Electronics Protection - Summer 2014 - (Page 22)

Feature Cooling and Shielding in the Right-Sized Enclosure Walter Podbelski, Director of Enclosures & Component Group Elma Electronic, Inc. Once considered merely a containment vessel for a system's electronics, enclosures have evolved to incorporate more intelligent features and provide overall benefits that contribute heavily to system optimization. Now more than ever, every system designer who has space constraints must consider the classic parameters of SWaP-C - size, weight, power and cooling. Add to that EMC considerations and one has a veritable abundance of design considerations to address. To satisfy the demands of any given environment, and especially as electronics get increasingly smaller, an engineer must consider a myriad of design elements from mechanical constraints, cooling requirements and power distribution, to system monitoring, reliability (MTBF) and maintainability (MTTR). Addressing any one of these issues can be a difficult design task, but balancing these requirements, while hitting a specified cost target under time-to-market pressures, illustrates the importance of experienced packaging design. Two noteworthy areas of enclosure design have seen significant developments in recent years. First is the improved use of the enclosure to better mitigate environmental elements, such as heat and EMI. Next is the ability to incorporate flexible sizing parameters that enable systems to fit into a wider variety of spaces and keep costs down on small volume applications. The Heat is On Increasing density of electronics and faster processor speeds (akin to Moore's Law) has placed tremendous amounts of heat into smaller packages, making the demand for proper cooling a priority. As more systems are designed for mobile use, they are faced with tougher thermal challenges, often having to survive outdoors, where dust and moisture can be an issue. This has led to an increase in the need for conduction cooled packaging. Fortunately, there are standard enclosures available already designed to certain environmental parameters, such as NEMA and IP ratings. Figure 1 illustrates a standard Elma enclosure that can be designed for ratings such as IP65/ NEMA 3 (dust & light water pressure), and Figure 1. Many standard enclosures meet IP68/NEMA 6 (dust & environmental parameters specified in key inwaterproof). dustry ratings, such as NEMA and the IP code. On the back end, thermal simulation software enables the designer to input all of the variables into a program to verify that the cooling for the system is adequate prior to fabrication. Better yet, if time and budget allow, building and testing a thermal mock up unit will give the best evidence that the goals have been achieved. But far before the testing phase, understanding the system's total power dissipation and localized hot spots is critical. Although new technologies such as liquid or vapor cooling are emerging, 22 Summer 2014 * www.ElectronicsProtectionMagazine.com forced air using fans or blowers still cool the vast majority of systems. Knowing the common pitfalls that tremendously impact the overall utility of this tried-and-true cooling method can set designers on the right path from the get-go. For starters, minimize restrictions in the airflow path and employ air baffles or plenums to optimize air flow, thereby eliminating hotspots. Radical bends in the air flow path will impede air movement, for example. And make sure air leakage in fan mounting area is kept low, so the air coming in is directed where it needs to go. The air flow cutout on a fan mounting plate should be larger than the inlet diameter of the fan, and objects in the air inlet area should be located more than 1/2 inch from the fan diameter. Since fans and blowers are measured in static pressure, a good rule of thumb is to select a model that will perform at greater than 60 percent of its "free air" maximum, based on estimated static pressure. Certain tradeoffs need to be made during cooling design, such as the use of a honeycomb filter for EMC that allows more than 90 percent airflow at the opening, but is more expensive than simple perforations. Systems could require the use of an air filter to protect electronics, which can hinder airflow, and if noise attenuation must also be considered, it could conflict with maximizing airflow. There are some simple solutions to mitigating these challenges and achieving an optimum enclosure design. To reduce noise, for example, variable speed temperature regulated fans can be employed. Tachometer output fans can monitor fan fail conditions and increase the maintainability of the system. Bring on the Noise Packaging designers are tasked with ensuring that the developed system won't interfere with, or be susceptible to, surrounding equipment emitting its own electrical disturbances. Shielding effectiveness (dB) is a measure of the degree of attenuation that an enclosure provides. Many application-specific standards, including FCC (class A, B), CISPR class B, CE, NEBS (L3) and MIL-STD-461, ensure equipment compatibility in a given market. A relevant EMC design should not only focus on compatibility with the necessary industry standard, but should address: * Suppression: Reducing the interference at its source * Isolation: Isolating the offending circuits by filtering, grounding and shielding * Desensitization: Increasing the immunity of any susceptible circuits To control EMI, proper shielding requires a step-by-step approach that looks at immunity (susceptibility) and emissions, both conducted as well as radiated. Good design practices to enhance shielding effectiveness (dB) will also consider all aspects of shielding from the walls themselves to the individual components as well as the final assembly. Various methods for EMI protection can be employed in electrical enclosures. For example, integrating shielding gaskets into the enclosure design will provide a continuous conductive/ground contact surface for all removable panels, thereby restricting the aperture size of any seams. These gaskets should be selected based on their attenuation characteristics, RF impedance, material compatibility (for corrosion control), compression range, compressibility and environmental sealing. Metal finger or metal- http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Summer 2014

Editor's Choice
New High Efficiency LED Technology Benefits From Best Practice Thermal Management Design
Passive Thermal Management of Lithium-ion Batteries Using Phase Change Materials
Considerations for Powering Military Applications
How Thermal Ground Plane and Compact Air-Cooled Heat Sinks are Revolutionizing Thermal Management
Access Control Solutions for Railway Infrastructure
20 Data Center Downtime Study Puts Focus on Maximum Protection
Cooling and Shielding in the Right-Sized Enclosure
Enclosures
Thermal
Power
Hardware
EMI/EMC/ESD
Industry News
Calendar of Events

Electronics Protection - Summer 2014