OpenVPX (VITA 65) has enabled significant improvements in system speeds, reliability, upgradeability, packaging and SWaP-C (size, weight and performance-cooling*) for critical military applications. It also provides greater bus structure commonality across multiple boxes and even across services.
As system density has increased, the need for more aggressive and innovative thermal management techniques are needed. OpenVPX now boasts several standards that provide various means of dissipating heat within a system.
- 48.4, liquid flow-through (LFT), probably the most efficient up to 450 watts per card
- 48.5, air flow-through, which enables metering 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, (ANSI ratified October 2017)
Here we take a closer look at VITA 48.8, as it shows encouraging signs of becoming the most practical and likely most cost-effective option. It uses air-flow-through cooling, and its mechanical design supports air inlets at both card edges, while routing air flow 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.
Managing Increased Thermal Profiles
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 largely determined by the use of FPGA payloads in HPEC (High Performance Embedded Computing) systems, especially in applications like software-defined radios and radar systems.
This adds to the increased challenges of maintaining SWaP-C during the design process and has intensified the emergence of VITA 48.8 as one of the cooling strategies 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 air flow 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.
[See blog post on heat generation factors in OpenVPX systems]
Next-gen Systems Using OpenVPX
New hardware convergence initiatives within the OpenVPX 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 application for next-generation boards and backplanes.
Supporting those initiatives, Elma’s 3U OpenVPX CMOSS backplane and development chassis provides 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. (Figure 2)
Follow-on standards such as SOSA (Sensor Open Systems Architecture) also are adding the need for cooling schemes beyond the standard VITA 48.X-based conduction cooled standards.
Heat and density will continue to increase within embedded systems, and VPX-based electronics are not immune. Understanding the choices available to manage the thermal profile and still stay within the defined parameters of the VPX standard will provide a useful means of developing systems that can handle these increasing design pressures.
* For purposes of this discussion, the “C” in SWaP-C refers to “Cooling” whereas some definitions determine the “C” to mean “Cost”.
