Forced convection cooling
Forced convection cooling applies air in direct contact with boards and system components via the use of chassis fans or a host platform environmental control unit (ECU). Forced air impinging directly on system boards, power supplies and other components, absorbs heat and then exits the system via exhaust vents.
Forced convection cooling is functionally acceptable if the air is clean and dry, and is ideal for slightly more benign military environments (see figure 2). Assuming a maximum air temperature of 55°C, power dissipation can be as high as 100 W per board.
Figure 2: The Kontron FS-1290 provides an example of forced convection cooling. Internal fans draw air through an inlet located on the front door of the chassis to cool the card. The inlet is routed up through the VME/cPCI card cage, around the power supplies and is exhausted through the upper rear of the enclosure.
To increase thermal dissipation, designers can simply use more or bigger fans to move air through the system enclosure, assuming space, cost and noise are not design issues. It is important to note however, that raising the number of fans increases cost and weight while adding points of failure. For more sophisticated applications such as unmanned aerial systems, SWaP constraints force the designer to explore other means for getting rid of heat.
Unlike forced convection systems, conduction systems cool by transferring heat energy through direct contact of the heat-generating components to a heatsink such as the system enclosure. Thermal energy then transfers to either a moving air stream or liquid inside hollow side walls, or to external fins for passive convection. In contrast with forced-convection solutions, air moving through a conduction design never physically touches system components.
In a typical conductively cooled system, boards, power supplies, and other components are sealed inside an air tight enclosure. Wedge locks or other mechanical elements clamp the edges of each component to the enclosure’s structure. As the wedge locks expand when tightened, they create a primary cooling path from the heat-producing elements to the chassis. The wedge locks assure the boards are mechanically secure and offer excellent resistance to shock and vibration. The drawback to conduction cooling is that designs tend to be more costly than their forced convection-based counterparts.
Pairing conduction with passive convection
Conduction cooling paired with passive convection offers military designers an alternative for applications in which fan-cooling is impractical. These types of environments may also demand higher MTBF, due to mission critical aspects and/or limited accessibility for maintenance. As a result, a thermal solution with no moving parts may be required.
Conduction-based passive convection solutions do not use a fan in the system. The boards are confined and completely isolated from the ambient environment. The designs remove heat through conduction to a passive cold wall that then convects or radiates the heat away. As the box itself physically heats up, it heats up the air immediately surrounding it. The air’s reduced density causes it to rise and pull in cooler air from beneath, resulting in passive convection. Conduction-based passive systems are available in either standard ARINC 404A style form factors or custom enclosures (see figure 3).
Figure 3: Thermal performance curve (top) for ruggedized passive convection-cooled enclosure shows the effectiveness of the approach. The enclosure (bottom) holds five 3U System boards.
A common myth in systems packaging is that radiation plays a marginal role in cooling electronic equipment. That holds true for higher-power systems with greater levels of power to be dissipated. Engineers should pay attention to the effects of radiative cooling in passively cooled convection systems that operate at low power, however. These smaller, lower-power systems are carving a path in military electronics and radiation can play a significant role in their growing applicability. A generic 8 in. x 12 in. x 7 in. metallic box in an environment at 100°F and 0 Pa air pressure (i.e., a perfect vacuum), for example, can dissipate more than 27 W by radiation effects alone. This is significant for designers, providing additional cooling for enclosures in unmanned aerial vehicles flying at very high altitudes where the air is thin or where there is a lack of infrastructure to support liquid or air cooling.
A passive convection approach can offer scalability and good power dissipation in a sealed system (see figure 4). Efficient thermal design can support fanless operation in severe environments, making the approach compatible with a full range of ground vehicle, UAV, manned airborne or shipborne requirements.
Figure 4: Able to integrate three single-board computers based on computer-on-modules into a fanless system, the VETRONICS enclosure can dissipate up to 85 W via passive convection through external fins on the side of the box.
Conduction paired with forced air
In using conduction cooling methods with a fan, the boards are again sealed off from environmental elements. The types of seals used in conduction cooling offer not only protection from environmental contaminants but also protection from EMI effects for conducted, radiated, and emitted electric fields. A rear-mounted fan pulls air through hollow side walls and exhausts it out of the enclosure. The reason air is pulled rather than pushed is to prevent fan-generated heat from being introduced into the air stream and reducing the system’s overall thermal efficiency. Similar to the passive convection enclosures discussed earlier, these conduction-cooled forced-air systems are available in either standard ARINC 404A styles or custom form factors (see figure 5).
Figure 5: Thermal performance curve for a chassis cooled by conduction with forced air shows the effectiveness of the approach. Able to house five 3U CompactPCI or VPX system boards and a 200 W AC or DC power supply, this design meets the requirements of MIL-HDBK-5400 for power dissipations of 100 W or less.