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December 2005
library  >  Application Notes  >  General Articles

Thermal Modeling Reduces Costs In Telecommunications Design


by christopher j. scafidi, p.e. adc


gone are the days, when cooling technology referred only to the air conditioning unit in your office, not heat sinks on printed circuit boards. today's electronic circuits are faster, more powerful, and hotter, and components on the board require cooling to remain reliable.

 

the increased heat generated in electronic enclosures can greatly cut the lifetime of these components and increase the likelihood of malfunction and damage to the equipment. because of this, the dissipation of this heat, via thermal design management, has become a critical issue in the design process of electronic equipment, and has created a growing market for modeling software.

 

software developers have created thermal modeling and virtual prototyping programs that, if applied early in a design project, can save companies considerable time and money. these software programs allow an engineer to create a computer model of the product design, apply different conditions based on specifications or customer requests, and confirm or make changes to the design before building a prototype. this procedure results in fewer modifications during the prototyping phase - an expensive step - and a shorter time to market period.


when deciding on a cooling system for electronic equipment, an engineer must analyze various methods and elements of cooling. a key element in designing the equipment's cooling system is the airflow within the unit. the molecules of the moving air pick up the heat generated from an object and carry the heat away. the air's direction and velocity directly affects the temperature of components in an enclosure. different types of heat sinks exist, differing in cost, thermal performance, and ease of assembly. there are different types of interface materials, such as thermally conductive elastomers and phase change materials, that engineers must consider, as well as the temperature requirements of the equipment.


computational fluid dynamics (cfd) software, such as fluent's icepak program, models the airflow velocity and direction in designs, and calculates its effects. first, the engineer enters the physical design of the model, either by interfacing with an existing cad model, creating the model in icepak, or by using macros that simplify entry of heat sinks, fans and fpbgas. fans and basic heat sinks can be added, removed, and maneuvered to meet specifications or customer conditions. using this data, the software then calculates the direction and velocity of airflow within the unit and around each component, predicting the temperatures of these parts.


adc used icepak to analyze a modular transmission platform, a product that delivers analog and integrated digital services over broadband hfc (hybrid fiber-coax) cables. the software calculated the velocity (ft/s) and direction of the airflow through the system, as shown in figures 1 (front view) and 2 (side view). we entered the fan resistance curves, and icepak calculated the system backpressure to determine the airflow. figure 1 shows the air being forced up by fans under the modules, through the power supplies, and out the vents. the dark blue color shows where airflow is blocked or slowed by modules in the system. green and yellow areas show more rapid air movement.



 

 


 

figures 3 and 4 illustrate the predicted temperatures of critical components as a result of this airflow. the large dark orange area shows the higher temperature of the power supply plate on which three power supply modules (rated at 85°c), indicated by the green areas, are mounted. the platform has a 50°c maximum specification for its environment, the hybrids are rated at 100°c and the lasers are rated at 65°c.

 

these images show that all of these critical components will be cooler than their rated temperatures (figures 3 and 4). an earlier iteration of this analysis showed that the increased module density required a higher quantity of smaller fans. by discovering this requirement before building the initial prototype, we will be able to save the cost of the prototype and cut our time to market significantly.

 



 

 


 

cfd is still evolving-becoming more functional and more powerful with each new release. icepak, in particular, has evolved through adc's relationship with it. adc's modular isx nodes, part of the optiworx? broadband hfc transport system product line, are outdoor units "strand-mounted" to telephone poles. adc uses angular fin heat sinks in these units, where the fins are at a 45? angle to the heat sink base, instead of the normal 90? angle.

 

the original icepak software we used could only model for straight fins, not the angular fins in these nodes. to compensate, we used another software package in which we created a model of the heat sinks with angular fins. this software is fea-based (finite element analysis), so while it can model the effects of air, it can not model the airflow itself. this makes it difficult to match results due to changes in the convection coefficients that vary with airflow velocity and fin orientation.

 

the current version of the icepak software, however, now can handle angular fin heat sinks. because icepak is cfd-based, it enables analysis of both airflow velocity and direction, enabling analysis of the thermal performance of the equipment. in our experience, the results of the analysis are very close to the test results. figures 5 and 6 show the temperatures of the external housing surface and internal modules of the isx node, respectively.

 

for this analysis we assume uniform power dissipation on all pcbs. table 1 specifies the power for each heat-dissipating component. the analysis and test chamber conditions were 62.3?c with an air-wind velocity of 1.7 mph (150 lfm). areas in dark orange indicate high temperatures, which correspond to the blue color areas shown in figure 7, illustrating the decrease in the velocity of the airflow.


we tested our node and measured the temperature of the critical points. table 2 shows the minimal difference between the test data and icepak results. with these results, we will be able to expedite analyses of new node modules with a high level of confidence in our predictions.

 



 

 

 

 

 

 

 

 

 

 

thermal design software such as icepak helps engineers analyze airflow and direction, and predict the temperatures of critical components within electronic equipment. this analysis helps to determine problems in the heat dissipation performance of these systems. prototype testing is still essential in any product development, but with thermal design software, there is no need to prototype at each design phase.

 

by modifying a design to resolve problems before beginning the expensive stage of prototyping, a company saves money and time to market, increasing the competitiveness and marketability of their product. cfd-based software is now making it easier and more accurate than ever to calculate to near-test results by virtual prototyping.

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