Thermal and rugged considerations for horizontal-mount chassis platforms

Horizontal-mount enclosures can be effective solutions in military embedded systems, particularly for smaller systems with thermal-management challenges.

Embedded system designers who go right to a 19-inch rackmount chassis with the boards mounted vertically – even for a smaller system – are going to lose a lot of space. Solving this problem will also mean overcoming some thermal hurdles.

If the backplane is only between two and six slots wide, then in a typical one-inch OpenVPX slot pitch there is likely 10 to 14 inches of mostly unutilized space. With front-to-rear airflow, the chassis will typically be taller than needed. For example, it would be 5U-6U high to accommodate 3U boards or 8U-9U high for 6U boards.

By using a horizontal-mount approach, however, the chassis height can be greatly reduced: 6U designs can fit two slots in a 2U high chassis. When using 3U cards, they can be mounted side-by-side for four slots in the 2U height. Theoretically, even three cards can be stacked in line, having the boards go 9U across. This would allow for six boards in a 2U chassis, but the thermal requirements would have to be considered very carefully in this configuration.

The horizontal-mount approach also easily facilitates a mix of 6U and 3U OpenVPX boards, with the boards set in line. Alternatively, a VITA 62 or other 3U PSU can be placed next to the 6U board.

Thermal management

The horizontal-mount approach does bring some challenges in providing effective cooling. Side-to-side cooling is an effective approach, as the air can pass straight across the boards without bends. There can be one set of fans or a set on each side of the cards for a push-pull airflow configuration. See Figure 1 for an example of a push-pull cooling design.

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Figure 1 | Example of a push-pull airflow configuration with fans on each side of the card cage in a horizontal-mount chassis.

Many applications don’t accommodate a side-to-side cooling airflow approach. In front-to-rear cooling, the airflow for a horizontal-mount system often has to make two 90-degree bends. While horizontal-mount enclosures have been around for a long time, it’s a little more challenging for OpenVPX due to the additional power/cooling and the architecture’s wider pitch (typically an inch) and the special spacing offset of the panels for the solder side of the circuit board. This configuration requires specially designed components for the architecture. For OpenVPX and other high-power systems, proper chassis cooling can be a challenge. (Figure 2.)

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Figure 2 | Front-to-rear airflow can also be achieved in a horizontal-mount chassis. However, the cooling typically takes a fine-tuned approach for higher wattage systems.

When the airflow needs to bend around corners, the chassis designers can employ air baffles or angle the airflow feeds. This approach can lessen the bends, providing a more efficient path. Thermal simulation can be performed to find hot spots and adjustments can be made to optimize the cooling. Figure 2 shows the air intake on the front side of the enclosure, then pulled across the boards, typically a very efficient cooling approach.

But what do you do if the boards need to be in the rear of the chassis? It’s less common, but sometimes the user will want the cards in the back so that the cables can reside in the rear of the chassis without a complex (and expensive) cabling approach.

To overcome this design challenge, the air can be pushed from the front to the rear. In the example in Figure 3, the requirement was that the boards be mounted in the rear of the chassis so that the cabling would be in the back of the rack. Most racks are set up to have the cool air come from the front of the unit and the heat exhaust to go to the back of the rack. Therefore, the 2U horizontal OpenVPX chassis had boards mounted in the rear with the air pushed directly from the front to the back.

In this particular case, the designer and the user had control over the front panels of the plug-in cards, which is not always the case, as third-party commercial off-the-shelf (COTS) boards are sourced. The designers were able to take advantage of the front-panel area by adding holes for air exhaust, which also enabled more precise airflow control. With the use of thermal simulation, carefully designed holes in the backplane provided a pathway for the air to go between the boards. The design of the backplane/chassis had the top board oriented to be plugged in upside down, ensuring that the airflow flowed through the hottest area of the boards.

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Figure 3 | To achieve high-wattage cooling in this 2U OpenVPX chassis, an airway path goes through cutouts in the backplane. One plug-in module is inverted (making the two boards face-to-face) to optimize airflow over the hottest components.

With the boards in the rear of the chassis, there was also inherent mechanical protection for the cabling. There was not anything in the front of the chassis for someone to bump, bend, or break.

However, this airflow approach is less common. How can a designer overcome the mechanical challenge in a typical front-to-rear or side-to-side cooling approach?

Mechanical protection

Protecting the front boards in a system in a common concern. The design of the OpenVPX handle solution keeps the boards fairly well protected, but when there is a lot of RF or other cabling, the cabling is prone to get bumped or snagged. The design could specify a full MIL-grade chassis, but this greatly increases the expense of the chassis.

Another solution is to recess the boards inside the chassis. The front cover of the enclosure provides the mechanical protection, while the boards and cables are safe inside. The handles for MicroTCA protrude out a couple of centimeters, so it’s best to protect the boards and cabling inside the enclosure. The cabling can be routed under the card cage to the rear of the chassis. In a front-to-rear-cooled chassis, an air intake area can be placed on the front panel along with any customized I/O. With standard sheet-metal thickness and commercial grade fans and power, this approach is more cost-effective than a full military system while still providing enhanced protection. Of course, a similar approach can be employed for OpenVPX or other architectures.

Ruggedized design

Ruggedizing the horizontal chassis can be achieved at various levels: military-grade fans, air, and power filtering, plus military 38999 I/O connectors as required for full MIL-461 EMI, and MIL 810 and 901D for shock/vibration/environmental requirements. Some applications require a hybrid approach, where the chassis needs to meet lower shock requirements, without all of the extremes in temperature, environment, etc. In these cases it’s possible to use thicker extrusions and sidewalls, extra screw/assembly points, a ruggedized PSU, and the like and still meet the application needs. Dampeners, spot welding, and isolators can be employed to provide additional rigidity.

Versatility for horizontal-mount chassis platforms

Horizontal-mount chassis platforms are ideal for small to medium systems where rack space is at a premium. There are a wide range of options a designer can choose for front-to-rear or side-to-side cooling, mechanical protection, ruggedization, etc. As the wattage levels vary greatly in each application, it’s important to work with the chassis designer to factor in a proper cooling solution. The cooling needs to be balanced with the power requirements, space for cabling or RTMs [rear transition modules], physical space limitations/constraints, ruggedization level, and so on. No matter which configuration is chosen, a horizontal-mount system can provide a powerful solution in a compact space.

Justin Moll is VP, U.S. Market Development for Pixus Technologies. He previously served as Director of Marketing for VadaTech and Director of Marketing for ELMA Bustronic. Jacques Houde is Principal Design Architect for Pixus Technologies.

Pixus Technologies www.pixustechnologies.com