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Cooling Densely Packaged Chassis
Typical Vertical ATCA telecom cabinet with 14 slots.
By Bahman Tavassoli, Ph.D, Chief Technologist, Advanced Thermal Solutions, Norwood, MA
In 2001, PICMG, the PCI (Personal Computer Interconnect) Industrial Computer Manufacturers Group released the ATCA (Advanced Telecommunications Computing Architecture, or AdvancedTCA) standards to help develop a modular, PC-style approach for next-generation telecom equipment. Over the past 25 years, PICMG has provided the PC industry with standards for the physical design of hardware such as motherboards and hard drives, and for interfacing with peripherals.
Among the benefits have been a huge cost savings in volume production, and the ability of software and hardware developers to put systems together quickly and efficiently.
In contrast to these improvements for the PC industry, the telecom market has remained fragmented, with many vendors making multiple, proprietary systems.
Typical horizontal ATCA enclosure with 5 slots.
But now the ATCA approach allows telecom OEMs to build switch/router cards and boards for different applications and multiple product lines. This will improve product reliability, for example, introducing industry-standard hot swappable hardware and software, and drive down prices as less time is spent with power, mechanical design, and connector issues.
The ATCA design was considered for carrier offices and switching centers with a large number of wire cables and peripheral devices. In 2004, a subgroup within ATCA was established to consider a smaller scale version of the ATCA standards. The result is called MicroTCA (Micro Telecommunications Computer Architecture). By these low-cost standards, AMC cards can be directly plugged into a backplane without the need for an intermediary carrier card. MicroTCA was envisioned to be suitable for remote carrier locations and enterprise applications. Its cost efficiencies are very attractive for telecom service carriers.
Typical microTCA package.
The estimated market for ATCA, including MicroTCA, has been predicted to be anywhere from $7 billion to $20 billion. Major telecom players supporting the ATCA standards include Ericsson, Motorola, Intel, Lucent, Sun, and HP. Huawei Technologies is among the largest ATCA vendors. Established companies that have not supported ATCA include Cisco, Juniper Networks, and IBM. These firms claim that moving from custom designs to ATCA will adversely affect their products' capabilities.
Different configurations are possible with ACTA standards. ATCA designs can be vertical or horizontal with different slot numbers and sizes.
As with all electronic devices, ATCA-compliant systems need adequate cooling to keep components running below their critical junction temperatures. The conventional approach is air cooling. Typically, a push/pull system of fan trays delivers air into the chassis for convection cooling. A pull system will generally outperform a push system because the entire chassis is used as a plenum for the entering air. The push fan tray is for redundancy and increasing the overall static pressure of the combined fan trays.
The operating point of a system is the intersection of the combined fan tray static pressure curve with the system curve.
Hermetically sealed PCB attached to a backplane and liquid ports.
After calculating the flow, individual boards must be analyzed to identify critical components and possible heat sink designs. If there are empty slots, dummy boards with face plates at the upstream will prevent air flow leakage into these least resistance paths. Given the steady increases in component power dissipation, air cooling is approaching its limitations and the need for liquid cooling is fast approaching. Telecom central offices are gradually moving to liquid cooling, including the use of a facility?s cold water to remove the heat from the building.
As an example, a MicroTCA-compliant modular cabinet from Optima EPS (an Elma Company) has 10 MicroTCA cards for a total of 6kW. The top and bottom of each card is open, letting the main air flow travel through each subsystem. Each subsystem has its own air-to-liquid heat exchanger. As hot exhaust air from each stage goes through the heat exchanger its temperature drops by 15°C to compensate for its temperature rise due to heating. Cold water is supplied by the office facility. This method allows for extremely dense cabinet packaging with very low noise.
Liquid-Cooled PC Boards
Another application of liquid cooling was demonstrated by the Liquid-Cooled Embedded Computing (LCEC) initiative, a privately funded group tasked with creating an open standard for liquid-cooled embedded computing architecture. In this design, the individual boards are hermetically sealed and filled with an inert fluid. The facility's cold water is fed into the cabinet, and a liquid-to-liquid heat exchanger transfers the heat from the PCBs. The boards are all line replaceable units (LRUs). When they're plugged into the backplane, inlet and outlet liquid tubes are connected to the PCB. Special connectors allow the PCB to be disconnected without spilling the liquid. At optimum performance, this technology will enable 350 to 500W of heat dissipation in each PCB, and a total heat dissipation of 5000 to 7000W per subrack.
Another cooling technique used in military applications is spray cooling. Due to the complexity and expensive nature of this technology, the defense industry may remain its main marketplace. ATCA-compliant designs are quickly becoming important in the telecom industry. Conventional air cooling for these systems will be needed for the foreseeable future on an individual basis, but liquid cooling will soon be the dominant method for thermally managing ATCA cabinets in data centers.
Contact: Advanced Thermal Solutions, 89-27 Access Rd., Norwood, MA 02062
781-769-2800 fax: 781-769-9979 Web:
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