CP-TA: Addressing the thermal interoperability of ATCA shelf and boards

CP-TA members are developing an Interoperability Compliance Document (ICD) and Test Procedure Manural (TPM) that define objective and verifiable interoperability criteria. Both will be developed for hardware and software building blocks based on existing open industry standards from organizations such as PICMG, OSDL and the Service Availability Forum, and to the system-level profiles developed by SCOPE. Initially, CP-TA is focused on certifying building blocks to interoperability requirements based on PICMG's AdvancedTCA.

This paper describes the AdvancedTCA (ATCA) shelf requirements to cool all the boards and facilitate the interoperability of shelf slots. It also covers the flow impedance issues of boards and introduces a methodology of impedance balancing. Further, to date, there is no standard measurement technique that can be adopted to compare shelves in terms of air flow volume through each slot in the shelf. This paper also describes a methodology of evaluating airflow volumes through each slot in an ATCA Shelf that can be used to benchmark each Shelf. The recent designs that populate the Board with mezzanine cards (AMCs) demand air flow at a higher flow impedance. The cooling challenges posed by the introduction of AMCs on a board are enunciated as well.

Thermal interoperability
PICMG guidelines for ATCA define thermal interoperability for shelf and boards as any and all shelf slots may be populated with any and all boards in any usage model whose power dissipation is within 200 Watts in the front board slots and 25 Watts in the RTM slots.

The air cooling of a board in an AdvancedTCA Shelf may be compared to the air flowing over a hot plate which may be represented by the following equation:

This equation yields about 29 CFM of air flowing over a plate with 200 Watts of thermal power to increase the air temperature by 12 degrees.

Several flow tests and simulations indicate a pressure drop of 0.15 to 0.2 inches of water at roughly 30 CFM of airflow across sampled compute boards, the most populated board in the shelf. An example of the P-Q curve for a compute board or an SBC (Single Board Computer) is seen in Figure 2.

It is logical to assume that, as the power of a board increases, the pressure drop across the board increases due to the combination of additional components and heat sinks. The theoretical and experimental evaluations show that the Shelf should provide 30 CFM of air flow at 0.15 inches of water pressure drop to each slot to be able to cool a 200 Watt board. Further, a shelf needs to provide a total of (nx30) CFM of airflow at 0.15 inches of water pressure drop where n is the number of slots in the shelf.

As a fundamental requirement, it is imperative for the shelf to provide at least 30 CFM per slot at 0.15 inches of water to facilitate any telecom usage model.

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