Addressing thermal considerations in PXI/PXIe system design

17 November 2014

PXI/PXIe is an ideal deployment platform for measurement and automated test systems, environments in which reliability is critical. 

In the development of a reliable PXI/PXIe platform, however, thermal issues demand maximum consideration. 
Priorities to ensure these thermal issues are properly taken care of include: ensuring that airflow is sustained when passing over elements in need of cooling; minimising the take-in of unwanted additional heat; successfully balancing maximisation of exhaust port dimensions with safety considerations, and optimising the PQ curve of fan placement. Of equal concern is power supply disposition. Providing a dedicated heat dissipation path for the power supply enables an optimum thermal solution.

Reliable PXI/PXIe chassis design

Effective PXI/PXIe chassis design must take into consideration aspects of module deployment that can affect airflow. PXI/PXIe chassis generally provide 3U PXI modules for 4U chassis. In this confined space, thermal management and sustained reliability present significant challenges.

Fan positioning and configuration considerations

Fans are frequently deployed in PXI/PXIe chassis on the bottom of the chassis interior. However, such a configuration has a tendency to impede or interrupt airflow, adversely affecting overall cooling efficiency. For example, connection slots between a pair of fans can easily experience higher temperatures than connection slots in other locations due to uneven airflow. This presents a challenge in the planning and configuration of the system. Thus, disposition of the fan at the rear of the chassis can improve airflow integrity.

Irrespective of location, airflow can normally be generated by fan outflow or fan intake, based on the direction of rotation. If outflow is employed, airflow velocity is increased. But control of the airflow's path can be difficult, and, according to PXI specifications, airflow must pass the module from bottom to top so that heat is dissipated through top vents, potentially compromising critical environments. If fan intake is employed, airflow velocity is lower, but airflow is steadier and more easily controlled.

The airflow pathway must also be considered in fan configuration. Avoiding additional sources of heat and effectively exhausting cool air are major considerations for chassis design. Many users adopt hybrid test systems, installing their PXI/PXIe systems in a system rack cabinet. In this case, potential heat sources across the entire hybrid test system demand attention.

When fans positioned in the rear of the PXI/PXIe chassis draw air in from the exterior and then exhaust it through the front of the chassis, the possibility exists for air heated by neighboring systems to be drawn in and forced over the PXI/PXIe modules, thereby creating an obvious disadvantage in the cooling of the overall system. Newer PXI/PXIe chassis designs use rear-mounted fan designs that channel unheated air from the front through the PXI modules, into the back of the chassis, and then out of the system, thereby maintaining intended levels of dissipation efficiency.

Optimal vent design

To effectively guide airflow and obtain optimal cooling results, the disposition of chassis vents is critical. Balancing safety requirements and structural limitations can present significant design challenges. Placement of vents is restricted to actual air intake/exhaust locations in compliance with PXI specifications, and height is limited by the bottom line of installed PXI modules. Misplacement is likely to impair airflow at the two rear sides. As a result, the new PXI/PXIe chassis designs not only provide vents in corresponding locations on the front and back, but further enhance dissipative efficiency with maximised apertures on both the sides and front panel.

Inherent environmental restrictions on PXI/PXIe chassis require precise calculations to be made to achieve optimal configuration for PQ curve and airflow quality. Ideally, flatter paths over longer distances provide maximum fan performance. Space restrictions, however, require extensive simulation trials to determine the optimum slope of the backplane baffle.

Power supply module configuration

Selection and configuration of a power supply can be equally challenging. Often overlooked is the fact that the power supply itself also produces heat, potentially affecting PXI/PXIe chassis performance.

Most conventional designs fail to separate flow of heat dissipated from the power supply from that of the chassis itself. Also, forceful ventilation from the power supply can frequently produce a chaotic airflow condition within the chassis. Accordingly, separation of heated airflow from the respective elements is critical, with independent vents and fans for the power supply contributing considerably to improved overall cooling.

Intelligent monitoring and automatic adjustment

Inclusion of intelligent monitoring to provide protection, adjustment, and moderation of the system in the design can ensure operational stability. By including sensors in the PXI/PXIe chassis, for example, five sets of sensors on the upper portion of the backplane, the temperature inside the chassis can be monitored and controlled. Through programmable settings, the system can be configured to increase fan speed when the temperature rises, maintaining temperature control and conserving energy.

Portable accessory considerations

In response to the portability requirements of PXI/PXIe measurement systems, accommodating installation of complementary monitor/keyboard accessories enhances convenience and usability. Such external monitor/keyboard kits easily convert rack-cabinet PXI systems into portable PXI systems. Even so, if cooling of the PXI/PXIe chassis is not optimised, addition of monitor/keyboard accessory kits may adversely affect original cooling capabilities. Improved fan placement in the rear section of the chassis, with cool air drawn through lower apertures and exhausted through the back, allow addition of the accessory kits to the system with no obstruction of cooling system components. Spacers on the underside of the keyboard further allow the independent cooling design of the power supply to function effectively, significantly increasing the convenience of the PXI system without system stability concerns.

PXI/PXIe chassis design must not only take into consideration design of the machine chassis itself, but also the inclusion of other measurement systems in common working environments. Fortunately, today's PXI/PXIe-based systems are being designed from spec-out to build-out to directly address system demands, presenting a definitively reliable platform for mission-critical applications.

ADLINK Technology brings many years of experience in the research and development of PXI/PXIe systems and modules, with its latest PXES-2590 specifically designed to meet high-performance, high-bandwidth PXI module requirements, and the industry's first all-hybrid 9-slot PXIe chassis.

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