For OE designers of today's advanced electronic equipment, two growing equipment protection concerns are thermal management and shielding from electromagnetic interference (EMI). Unfortunately, protection in one area often means forfeiting some performance in the other.
A completely sealed, shielded enclosure would ward off EMI across a wide range of frequencies. However, the need for airflow means there must be openings in the enclosure, which means opportunities for EMI. To prevent EMI while maintaining airflow, the enclosure designer must find a happy medium by selecting the proper EMI filter at the fan exhaust. EMI filters often take the shape of a honeycomb. The size (or waveguide) of the openings affects both airflow and shielding effectiveness, so waveguide is a very important design decision.
Comparison of EMI wavelengths to recognizable objects.
This image compares EMI wavelengths to recognizable objects. On one end of the spectrum, a radio wavelength is comparable to the length of the Titanic. On the other end, the wavelength of an X-ray is comparable to the length of an atom. This is why an X-ray can travel through skin (just as an atom could) but not through dense lead. Similarly, a radio wave can travel through the air but could not be received inside a car without the use of an antenna.
These same principles can be applied to the size of the honeycomb. The larger the openings are, the more susceptible the electronics are to longer wavelengths and lower frequencies of EMI. However, the smaller the openings are, the more airflow will be impeded. Fortunately, adding depth to the openings (the thickness of the filter) improves shielding effectiveness without greatly impeding airflow. Furthermore, certain materials offer more effective shielding than others but are much more expensive (e.g., copper versus aluminum), which can drive up cost significantly for high-volume products. It's a balancing act between cost, airflow, and shielding, and finding the perfect balance for your equipment first requires you to identify the tolerable operating parameters.
As it applies to filter selection, we must look at both the temperature range and the EMI frequencies related to equipment performance.
If the OE electronics designer can identify the ideal temperature range, the enclosure designer can use the wattage of the equipment to determine waste heat generated, as well as the estimated ambient temperature, in order to recommend the proper fan size in terms of cubic feet per minute (CFM) of air moved. The enclosure designer would then subtract the amount of static pressure lost due to the impediment created by the honeycomb. If the resulting CFM output were determined to be sufficient for the temperature range, and the honeycomb waveguide were small enough to limit the frequencies that could put the equipment in danger, the balancing act would be successful.
This is just one example of how enclosure designers can help OE designers protect their equipment with common-sense solutions. As electronic equipment becomes more demanding and is subjected to harsher environments, it's becoming increasingly important for OE electronics designers to collaborate with enclosure designers throughout the design process.
Although the short article invites to see this matters in more detail, a little more info would be appreciated...
Like: Comparing between std. aluminum vs. copper enclosures, can we use thicker aluminum wall enclosures, or using a copper layer or sheet under the aluminum box acomplish a better result? Then, how thick?... Are there any thumb rules regarding ventilation hole sizes/pitch and area, size of the perforated section? Location of perforated areas? and so on. Amclaussen.
This is a new concept for me and I can't help think about the wider implication of extending the principles to portable items and weatherproof vehicles.
In aircraft and outdoor portable units a small bore honeycomb would be vulnerable to water ingress due to capillary action. Exhaust blown air would help remove damp and condensation moisture but any air intake would be vulnerable. There would also be the issue of corrosion in residual trapped water in untreated copper or aluminum honeycomb. This could set some minimum diameter guideline.
The other out-door issue common to aircraft owners, is the minimum diameter of openings that resist the entry of insects. At certain time of the year insects find tubular openings that they can enter. They make silky nests for their larvae to grow in and can obstruct critical airflow. The maximum diameter seems to be about 0.125", which must be too small for a wasp's whiskers and head.
In common households, and maybe some industrial environments, there is the problem of dust accumulation. Think of the PC CPU cooling fins that get clogged with dust (and fine cat hair if you are a pet lover). Again with input airflow, smaller orifices attract more significant obstruction over time.
So there seems to be some mechanical Max/Min guidelines to consider in a non-sterile existence.
I wonder what has been the experience of the Laser printer manufacturers with their honeycomb blocks. This maybe synthetic, thermally and electrically non-conductive material though. Would plated plastic work?
Almost everything the military and NASA designs is in a metal enclosure afterward. It is fairly common practice to do so, I have found. I place a lot of my more sensitive projects inside a metal box routinely. So often the 60hz at the wall has been an issue with my testing, it was a necessity after.
I agree that sometimes a completely enclosed metal box must be used. In these cases, more expensive heat pipes and heat sinks can also be used for thermal management.
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