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Optimizing Thermal Controls in Pre-Validated Systems

Choosing a cooling solution for a rugged box system is a complex task. Everything from the ambient environment to the power dissipation of sub-systems must be analyzed.

SIMON PARRETT, CONCEPTUAL/STRUCTURAL/THERMAL ENGINEER KONTRON

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Pushing the environmental limits of performance is essentially the definition of mil-aero design. While box-level computing systems create an advantage with Size, Weight and Power (SWaP) pre-validated for a range of rugged application needs, developers must also possess a comprehensive understanding how thermal factors affect their designs. Design challenges can involve everything from managing the ambient environment and component and sub-system power dissipation to accommodating the "as installed" thermal factor. Each of these considerations must be addressed keeping in mind greater performance capabilities coupled with continued reductions in system footprint to determine the optimal cooling option.

Using the right tools and optimization techniques, developers can readily validate a box-level system's thermal performance for their specific application. Sophisticated tools such as thermal modeling and computational fluid dynamics (CFD) evaluation enable the most successful designs, based on highly accurate projections of airflow, temperature distribution and heat transfer between components, boards and ultimately within the fully-deployed system.

Pre-Validated Systems

Small form factor, application-ready systems enable development versatility. These ruggedized systems pack a lot of performance into tight quarters, offering features that include highly integrated imaging, sensors, networking, avionics, multifunction displays and communications. Suitable for harsh environments such as ground vehicle systems, shipborne computing, manned aircraft and UAV payloads, these small footprint systems capitalize on extended thermal characteristics 'by design'-or via a Computer-on-Module (COM) that can be specifically engineered and optimized for extended temperature applications.

By addressing significant power densities generated at the board, chassis and system levels, application-ready systems both reduce risk and streamline deployment. A sealed IP67-rated housing and fanless operation further support operations in severe environments. These systems meet Size, Weight, Power and Cooling (SWaP-C) needs through extreme ruggedization and offer processor performance/power options ranging from low power Intel Atom to high performance Intel Core i7-based systems.

A box-level system optimized for SWaP-C is the Kontron COBALT offers a complete, rugged small form factor system with operating temperatures -40 to +71 degrees C (Figure 1). As a sealed IP67 system, Kontron COBALT is a complete, rugged small form factor system. Its rugged features include a special Rapid Shutdown circuit design, an on-board mechanism enabling survivability from episodes such as high energy electromagnetic pulse (EMP).

Figure 1
Developed for such rugged platforms as vehicles or helicopters, COBALT provides a performance advantage for graphics-heavy imaging and sensor data processing applications.

Solving Thermal Challenges

In addition to reducing resources needed for design and development, application-ready systems also empower innovation-enabling designers to focus on cooling solutions created to handle the most extreme deployment scenarios. Designers must solve four primary areas to optimize thermal performance of rugged box-level systems: They must optimize the enclosure fin interface with the ambient environment using computational fluid dynamic (CFD) driven parametrics; They must conduct a system-level thermal analysis to determine any power dissipation trade-offs and how one internal sub-system impacts another; They must perform an examination of the primary internal thermal conduction paths to the enclosure; and, the must look at how the installed thermal platform affects overall system performance.

To perfect the thermal performance of a natural convection cooled product, designers need to first focus on the design of the cooling fin geometry. A proven baseline design can be used to incorporate finning formed into the upper surface of the enclosure housing. Maximum heat is removed from the circuit board and processor, commonly designed to sit near the housing surface. Upper surface fins are able to be supplemented with modular rear and side wall mounted heat sinks if additional cooling is deemed necessary (Figure 2).

Figure 2
Graphic shows the Response Surface Evaluation CPU Temperatures vs. Fin Parameters.

There are numerous fin design parameters to consider along with the four finned surfaces of the enclosure. This is a time-consuming process as repeat iterations may be required to solve the application performance goals. Risk is a factor as well, with the potential for wasted resources if the design proves incapable of meeting application needs. Alternatively, new and more advanced software tools provide designers with more detailed data than traditional CFD software tools. These new software tools should be applied in tandem with analyzing a broad base of "Design of Experiments" design scenarios, creating an environment capable of quickly guiding engineers to fin geometries considered most advantageous for a given design.

Demonstrated by a design optimization tool that compliments the existing ANSYS Icepak CFD software, these more powerful algorithms reduce risk and accelerate design by evaluating sensitivities against a number of design and performance variables unique to each application. Iterations are reduced and the entire design process is streamlined significantly, using the final CFD analysis as a final validation of fin design.

Power Dissipation Trade-Offs

CFD tools are capable of providing system level thermal analysis and to test thermal relationships between the other various electronic sub-systems within the enclosure. This will reveal performance trade-offs that may be required, for example considering power dissipated by an optional XMC expansion card. It also exposes the potential for increased operating temperature from the COMs processor designed in close proximity.

CFD analysis provides invaluable information, particularly in conjunction with finite element analysis commonly executed as a best practice in design and development. Designers can gain insight related to maximum operating temperatures for each component, processor thresholds for both low and high temperatures, power and power density of components, and sidewall versus internal to external wall conductive path components. By compiling comprehensive data, designers are better able to not only select the best product profile option for a specific application, but also to more easily maintain compliance with customer-specified MIL or RTCA test standards.

Validating the efficiency of thermal paths between higher power, solid state components and the enclosure walls is also important. Using thermal simulation tools offered with CAD design software, designers may determine the width of a heat spreader. Adjustments may be required in order to optimize its gradient thermal path to the uppermost surface. Once validated, the most effective final solution will ensure minimal thermal resistance while maintaining a low mass.

Evaluate System Performance

The final area of evaluation is the specific operating environment where the box-level platform and full system will be deployed. For instance, how much headroom remains in the performance envelope if the system is deployed in the thinner air conduction conditions common to an UAV or other aircraft? While a system onboard a UAV may benefit of a colder environment, atmospheric effects at higher altitudes can affect cooling fans and must be considered. Because of the powerful impact of local environmental factors, careful evaluation of elements such as mounting platform material, mounting orientation, vicinity to other electronic equipment, altitude and potential solar loading are critical maximizing thermal performance.

Questions about two highly relevant factors are necessary. First, what is the possible performance impact of a small form factor, box-level platform mounted on an aluminum cold plate? Second, is there potential for radiation exchange between nearby electronic enclosures using comparable power? These influences underscore the need for a comprehensive understanding of contributing thermal factors, ensuring that reliable performance "as designed" is consistent with the "as installed" environment.

Optimized Thermal Management

Optimization tools and techniques play an essential role in establishing a box-level system's thermal performance for a given application. Sophisticated tools such as thermal modeling and CFD help designers evaluate many thermal methodologies. These tools are valuable resources in verifying optimal fin geometries, the conductive relationships between sub-systems, and the effect of power dissipation paths within the enclosure. Critically, the same tools also validate the "as installed" environment will perform as expected and required. The sidebar "Evaluating Thermal Management Options" in this article explores various approaches to thermal management.

Pushing the limits of performance-accommodating severe temperature ranges, jarring shock and vibration, rapid decompression, high altitudes and exposure to corrosive materials - remains an ongoing challenge for military systems developers. By optimizing thermal controls in the latest application-ready systems, developers are applying development resources wisely and ensuring long-term survivability for mission-critical applications.

Kontron
Poway, CA
(888) 294-4558
www.kontron.com

 

SIDEBAR

Evaluating Thermal Management Options

Pre-certification assures all required system functionality has been implemented in a chassis; as opposed to deploying a chassis categorized as "designed to meet," a sealed and temperature-controlled environment is certified to protect and ensure reliability of the electronics inside. For example, systems manufactured and validated to meet MIL-STD-810G's various environmental requirements are ready to withstand specified extremes of temperature, vibration, shock, salt spray, sand and chemical exposure.

Another approach includes evaluating potential impact of radiative cooling in passively cooled convection systems, often operating at low power. Because of reductions in size, weight and power, radiation can significantly influence placement of components, as well as locations where the completed system can be reliably deployed. To remediate these challenges, developers may instead opt for a sealed system with a natural convection design as a means to achieve scalability and excellent power dissipation.

A typical small form factor aluminum chassis may have an ambient temperature of 15 to 20 degrees C. This same system may also dissipate up to one third of its power through the effects of radiation; this is a significant measurement relative to overall power dissipated and may become even more significant depending on the end-use application, for example performing at higher altitudes common to UAVs. Embedded computing suppliers such as Kontron are addressing these market requirements, investing in technologies to optimize thermal management in small form factor platforms.

 

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