Overcoming the Challenges in Designing High-Performance ATRs for OpenVPX

With a modular design approach and using thermal simulation to optimize each design, you can balance high power OpenVPX modules in as small a size and weight as possible.

There are two diverging requirements for most OpenVPX Air Transport Rack (ATR) applications. The first is keeping SWaP low, as nearly all of these designs desire lower Size, Weight, and Power consumption. On the other hand, the performance demands are continuously increasing to provide the capabilities for C4ISR, electronic warfare, RADAR, and similar systems. This typically means hotter boards. The challenge for the chassis designer is cooling these heat-intensive modules effectively in as little space as possible.

Figure 1: This model shows a rear-loaded ½ ATR for 3U OpenVPX with a compact size.

Key Elements of ATRs
Each ATR application is a little different, and there is nearly always some customization needed for the specific I/O the customer requires. But, there are some basic elements that most projects have in common:

  • Conduction-cooling (which may or may not be enhanced with air or a liquid)
  • Mechanical protection against shock/vibration
  • Sealing/protection against environmental elements (liquid, sand, salt fog, dust, etc.)
  • Protection from EMI

To meet the requirements, the applications typically are required to meet MIL standards such as MIL-461 for EMI and 810G or 901D for shock/vibration/environmental. Some of the more variable factors include supplemental airflow availability, the power input/redundancy/ conditioning (whether MIL 704A, for example, is required), the I/O configuration that can be used and of course the available dimensions for the enclosure. Some of these factors are tied to the reliability required and whether any active parts such as fans can be incorporated.

One of the key factors for an ATR design is whether airflow will be available or allowed in the application. Where the ATR provides only natural convection, the board wattage options are limited. Years ago, the slots would typically max at about 20W each. But, OpenVPX customers often want a processor and an FGPA that may be up to 45W, for example. With a PSU inside, that doesn’t leave a lot of headroom for many other modules in a 3U OpenVPX system. Without any supplemental airflow, cooling even these mid-grade modules is challenging, let alone keeping the enclosure very compact in size. There are several elements that the chassis manufacturer needs to review to optimize the cooling. This includes:

  • Material
  • Finish
  • Fin characteristics
  • Ambient temperature, altitude, humidity
  • Cold plate temperature
  • Wall thicknesses
  • Size of the enclosure

The design elements of the enclosure may include heat exchangers, heat pipes, liquid channels, etc. Figure 1 shows a model of a ½ ATR for 3U OpenVPX boards in a 3-slot size. The chassis can dissipate at least 125W in a very compact size in a natural convection format. In this design example, a rear-loaded approach was utilized for extra reliability and stability of the system as well as preventing a sharp bend in the cabling to the front panel. The rear-load approach allowed less space to be required for the bend and kept the chassis smaller. 3U OpenVPX board spacing leaves little room to widen the fins for enhanced heat dissipation in the ½ ATR size (4.88″). Therefore, fine-tuning the other variables becomes increasingly important. Performing thermal simulation can help a designer optimize the design.

Figure 2: Top view of a 6 payload, 1 PSU slot 5/8 ATR for 3U OpenVPX with heat exchange.

The I/O required in the system also affects the size required for the chassis. The designer needs to provide enough space for the connectors, as well as ensure there is significant stability and enough bend radius for any cabling where applicable. The cabling was also placed at the top of the chassis, as there was the desire to keep the hot modules on the bottom of the enclosure. This maximized the distance in the small enclosure for the heat to travel before it hits ambient air.

Further Improving Chassis Thermal Management
Even as processors are providing more performance with lower Thermal Design Power (TDP), the desire for higher wattage boards in conduction-cooled ATRs remains. It is not uncommon for OpenVPX boards to be selected that range from 75-100W. Where an external air source is supplied, it helps the chassis provider with a lot more options to cool the chassis. But, let us assume the chassis would need to generate its own supplemental airflow. On the higher end, some OpenVPX ATR design requirements are ranging from 500-700W. These designs often have several cards with 7-10 payload boards. One example of such an application was a requirement for 8 OpenVPX 3U slots and 2 VITA 62 PSU 3U slots with a heat dissipation requirement of approximately 625W. We had previously provided 6 payload slots and 1 VITA 62 slot in a 5/8 ATR for 3U OpenVPX at 375W. See example of the top-loaded chassis in Figure 2. The unit incorporated dual heat exchangers in an external shell that encompasses the internal sealed frame. The airflow passes over the fins and is pulled out the rear of the system. So, with a longer enclosure, would it be possible to cool up to 625W?

With FloTherm thermal simulations, the chassis parameters mentioned earlier can be adjusted within proper constraints to provide optimal cooling. In this example, the modules were spaced to prevent the hottest boards from creating hot spots. The fan configuration was modified, the chassis fins were widened, and the enclosure height was modified slightly. After re-simulating to the new configuration, we were able to get the chassis to a max of ~70 degrees C at the wedge locks. See Figure 3. With minor tweaks, the chassis thermal management was further improved.

6U OpenVPX
As 6U boards extend more heat to the wedge locks and a larger chassis is utilized, you can typically cool more overall wattage in the system than a chassis for 3U boards. However, you cannot load the modules in a ½ or ¾ ATR unless you are loading them vertically so that the wedge locks are facing the front and the rear. The designer can create a wall in front of the front panel area to absorb the heat from the wedge locks and draw them to the fins. But, this approach isn’t as efficient as the 3U configuration of loading the modules horizontally. If a customer can utilize 3U OpenVPX modules to accomplish their needs, it not only provides a smaller and lighter form factor, but the sizing allows a more efficient cooling approach.

Figure 3: Thermal simulation of an 8 payload, 2 PSU slot 3U OpenVPX ATR dissipating 625W.

Enclosures Small and Large
We’ve illustrated dual approaches for handling higher power requirements for 3U OpenVPX. For a very compact design without the benefit of supplemental airflow, one can find the right size and heat dissipation (along with other factors such as I/O) to properly cool up to mid-range OpenVPX processors and FPGAs. Going to the other extreme, for applications that have more space for a larger system and can allow the use of heat exchangers, users can load several 3U modules with higher wattages.

Justin Moll is Vice President, US Market Development Pixus Technologies. Justin also serves as Vice President of Marketing for PICMG.

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