Tips for Selecting Electronic Enclosures

Electronic enclosures for VME, VXI, PXI, Com- pactPCI, and proprietary bus systems are steadily growing in features. Today, you can purchase enclosures complete with power supplies and cooling, monitoring, and chassis-management functions. Additionally, enclosures are available for development applications which provide easy access to board components for engineering tests.

Enclosure manufacturers offer complete system solutions as either off-the-shelf or customized designs for test and measurement, telecommunications, medical systems, military/aerospace equipment, and industrial control. Still, you should understand the cost and delivery consequences of a given feature set before specifying your packaging system. It’s important to review:

  • Application environment.
  • Physical/electrical features.
  • Power supplies and cooling.
  • Chassis monitoring and management.
  • Standards compliance.

Know Your Application

Defining the application environment is the first step. This includes the boards in the enclosure and their functions plus operational conditions. The information should be communicated to the enclosure manufacturer. The aim is to specify an enclosure that meets your requirements while avoiding excess cost.

In many applications, you have both product development and production to consider. While you may need a customized design for production, it is best to use an off-the-shelf model for development to reduce engineering and delivery times. Different enclosure characteristics may be needed for these two project segments.

For development, you may want direct access to installed boards to probe voltages and signals. On some chassis, this requires extender cards, which can degrade signal and board performance. However, enclosures created specifically for development applications have removable side covers for full test access to the component side of installed boards without using extender cards (Figure 1).

Also during development, load simulation boards may be needed for thermal analysis, which helps determine cooling requirements. As a result, comprehensive monitoring of all backplane voltages, currents, exhaust air temperatures, and fan speeds is important. It is convenient to have this information as well as the control of fan speed and voltages on the enclosure’s front panel.

Physical/Electrical Specifications

The physical and electrical specifications include the data bus type and the number of card slots and the sizes they accommodate. The card cage may be divided into front and rear sections, each of which could have a different number of slots and accommodate different board sizes. Card access may be at the front or rear of the enclosure.

Card height typically is specified as a rack-height unit, U, where 1U equals 1.75². This actually is the height of the card cage, not the cards. Length usually is specified in millimeters, excluding connectors and other components. A card cage might be specified as:

  • 12 front-loading, 6U × 160-mm slots, IEEE 1101.1 compliant.
  • 12 rear-loading, 6U × 100-mm slots, mirror image alignment with a front card cage.

In the example, 6U equals 10.5² (card cage, not board dimension). Typical card-cage sizes are 3U, 6U, and 9U with depths ranging from 160 mm to 400 mm in 60-mm increments. The enclosure manufacturer or standards agency also may specify spacing between boards, which is described in terms of horizontal pitch (HP) or T units. One HP or T unit is equivalent to 0.2².
A horizontal board orientation can help reduce rack space when there are few slots and a relatively low heat load. However, this configuration often is more difficult to cool than a vertical orientation, which tends to be better for larger loads.

Slot hierarchy also may be important. Some data buses require the system controller board to be at one end of the backplane, designated as Slot 0 (VXI) or Slot 1 (VME). On Com-pactPCI backplanes, the system slot can be at either end of the bus. You also may be able to specify whether the system slot is right or left to facilitate component access for testing.

You may want a benchtop unit for testing but a rack-mounted enclosure for production. Some benchtop enclosures can convert to the rack style by using an adapter kit with rack-mounting ears.

Specifying Power Supplies and Cooling

The total power required by the system is the first consideration in specifying the number and wattage ratings of power supplies. Although power supplies are very dependable, system reliability can be increased by spreading the power load across multiple supplies.

Two common schemes are redundant operation, where any single supply can carry the entire load, and N+1. In N+1 operation, the load is shared among multiple (identical) power supplies, but no single supply carries the entire load. For example, if you specify a 1,500-W N+1 system, you could get three 750-W power supplies. If any one fails, the other two still can carry the load.

To accommodate different power-line voltages and frequencies, specify power supplies with universal input, such as voltage from 90 to 264 VAC and frequency from 47 to 63 Hz. Some manufacturers extend the frequency range to 440 Hz for military and airborne applications.

Besides input and output power requirements, other critical supply characteristics should be considered to ensure system needs are satisfied. For ease of service and quick replacement, specify a modular design with front or rear plug-in access. Hot-swap capability also may be desirable.

Since a typical power supply is only about 80% efficient, its losses represent a significant portion of the system’s thermal load. Ideally, power modules have their own cooling fans for increased reliability. Otherwise, they must be included in your thermal analysis and calculation of cooling requirements. Also, the enclosure manufacturer must provide additional airflow paths from central cooling fans to the power-supply bay.

Modeling and calculating accurate airflow requirements for a chassis can be very time-consuming. However, there is a known starting point based on the specific heat of air:

F = 1.756 × Q/dT
where: F = airflow in cfm
Q = heat load in watts
dT = temperature rise in 
degrees Celsius

This assumes 100% of the airflow contacts all heat-generating components. This is not realistic and only establishes a baseline for cooling needs. Don’t be surprised if the enclosure manufacturer’s recommendation is much larger than these calculations suggest.

For example, consider a 400-W system load, an 80% efficient power supply, and a specification requiring a maximum temperature rise of 20°C above ambient. The total heat load is 500 W. While the equation suggests an absolute minimum volume of 44 cfm, it probably is inadequate because of airflow restrictions.

Dust filters, mounting brackets, changes in air direction, and board components create backpressure, which reduces a fan’s effective airflow. In a typical 6U system, combined tube-axial fan ratings of 200 cfm or more may be necessary to properly cool 500 W.

Other considerations are airflow path and distribution. The two most common airflow paths are bottom-intake/top-exhaust and front-intake/rear-exhaust. Both can be accomplished using either positive (pressurized) or negative (evacuated) pressure airflow. While positive pressure systems provide a constant stream of air in the presence of vacant slots, evacuated systems generally supply more evenly distributed airflow. To simplify service and replacement, a modular fan design is desirable.

Chassis Monitoring and Management

In bus-based systems, chassis monitoring and management (CMM) functions have been an integral part of enclosures for some time. They let you know about problems with power supplies and cooling fans and may allow remote management of these functions.

Monitoring cooling functions is crucial to ensure reliable system performance, but no single chassis variable provides 100% assurance. If you specify just one variable, the best indicator of adequate cooling is exhaust air temperature. However, fan rpm and other temperature sensors provide more complete information. To control cost, consult the enclosure manufacturer before writing specifications for the type, the number and locations of sensors, and other monitored variables.

Advanced CMM functions monitor variables outside the electronic enclosure, such as those associated with the system environment or other test and measurement hardware (Figure 2). With a suitable data communications interface, you can transmit this information in real time to a central monitoring location. While this adds cost to the enclosure system, the investment is repaid with reductions in system hardware and software development costs, higher reliability, and reduced downtime when something goes wrong.

EMI/RFI and Other Standards Compliance

Since boards are designed for a specific data bus, such as VME and CompactPCI, you probably will specify compliance with the appropriate industry standard. However, the way you do this has a major impact on cost, particularly when EMI/RFI compliance is involved.

A major issue is whether to ask only for design certification or insist on performance testing and documentation. Keep in mind that it makes little sense for an enclosure manufacturer to qualify the base chassis without a board-set, cabling, and running software.

Generally, enclosure and overall system costs can be reduced by designing in EMI/RFI control at the subassembly level such as boards, power supplies, and backplane. When the enclosure must be designed for EMI containment, don’t automatically assume that EMI gaskets and shielding must be used around every access point.

Depending on application requirements, the manufacturer may be able to provide operational access with integral shielding that is part of the sheet-metal structure. For example, sheet metal can be formed to act as a waveguide at high frequencies and minimize radiation at enclosure joints.

With forced air cooling, the manufacturer can suggest appropriate intake and exhaust apertures to reduce RF leakage and the cost of shielding materials. Metallic filters of mesh or honeycomb screen can replace foam or fabric to restrict the entrance of contaminants and suppress EMI. As a rule of thumb, the maximum aperture dimension should be less than 0.05 times the wavelength of the highest frequency being shielded.

Also, remember that EMI/RFI containment requires appropriate connectors to ground cable shields where they attach to the enclosure. Front and rear enclosure covers add EMI/RFI control and help protect installed components.


Ultimately, flexibility tends to have a favorable effect on price and delivery. A manufacturer may be able to modify a standard enclosure to meet application requirements and deliver it at lower cost with a shorter lead time than a fully customized solution.

About the Author

David Angelo is a project engineer at Tracewell Systems. As both a consultant and an employee, he has been affiliated with the company since it was founded in 1972. Mr. Angelo received a B.S.E.E. from The Ohio State University. Tracewell Systems, 567 Enterprise Dr., Westerville, OH 43081, 800-848-4525, e-mail: [email protected].

Return to EE Home Page

Published by EE-Evaluation Engineering
All contents © 2000 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

September 2000

Sponsored Recommendations


To join the conversation, and become an exclusive member of Electronic Design, create an account today!