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April 2005

Redesign of a wireless communications system for improved thermal management

a major oem was experiencing cooling problems in their flagship product, resulting in the product being rejected in trial tests due to overheating. the objective is to redesign the chassis for better thermal management while meeting environmental requirements. specifically, the re-design effort must deliver on the following requirements:

  • improve on the total airflow and flow distribution
  • reduce acoustic noise of the unit through cooling efficiency gains
  • improve heat sink design and verify that current 400mhz processor can be upgraded to 600mhz and ultimately 800mhz performance levels without over heating
  • the new design must retain the same form factor: the board pitch and backplane cannot change

the 3g network switch features a unique integration of data, voice, and multimedia into a single platform and offers mobile subscribers unparalleled communication flexibility. however, this array of features also results in extremely dense packaging within the 14u form factor, which the oem is not willing to exceed. a number of other requirements must be met through the redesign including:

  • the system must operate up to 55c external ambient. the current system can only cool to 35c ambient.
  • acoustic noise must be limited to 60 dba under normal operation.
  • the new design must offer a fully redundant cooling subsystem.
  • the cooling subsystem must interface with the host system through fault-tolerant i2c communication.
  • the new system must cool under a single (worst case) fan failure

total power dissipation in the system is 3.7kw. this model was benchmarked against data from lab tests. severe distribution and recirculation problems limit the system to a maximum operating ambient temperature of only 35?c, whereas the system is required to operate up to 55?c.
the chassis is equipped with an axial fan tray at the bottom and an impeller tray at the top. neither fan tray, operating alone, can cool the chassis and thus the required redundancy is also not achieved.

a cut-away view of the thermal model shows the redesigned chassis. the total power dissipation in the new chassis has risen to 4.5kw due to processor upgrades on all the circuit packs.

the bottom fan tray was upgraded to higher speed fans. a better thermal management controller was designed to handle the higher power of the fans. the top impeller tray was completely replaced by a tray of very low profile axial fans, which considerably improved the flow distribution among slots.
a host of other changes were made on the chassis: honeycomb filters to provide emi shielding as well as better percentage opening at vents, vent locations and sizes, plenum redistribution, card guides, special modifications to slot 1 which had the most severe cooling problem, etc.

air velocities in the old and new chassis, superimposed on a map of air temperature, are shown on the right. the picture on the left shows the airflow and temperature in the old chassis. notice the recirculation zones in both the front and back of the chassis. these dead air pockets have been eliminated in the redesign, as shown by the picture on the right, which also shows a considerably more uniform airflow. consequently the air temperature dropped significantly.

component temperatures between the old and new chassis are dramatically improved. in the old chassis, shown on the left, a majority of the components were failing because they were running too hot. in the re-designed chassis, shown on the right hand side, all components are running considerably cooler and within their respective specifications.

engineered airflow. intelligent cooling.


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