Offsetting Thermal Challenges in High-Performance VPX Systems

It’s no secret that higher performance means higher thermal management requirements. Denser electronics packed into smaller spaces often leaves designers with the challenge of finding more creative ways to dissipate the increased amount of heat for conduction-type cooling methods.

But system density is only continuing to increase, and end users are still searching for ways to fit smaller boxes into more compact spaces so that they can put even more electronics into their applications. Which, of course, means more heat.

Factors of Increased Heat Generation
With enhancements to technology architectures, like VPX (see sidebar), increasing the abilities of embedded systems and continually pushing the envelope of high-performance, thermal management becomes that much more complex. In addition to the effects of component density and smaller form factors, evolutions in system design are impacting thermal management as well.

OpenVPX has introduced optical and RF signals to the backplane, removing these otherwise discrete connectors from the front of the cards. While the new backplane connections eliminate what would otherwise be a jumble of cables, the aggregate high-speed signals that now traverse the backplane rapidly heat up the system, exacerbating the already difficult-to-manage temperature increases.

Some of the most complex cards are being used in applications such as signal intelligence for communications and recording signals on the battlefield – including enemy communications – taking in audio inputs and triangulating the source of enemy fire.

Many high-performance applications require processor and FPGA (Field-Programmable Gate Array) system bandwidth that drive up the thermal load on the inside of the chassis, necessitating the need for new thermal management strategies. One example is a recent aerospace application that required many RF inputs – 36 payload slots each with 16 RF signals and many large radar arrays that require vast amounts of RF I/O signals.

Tight Spaces Mean More Heat
Embedded sub-systems must sometimes be packaged to fit existing tight spaces in aircraft, ground vehicles, submarines, spacecraft, and other rugged compact environments; leading to the need for optimized SWaP-C (size, weight and power-cooling*). While OpenVPX offers significant improvements in field-deployed system signal integrity, speed, and capability, it has created new challenges in these space-constrained installations (see Figure 1).

Figure 1: The number of military and defense applications needing increased performance continues to grow.

As higher performance systems are implemented, the choice between 3U VPX and 6U VPX becomes a matter of what functionality can be packaged on the smaller card vs. the larger. And as processors and FPGAs enable more capability, the 3U VPX form factor is favored for reduced size and weight. This pushes the existing convection and conduction cooling techniques defined by the standard to their limits.

That concentration of power in a smaller board has heavily impacted chassis and backplane designs and complicated thermal management in systems using a 3U card, making heat dissipation a larger issue. However, new cooling options under the VITA 48 umbrella are working to accommodate the increased heat in these high-performance systems.

Beyond Traditional Convection and Conduction
Most current applications find conduction cooling, as defined by VITA 48.2, and its respected cohort convection cooling enough. But the added complexity and heat generation of new boards and connectors quickly push current system cooling methods beyond these defined limits.

As VPX has grown in popularity, the VITA standards committees have defined additional cooling methods under VITA 48 to ensure future thermal needs are adequately handled. Current iterations are:

  • 48.4, liquid flow-through, probably the most efficient up to 350 watts per card
  • 48.5, air flow-through, which has the advantage of being able to meter the air to specific cards
  • 48.7, air flow-by
  • 48.8, air flow-through cooling without sealing for small form factor 3U and 6U VPX modules, ratified by ANSI (American National Standards Institute) in October 2017

The environment in which the boards will be developed and tested is typically different than the final deployed unit, so a lab chassis, for example, can usually rely on just fan cooling, whereas a deployed unit might need conduction cooling. The proper cooling method for a deployed system should be based on the most practical design and take into account the housing, the card heat sink, and the chassis itself.

Managing Increased Thermal Profiles
Taking a closer look at VITA 48.8, it shows encouraging signs of becoming the most effective option. VITA 48.8 uses air-flow-through cooling and its mechanical design supports air inlets at both card edges while routing airflow across the entire top surface of the boards. Conduction cooling methods, in general, provide a better system cooling alternative to the complexity and infrastructure required by liquid cooling.

With higher speeds and compact designs, customer applications need to dissipate from 50 to 75 percent more heat than before in roughly the same amount of space. Heat output is primarily determined by the use of FPGA payloads in HPEC (High-Performance Embedded Computing) systems, especially in applications like software-defined radios and radar systems (see Figure 2).

Figure 2: HPEC systems are increasingly expected to handle multiple data streams simultaneously.

This adds to the increasing challenges of maintaining SWaP-C during the design process and has intensified the emergence of VITA 48.8 as the cooling strategy of choice in the design of next-generation OpenVPX systems.

Of value to military applications, especially helicopters and Unmanned Aerial Vehicles (UAVs), are the improved SWaP-C characteristics of VITA 48.8. Traditional card retainers and ejector/injector handles are replaced by lightweight jack screws and rely less on module-to-chassis conduction cooling, thanks to VITA 48.8’s improved airflow design, while allowing the use of lighter composite chassis. Designers can also incorporate fixed slot pitches of 1.0”, 1.2” and 1.5”, freed from the limited 1.52” in VITA 48.5, to enable alternate air flow arrangements and add an air inlet at the card edge in addition to the conventional top edge inlet.

Because the VPX architecture tends to be complex, the margin for error can be high, especially in a first-time implementation. Typically, a system architect works with embedded card suppliers to address the functional requirements of the target design, then partners with a packaging solutions and system integrator such as Elma to review power requirements and propose the best cooling method.

The Need for VPX
In response to industry demands for standardization, a high-speed serial architecture, VPX (VITA 46 and VITA 65) was introduced, bringing new opportunities to Mil/Aero designs. VPX uses high-speed MultiGig backplane connectors, vastly increasing potential system bandwidth.The result: high-speed serial I/O (for digital video, mass storage, and FPGA interconnects) and switched serial fabrics with performance up to 10 Gbps as well as rugged form factors with greater design flexibility.
Prior to OpenVPX, many C4ISR (Command, Control, Communications, Computer, Intelligence, Surveillance, and Reconnaissance) systems were built on parallel bus structures such as CompactPCI, VME, or largely custom implementations. Specific systems were available at the box level but did not support a common infrastructure or form factor across multiple boxes.VPX enables significant improvements in system speeds, reliability, upgradability, packaging, and SWaP-C for critical military applications. It also provides greater bus structure commonality across multiple boxes and even across services.

Next-gen Systems Using VPX

New hardware convergence initiatives within the VPX community driven by the Department of Defense (DoD) enable greater compute density, which in turn is driving the need for advanced cooling methods. While VITA 48.8 is still a new tool for design engineers, it is expected to grow rapidly in applications for next-generation boards and backplanes.

Supporting those initiatives, Elma’s 3U OpenVPX CMOSS backplane and development chassis provide the foundation to create systems optimized for performance, reduced SWaP, and lower lifecycle costs with rapid technology insertion. The backplane includes precision radial network timing, plus slot profiles for SBCs, switches, radial clock(s), and expansion (see Figure 3).

Figure 3: The above CMOSS development platform enables system engineers to test a range of boards that meet profiles designed for use in various DoD program requirements, significantly streamlining engineering efforts and reducing time and cost to deployment.

Follow-on systems that will use derivative CMOSS architecture requiring high-power implementations are expected. This will drive the use of alternate cooling techniques, such as VITA 48.8. Follow-on standards such as SOSA (Sensor Open Systems Architecture) also are now considering the need for cooling schemes beyond the standard VITA 48.X-based conduction cooled standards.

OpenVPX into the Future
OpenVPX has allowed new definitions for VPX backplanes and systems, giving system architects and end users a far wider range of choices in critical high-speed applications, thus paving the way for more open architecture and multi-vendor interoperability in the future. It fosters technology growth over time without requiring changes to system architecture. It uses adaptations within the standards themselves to enable new capabilities and build HPEC hardware.

Finally, OpenVPX enables extraordinary leaps in aggregate system bandwidth and processing speeds that mandates new methods to meet the resulting thermal challenges. VITA 48 and its underlying cooling standards show promise of providing the advanced heat management methods that will alleviate the thermal burden now facing these high-speed systems.

* For purposes of this discussion, the “C” in SWaP-C refers to “Cooling” whereas some definitions determine the “C” to mean “Cost.”

Steve Gudknecht is Product Marketing Manager at Elma Electronic. He has held positions in field applications and marketing in high technology industries for nearly three decades. Steve’s responsibilities include product development, product marketing, training, and sales support.

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