Published in Military Embedded Systems
Authored by Ivan Straznicky of Curtiss-Wright and David Vos, Lockheed Martin
The ongoing challenge for commercial off-the-shelf (COTS) system developers is to balance the competing approaches to reduce the system's size, weight, power, and cost (SWaP-C) while trying to deploy the most modern technologies. Today, system integrators are confronting a rapidly narrowing margin for achieving that balancing act. While the power and density of devices has increased, platform ambient boundary conditions haven't changed. The result is tighter and tighter margins. The only option: Become more efficient in removing heat from the system. While thermal management for deployed systems is becoming harder, platforms’ weight constraints are getting more severe. Many modern platforms, such as rotary-lift helicopters and unmanned aerial vehicles (UAVs), are increasingly weight-constrained, with every additional pound on a system affecting mission duration and range.
As system integrators get squeezed from every direction when dealing with SWaP-C, there is some good news. The recently ratified ANSI Standard ANSI/VITA 48.8-2017, “Mechanical Standard for Electronic VPX Plug-in Modules Using Air Flow Through Cooling” (ANSI/VITA 48.8) represents an approach for cooling embedded systems using an air flow through (AFT) technique that significantly reduces the SWaP-C of deployed electronics while increasing the reliability of avionics systems and enabling the deployment of hotter contemporary devices. As the ability to cool today’s hotter modules using traditional conduction-cooling methods becomes less viable, VITA 48.8 greatly widens the thermal management margin while providing a better system platform alternative to the complexity and infrastructure required by liquid cooling.
VITA 48.8 is the first open-standard AFT technology to support small-form-factor 3U VPX COTS modules, which are preferred for use in SWaP-C-sensitive rotorcraft and unmanned platforms. Based on technologies developed by Lockheed Martin Rotary and Mission Systems, VITA 48.8 helps reduce weight and cost for high-density, high-power-dissipation 3U and 6U module-based systems by eliminating the use of wedgelocks and ejector/injector handles. It also supports alternative air flow arrangements, allowing air inlet at both card edges. Because VITA 48.8 does not use module-to-chassis conduction cooling, it also promises to help drive innovative use of new lightweight polymer or composite material-based chassis. (Figure 1 and Figure 2.) The ANSI/VITA 48.8 standard enhances previous design challenges for AFT cooling, such as pressure drop, flight altitude, air-cooling, air-flow intake, heat exchanges, and exhaust paths. ANSI/VITA 48.8-compliant modules use a finned heat-exchanger frame located within the central section of the assembly to top-cool primary circuit board and mezzanine board components.
At the system and platform levels, the key benefit of the VITA 48.8 AFT approach is size and weight reduction versus the infrastructure required to implement liquid cooling. At the electronics module level, the key benefit is greatly improved thermal management. Compared to module conduction cooling (VITA 48.2) or chassis liquid cooling alternatives, VITA 48.8 enables system integrators to nearly double the electronics functional density that can be deployed in a given chassis or even reduce overall avionics weight by hundreds of pounds per aircraft. For the warfighter, the reductions in SWaP-C provided by VITA 48.8 can deliver a significant increase in mission range, payload, and fuel economy, while enabling unprecedented levels of compute power to be fielded for new capabilities.
Figure 1: and Figure 2: Traditional conduction-cooling (Figure 1) methods are becoming less viable for today’s hotter-running modules. VITA 48.8-compliant AFT modules use a finned heat-exchanger frame located within the central section of the assembly to top-cool primary circuit board and mezzanine board components (Figure 2).
VITA 48.8 Preserves the Laws of Physics
The main reason that VITA 48.8 is so compelling for embedded system designers: Power and heat are rising on devices and modules and you can’t break the laws of physics. Over the last 20 or 30 years, as conduction cooling became established as the approach for the majority of the hottest VME- and VPX-based systems, the basic goal was the ability to effectively cool a 50 W card to platform ambient. It’s already commonplace today to have a 50 W processor, field-programmable gate array (FPGA), or general-purpose graphics processor unit (GPGPU) device on a host card.
Now consider the addition of a mezzanine card: It’s increasingly likely to find a 100 W 3U VPX module hosting a 50 W XMC card. Unfortunately, it’s just not feasible to cool the 150 W resulting from the combined host and mezzanine cards using VITA 48.2 cooling. That’s because it’s just not possible to hold 85 °C at the mezzanine mounting points when a 3U host card that’s designed to an 85 °C card edge hosts a mezzanine. Physics says there has to be a higher temperature for the mezzanine card than for the host’s card edge.
For real-time deterministic applications typical of intelligence, surveillance, and reconnaissance (ISR) platforms, the challenge of maintaining an 85 °C card edge is critical. Consider an FPGA device’s power dissipation over temperature. If the FPGA’s 85 °C junction temperature goes up to 100 °C, the actual current draw goes up by 30 percent, resulting in nonlinear power dissipation and the potential of a thermal runaway condition. To avoid thermal runaway, many multicore processors employ throttling, which – while it protects the device from overheating – can create a sudden reduction in performance that can prove critical. Due to the threat of thermal runaway there is a drive to maintain the module junction temperature at 85 °C, but (as pointed out earlier) this approach creates a physics impossibility without adding active cooling/refrigeration to the platform.
Dispelling the “Heat Sandwich” Problem
Another compounding challenge for system designers seeking to cool contemporary 3U cards and mezzanine modules is the problem of heat being “sandwiched” between the two boards. Typically in an open architecture system, the primary side of the mezzanine card is positioned to face the primary side of the host card. That design results in the high-power-density components designed for top cooling on the two cards being positioned directly against each other. VITA 48.2 attempts to pull the resulting heat out through the wedgelocks, but there are additional conductive and interface thermal resistances that limit this method’s effectiveness and efficiency. The VITA 48.8 heat exchanger approach provides an ideal way to solve the sandwiched heat problem, because it brings cool air as close as possible to the devices’ primary thermal path, effectively decoupling the sandwiched heat between the cards and driving it out via the air flowing between the two cards’ primary sides.
While efforts have been made over the years to improve the efficiency of conduction cooling through the use of exotic materials for either the module frame or wedgelocks, there have been diminishing returns in effectiveness with this approach. Another push has been to lower the boundary conditions, by driving the card edge temperature below 85 °C to 70 °C or even in some cases to 60 °C. From the system integrator perspective, that approach is problematic. Given a 49 °C exterior air temperature, plus the solar and other subsystems’ heat loads, that equates to 70 or 71 °C inlet temperature into the chassis when using ambient air cooling. Using a refrigerant-based cooling method to achieve a colder card edge comes with a high SWaP-C penalty and a significant amount of infrastructure at the platform level.
From a systems perspective, a very dense, high-power conduction module might be attractive in terms of size and weight, but when considered from the chassis and platform level, the additional weight required for a liquid cooling system can offset the benefits at the module level. In comparison, VITA 48.8 can significantly reduce overall system weight. Effectively cooling 3U conduction-cooled modules and XMC cards – assuming each has 50 W chips on-card – to an 85 °C card edge requires a chassis with liquid cooling in the sidewalls. Consider the following design scenario: A system architecture that starts with a single half ATR [Air Transport Radio] box to house conduction-cooled modules may need a second half ATR box that serves purely as a liquid-to-air heat exchanger in order to effectively deploy the solution.
With VITA 48.8, the entire second half ATR box is eliminated, saving considerable weight and system size. In comparison, implementation of AFT cooling and achieving the 85 °C junction temperature with VITA 48.8 typically requires only a 20 to 50 percent increase in size and weight of a select few high-power modules and power supplies. Instead of 1-inch pitch modules, either 1.2-inch or 1.5-inch pitch modules will be needed, depending on the power and power density of the specific modules. Consider this example: If there are over 100 boxes on a platform, and VITA 48.8 enables each chassis to save five pounds, the overall benefit is 500 pounds saved, per platform, at the vehicle level.
Condensation not an Issue with VITA 48.8
Another issue that system integrators need to understand when considering liquid cooling is condensation. When the system’s coolant temperature is driven below the dew point of the available ambient air on the platform, condensation problems arise. Condensation resulting from cold inlet air/coolant supplied below the dew point has even been observed in controlled laboratory environments during the systems integration and testing process. When the system is deployed on the platform, in an uncontrolled environment, the dew point can be at or below the ambient temperature, which may be way above the coolant requirement.
In one user anecdote, a platform gained more than 200 pounds of condensation after landing on a tropical island. In addition to adding unwanted weight on platforms where every ounce matters, condensation can damage or destroy fielded electronics equipment. VITA 48.8 delivers the needed cooling efficiency by providing the shortest path from the cooling air at the platform level to the junction temperature. It also eliminates the condensation problem because it uses an integral fan that ensures operation is always above the dew point (including in operational situations where the relative humidity is over 100 percent).
For system designers of deployed COTS systems for aerospace and defense applications, VITA 48.8 provides a big step forward in being able to design and cool today’s leading-edge 3U modules. AFT can provide significant longevity for 3U-based designs into the future; by enabling the use of today’s hottest, most advanced semiconductor devices, it will drive the deployment of upgraded and new capabilities and more compute-capable systems.
VITA 48.8-compliant plug-in modules will provide government and industry customers with significant cost savings and approximately 40 percent weight savings for avionics systems deployed in platforms such as Future Vertical Lift aircraft. Even better, this new cooling technology preserves investment in existing electrical and software architectures and protects electronic components from environmental contamination. The new cooling standard defines design requirements for platforms that need high performance processing, graphics. or electronic warfare capabilities, which means that AFT-cooled plug-in VPX modules – including both 3U and 6U form factors – retains the current VITA 46.0 and VITA 65 connector interoperability.