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Case Study: Simulation Overcomes Thermal Challenges

Computer simulation helped engineers at a Swedish firm quickly design a metropolitan area networking system that easily meets all of the company''s demanding thermal requirements. At the beginning of the design process, engineer Tommy Lindblad, with Stockholm-based Transmode Systems AB, wanted to make sure that the larger components in the Transmode coarse wavelength division multiplexing (CWDM) System TS-1100 did not generate recirculation zones that could prevent cool air from reaching smaller, heat-sensitive components. Lindblad''s first board-level simulation with Flomerics'' Flotherm thermal simulation software confirmed that in the original design concept, some components blocked others.

Lindblad corrected these problems by changing the board layout to eliminate the hot spots. He then simulated the airflow through the whole subrack, verifying that the temperature rise from bottom to top was as expected. As a final step, he stacked the subracks in a cabinet-level simulation that predicted the airflow through the entire enclosure. This simulation helped him calculate the minimum required air gap between the power supply on top of the rack and the row of subunits.

"Computer simulation helped us iterate to thermally optimized design in a minimal amount of time by allowing us to evaluate thermal performance and rapidly try alternatives prior to the prototype phase," Lindblad said.

Transmode Systems AB develops, markets, and sells managed optical solutions for metro networks. The System 1100 is an optical transmission system designed for metropolitan and access networking with a modular design that keeps first-in costs low and allows hitless capacity upgrades. Thermal design is critical to the performance of this product because of its high performance and compact size.

"We selected Flotherm because it is one of only a handful of thermal simulation products that models turbulent airflow and because it is by far the most popular thermal simulation tool," Lindblad said. He selected the various components in the design from libraries and located them on the device''s six printed circuit boards, avoiding the need to model them from scratch. The model was solved to generate thermal performance parameters including junction-to-ambient thermal resistance, junction-to-board thermal resistance, and junction-to-case thermal resistance, as well as temperature profiles within the package under various conditions.

"We learned how the air flows around the cabinet and determined the size of the gap that needed to be maintained between the subracks in order to maintain airflow at the required levels," said Lindblad. Based on his previous work with Flotherm, Lindblad was so confident that the results were accurate that the company moved directly into manufacturing without building any thermal prototypes.

"We tested the initial production units and discovered that they performed exactly as predicted," he said. "Thermal simulation significantly reduced the time required to get this product to market."

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