Designing Electronic Enclosures: A Balancing Act
Like the performer in a high wire act, balance is an important element of success in electronic enclosure packaging. The more the performer stacks on his shoulders, his balancing staff, or his arms, the more he needs to make critical shifts in other areas to maintain his equilibrium. Its similar in packaging design particularly when space is at a premium. A modification in one area can require changes in another to balance out the optimal design.
In order to satisfy the demands of a given environment the engineer must consider: mechanical constraints, cooling requirements, EMI/RFI restrictions, shock/vibration, power distribution, cable management, system monitoring, high availability (HA), reliability (MTBF), maintainability (MTTR), and a wealth of other concerns. Addressing any one of these issues can be challenging, but it is the balancing of these requirements, while working to hit a specified cost target under time-to-market pressures, that illustrates the importance of packaging design expertise.
Making sacrifices
Whether the unit is to be desktop, portable or mounted in a 19” equipment rack the engineer needs to be acquainted with applicable IEEE and IEC standards to ensure that the final design will be compatible with other off-the-shelf equipment. One ongoing trend in system design is trying to achieve higher performance in less space. Clearly the chassis must be able to accommodate all of the components necessary to the system to include: circuit cards, backplane, power supply, cooling system, AC/DC power components and cabling. The need to save space puts significant pressure on the designer to make sacrifices. However, a modular design approach and innovative engineering can prevent massive overhauls and high costs. The use of extrusions, side plates, covers, and front/rear panels in various lengths and sizes, allow the building blocks of modularity. The second key aspect is the creative design, engineering, simulation, and modeling tools to perfect custom designs. Let's look at each element of a chassis design and review the impact on other areas.
Power distribution
Fundamental to system operation is providing power to the electronics. This requires an understanding of the system's voltage requirements, total power consumption and available input power. If standard 90-260VAC power is available, a switching power supply is used to convert the AC voltage to the lower DC voltages typically required by today's silicon. For systems utilizing a 48VDC input DC to DC converters would be used to step down to the appropriate voltages. Once the system power has been determined the next challenge is to distribute this power throughout the unit in an efficient and safe manner. This means adhering to the agency requirements of UL and CE through the selection and placement of appropriate components such as terminal blocks, line filters, circuit breakers, fuses, and wiring.
Balancing power distribution
But, how does the power effect affect other areas in the system? Well, more power required often means increased cooling requirements. More power supplies can lead to making the chassis taller if placed above or below the card cage, or consume slot space if placed within the card cage. Placing the power supplies in the rear of the system is an option, but can affect rear I/O and MTTR. A packaging company needs to have flexible options and be able to make modifications easily to adhere to the customer's preferences. Figure 1a shows an example of a 12U standard Elma High Availability rackmount chassis where the power supplies can be located within the card cage or above or below it. The 600W power supplies can provide N+1 redundant power, or just one or two power supplies modules can be used along with drives. To save rack space, a 9U solution with the power supplies within the card cage can be utilized.
Thermal management
One of the most challenging aspects of packaging design is thermal management. The need for greater density of electronics and increasing processor speeds has placed tremendous amounts of heat into smaller packages, making the demand for proper cooling a priority. Although new technologies such as liquid or vapor cooling are emerging, forced air, convection using fans or blowers, still cool the vast majority of systems. Understanding the systems total power dissipation and localized “hot spots” is critical in selection and locating fans or blowers based on the CFM and LFM required. Also, the size of intake and exhaust openings, airflow path and dust filter must be considered. A useful tool available to the designer is thermal simulation software. This 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.
Balancing Thermal Management
A lot of areas can affect the cooling capacity of a unit. Take I/O cabling for instance. I/O cabling can impede airflow. Often, designers will reduce the slots in a card cage to allow room for cabling. This can also present airflow problems, as the open area in the reduced slot backplane may not allow airflow to be re-directed properly. When this type of cabling in required, air baffles can be incorporated to improve the cooling effectiveness.
Particularly for rackmount applications, push/pull cooling is an excellent approach. However, this can require sacrifices in other areas. If the blowers are located in the rear panel area, this can limit the I/O space. Or, if the fans and blowers are located above and below the card cage, you will be adding height to the chassis. Figure 2a and 2b illustrate how an 8U Rugged/COTS chassis has front to rear cooling in a limited height (in the vertical card plugging format). Cooling, shock & vibration, and EMI protection are all achieved in an 8U height, along with a special rear I/O patch panel. The number of available drives and power supplies is flexible depending on the number of backplane slots.
Shielding
It is an important part of a packaging designer's responsibility to ensure that the system does not interfere with, and is not susceptible to interference from, other electronics equipment. Depending on the item's usage, electronics equipment must meet compliance standards like FCC and CE. The goal is to create ground continuity over the outer skin of the enclosure and to block given wavelengths from passing through any openings. In order to ensure compliance a wide range of issues must be considered. These include material type and thickness, conductive plating, vent hole size, seam length, gasketing, power filtering, access panels and AC cable routing. The primary focus of the design should be to use conductive material, limit the number and size of openings and eliminating seams to the extent possible.
Balancing Shielding
In conventional convection-cooled enclosures, strict requirements in shielding can greatly affect cooling capability. With the need to attenuate, for example 10 Ghz, the small apertures for less radiated emissions would affect airflow. The smaller holes impede airflow, increasing the static pressure buildup. Honeycomb filters may be needed to provide option EMI attenuation without compromising airflow. At times, the hole sizes can be reduced, while the number of perforations are increased. Testing has shown that the increased perforations can improve airflow effectively without hindering the EMC effectiveness. Modeling studies can help predict the optimum balance to create the desired shielding and cooling capabilities.
Shock/Vibration
Every piece of electronic equipment needs to be able to withstand the physical demands of the environment in which it is to be used. Because applications range from office and industrial use, to full military deployment the design requirements can vary greatly. Typical shock values for industrial use might be in the 5G to 10G range where systems located on Naval ships need to withstand shocks in access of 100G's. Based on the application, material types and construction techniques can be employed to ensure the unit can withstand the environment. Fixed mounted, screw fastened constructions are suitable for most office and industrial applications with shock isolators and welding used for severe military uses.
Balancing Shock and Vibration
Shock isolation devices such as rope-coil isolators are very effective damping tools. However, they can also reduce the slots of the backplane as they take up room in the card cage area. Further, they can also present challenges to maintain optimum airflow. Creative design for an experienced team in modular packaging and proper usage of thermal management techniques can minimize space consumption and provide sufficient cooling.
Reliability/Maintainability
Mean time between failures (MTBF) needs to be considered during the design of any electronic system. Good MTBF numbers can be achieved by selection of quality components with MTBF numbers around 100K hrs. Also, by ensuring adequate cooling and sizing of the power supply so that the system is not functioning at maximum power, the system will obtain a significantly longer life.
Mean time to repair (MTTR) is more important in today's designs than ever before. With information exchanging at staggering rates companies cannot afford to have their critical systems down. To maximize up time packaging must be designed to allow quick and easy access to failed items such as PSU's. “No tool access” is often needed to access cards and filters with plug in modules for fans and PSU's becoming common. For systems with 99.999% up time requirements, systems simply cannot go off-line. In order to comply, systems must be designed with N+1 redundant power and cooling systems with no one item being a single point of failure. To avoid and detect failures, system monitors are employed to status items such as power voltages, temperature and fan fail.
Balancing reliability/maintainability
As mentioned earlier, the use of N+1 power supplies can restrict space in other areas. Incorporating redundant fan trays can add to chassis height. Further, placement of the shelf manager can cause similar problems. The shelf manager could directly plug into the backplane. This increases the width of the backplane, but this extended part of the backplane can be placed inside or outside of the card cage. Inside the card cage, it would take up a slot of width. However, they can be placed outside the card cage along with other connectors for pluggable modules. For a flexible design, interface boards can be utilized which allow more options in the placement of the modules.
Design challenges
The challenges of the electronic packaging engineer are many. In an effort to maximize cooling he may jeopardize EMI integrity. The needs of maintainability may conflict with marketing's need for the unit to meet a specific size footprint. It is how the designer balances these tradeoffs and makes critical choices to optimize the final solution that separates an adequate design from a superior solution. By incorporating a modular design solution and with expertise in modeling and simulation for customization, one can limit the number of sacrifices and provide an optimal, low-cost solution, in a reasonable timeframe.
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