COTS conduction-cooled CompactPCI for military applications

5Jack describes why key must-haves for a wide range of military applications – wider operating temperatures, higher shock and vibration tolerance, and better reliability – are addressed by conduction-cooled CompactPCI systems.

Conduction cooling is the transfer of heat away from a circuit board and its components through a thermally conducting material. This conductive material ushers heat to the system enclosure and dissipates it to the external surroundings. Conventional cooling relies on airflow to carry heat away from the device, or in the absence of airflow, can cool low-power devices only. Figure 1 shows two 6U CompactPCI blades, one that relies on air cooling, and one uses conduction cooling.

Conduction cooling, on the other hand, can dissipate heat when airflow is impractical, such as in sealed enclosures and confined spaces, or when little or no air is available, as in high-altitude and underwater applications. The absence of moving parts such as cooling fans in conduction-cooled systems increases reliability and makes them suited for use in mission-critical applications and harsh environments. And higher resistance to shock and vibration typically results from using a machined metal cooling plate as the thermally conducting material.

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Figure 1: Conventional air-cooled 6U CompactPCI blade (left) and conduction-cooled blade (right).

Mechanical specification

The mechanical specification for CompactPCI is defined by the IEEE 1101 series of standards. Conduction-cooled 3U and 6U CompactPCI systems are further defined by the ANSI/VITA 30.1 standard, which evolved from the IEEE 1101.2 mechanical specification for conduction-cooled 6U VME cards. VITA 30.1 allows for mechanical compatibility of conduction-cooled CompactPCI blades and chassis from different manufacturers

A key rule is that all boards must be capable of insertion into standard, air-cooled CompactPCI chassis. This allows system integrators to begin prototyping conduction-cooled systems using commercial chassis and air-cooled peripheral cards, significantly reducing development time.

To accommodate I/O expansion on conduction-cooled CompactPCI blades using industry standard PCI Mezzanine Cards (PMCs), the VITA 20 specification defines the design rules for Conduction Cooled PMCs (CCPMC) and conduction-cooled PMC carrier boards. CCPMCs have a reduced component space that is occupied by thermal and mechanical interfaces to the conduction-cooled carrier.

Modifying a standard blade for conduction cooling

To modify a standard CompactPCI blade for conduction cooling, a machined aluminum cooling plate matching the component layout is secured to the board to conduct heat away from components to the card edges, as shown in Figure 2. The cooling plate also improves the structural rigidity of the standard commercial blade, minimizing flexing of the board and allowing it to meet shock and vibration requirements for rugged applications.

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Figure 2: Mechanical layout of a 3U CompactPCI conduction-cooled blade.

Cooling plate functions

Thermal

The cooling plate acts as a heatsink for the board components. The underside of the plate is machined to match the component heights and locations on the blade. Heat is carried away to the card edges where a wedge lock mechanism secures the blade inside the chassis and provides a thermal interface to transfer the heat to the chassis and surrounding environment. In conduction-cooled applications, the temperature at the interface between the wedge locks and the walls of the chassis’s card slot determines the blade’s operating temperature.

The mass and heat dissipation properties of the cooling plate effectively lower the operating temperature of the hottest components and average out the temperatures on the blade by transferring heat from hotter to cooler areas. Additionally, by reducing the temperature differences between regions on the board, physical strain resulting from differing thermal expansion coefficients of components and board materials is minimized. This results in improved reliability by increasing MTBF.

Mechanical

The cooling plate covers most of the component side of the blade and provides mechanical support to the entire board. Flexing is reduced and mechanical stiffening provided to increase resistance to shock and vibration. The wedge locks secure the blade to the chassis and minimize motion of the card inside the enclosure. (See Figure 3.)

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Figure 3: 6U CompactPCI conduction-cooled blade inserted into enclosure.

Conduction-cooled chassis

Conduction-cooled systems make using sealed enclosures and thus protection from harmful environments possible. Such systems can also operate in confined spaces with little or no airflow. Air Transport Rack (ATR) form factor enclosures have been a de facto standard for aircraft electronic equipment for over half a decade. ATR enclosures are available in a wide range of case sizes, and many COTS conduction-cooled ATR chassis for 3U and 6U CompactPCI systems are on the market today.

Case in point: remote sonar system

An example of a military application utilizing the advantages of conduction cooling is a remote sonar system towed underwater by a naval vessel. The sonar system detects vessels and other objects under water and consists of transmitter and receiver transducer arrays and a main transceiver. An ADLINK CT-31 3U CompactPCI conduction-cooled computing blade (Figure 4) controls the sonar system. COTS peripheral cards for data collection and signal processing accompany the CT-31 blade.

A custom sealed enclosure inside the main transceiver houses the system. The ADLINK CT-31 provides the required processing power in the limited space available in the towed unit and leverages COTS technology to make possible a high-performance system that costs less than Mil-Spec systems. The CT-31’s fanless conduction-cooling capability, rugged design, and low-power consumption meet the customer’s requirements for low noise, high reliability, and the ability to operate in a confined underwater environment.

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Figure 4: CompactPCI conduction-cooled application in a remote sonar system.

Mobile air defense radar system

Radar systems play a crucial role in air defense, producing vital data for timely location of enemy positions. A lightweight mobile weapons vehicle requires a radar system with high computing performance and data transfer rates in order to carry out automatic target recognition and provide computer-based decision aids to the operator. The system is installed in the limited space available in the vehicle with minimal airflow and must be able to withstand the harsh environments of the battlefield. An ADLINK CT-61 6U CompactPCI conduction-cooled computing blade controls the radar system (Figure 5). The blade’s Intel Core i7 processor enables the radar system to perform its mission with high speed and extended range and accuracy. The system is housed in a sealed ATR enclosure and meets the application requirements of compact size, ruggedness, reliability and wide operating temperature range.

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Figure 5: Mobile Air Defense Radar System utilizing a CompactPCI conduction-cooled system in ATR enclosure.

Blade series incorporates a number of conduction-cooling advantages

ADLINK has developed the “CT Series” of conduction-cooled CompactPCI blades in both 3U and 6U sizes. These blades share electronic designs with standard convection-cooled models and take advantage of their combined manufacturing volumes. CT Series blades feature an extended operating temperature range and significantly higher shock and vibration tolerance than their conventionally cooled counterparts.

The ADLINK CT-31 3U CompactPCI conduction-cooled computing blade is powered by a dual-core Intel Atom processor D510 and features up to 2 GB of soldered DDR2-667/800 memory and -40 °C to +85 °C operating range.

A 32nm process Intel Core i7 powers the ADLINK CT-61 6U CompactPCI conduction-cooled computing blade, which includes up to 8 GB of soldered DDR3-800/1066, two 64-bit/133 MHz CCPMC sites, and -40°C to 85°C operating range.

Conclusion

Conduction-cooled CompactPCI systems are well suited to military applications and are commonly deployed in land, sea, and airborne systems. Conduction cooling meets the unique needs of military environments by providing wider operating temperatures, higher shock and vibration tolerance, and better reliability than equivalent air-cooled products, as well as allowing the use of sealed enclosures and operation in confined spaces where there is little or no airflow. An approved conduction-cooling mechanical standard for CompactPCI provides for interoperability among vendors and allows system integrators to utilize COTS products for cost savings over Mil-Spec systems.

Jack Chang is Technical Writer for ADLINK’s Industrial Computing Product Segment. Jack has over seven years of experience in embedded computing product documentation. He previously worked as a computer programmer for embedded systems and has an M.Sc. in Physics from the University of McGill.

ADLINK Technology jack.chang@adlinktech.com www.adlinktech.com