violations of E911 Handset Requirements Commission Levies $1 Million Plus Fines for Unsolicited Fax Advertisements GR-3150 - Lithium Battery Certification Levels Based on Criteria for General Product, Safety, and Performance, Part 2 ESD Open Forum Machinery Field Evaluation for the United States Validation, Verification, and Immunity Testing Techniques for EMC for Functional Safety Thermal/EMC Simulation Addresses New Mechanical Design Challenges Product Safety Equipment Buyer’s Guide FCC Releases Report on Telephone Numbering Resource Utilization Canada Still Has PCS Spectrum to Assign Updated Standards List for the EU’s Medical Devices Directive Updated Standards List for EU Directive on Active Implantable Medical Devices Call for Papers for the 2008 EOS/ESD Symposium Announced Mobile Phone Use in Hospitals May Be Dangerous CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Product Reviews Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - November 2007

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