Integrating Electromagnetic and Thermal Design

 

Fierce competition and furious innovation drive the telecommunications industry. There’s no standing still even if you lead the pack. The explosion in the Internet demands ever increasing traffic capacity and Avici Systems, based in Billerica, Massachusetts, is developing tera-bit router systems to meet these needs.

 

 

 

The router system requires very high­speed digital electronics and multiple ASICs resulting in high power densities, so managing thermal performance and complying with electromagnetic compatibility (EMC) regulations is a challenge. All too often the thermal and EM designs have conflicting requirements. To meet these challenges, Avici has turned to a Design‑Class Analysis principle called Process Integration. Process integration defines an analysis environment that addresses multi‑disciplinary, inter‑related design problems concurrently. Flomerics enables Process Integration through its consulting services and design tools, FLOTHERM and FLO/EMC. Flomerics has pioneered the application of Design‑Class analysis software and services to the electronics industry. Thermal simulation now plays a central role in product design and development. Flomerics is repeating history with electromagnetic simulation.

 

The goal of the design engineers was to achieve NEBS thermal compliancy and FCC electromagnetic compliancy. Thermal and electromagnetic modelling enabled possible problem areas to be identified and visualized. This helped guide the design process by eliminating some of the ineffective designs, and costly experimentation. Tera-bit router systems, by nature, present significant thermal and electromagnetic challenges. This was reflected in rather high temperatures on some of the critical components and non-uniform flow distribution in the system coupled with electromagnetic emissions spread over a wide frequency range due to fast picosecond pulse rise times.

 

Dr. David Johns, Flomerics Technical Director Electromagnetics says “Electromagnetic analysis has matured sufficiently to accurately represent the physics of electromagnetic interactions at high-speed-digital frequencies. FLO/EMC is based on the time-domain Transmission-Line Modelling method (TLM) that is a prime example, where the broadband response of the system can be captured in one go. For EMC, this is advantageous since the emissions vary over a wide spectrum of wavelengths.”

 

AVICI’sintegrated-design was divided into three stages; component level, module level and bay level. At the component level Avici used simulation to characterize the air vents, heat sinks and seams. These components have a bearing on both the thermal and electromagnetic signature of the system. The air vent must provide a sufficient free area ratio for adequate air-flow, but at the same time the holes must be small enough to block electromagnetic radiation. FLOTHERM and FLO/EMC can be used to reach the optimal design.

 

 

Heat sinks are obviously intended as cooling devices, but they can become effective antennas at certain frequencies. FLO/EMC is used to determine the resonant frequencies of the heat sinks. If the resonance falls on the fundamental clock frequency or harmonics used to drive the ASICs, an alternative heat sink may be necessary.

 

The second stage involved characterizing one module. FLOTHERM was used to simulate the temperature and air velocity over the module. FLO/EMC was used to identify module resonances and explore design options. Finally, a baseline model was created, consisting of a shelf containing ten modules seated into a backplane. The conducted emissions via a cable feeding power supply to the modules was a primary concern. The near­field and surface current induced on the bay gives rise to electromagnetic radiation.

 

The goal was to achieve a good margin between the radiated electromagnetic emissions and the FCC limits. FLO/EMC simulation helped AVICIreduce emissions by predicting the effect of filtering the power cable, partitioning the module and backplane and introducing absorbing material.

 

Dr Fariborz Forghan, Avici’s principal thermal engineer says:

 

“Avici used both FLOTHERM and FLO/EMC to observe and reveal the performance of the thermal cooling system and the EMI shielding effectiveness.  Several iterations simulating the modification of designs led to the optimum cooling system and EMI shielding. In addition, the simulation specified design parameters for future modifications and improvements when it becomes necessary. The simulation saved cost and reduced the system size by avoiding over designing, passing compliance tests in the least number of iterations and in some cases just in one run; therefore, saving time and money to introduce the product into the market. Using these tools helped Avici increase the number of interfaces we could support in a 7-foot rack, giving our customers the industry’s highest port density”.

 

Since 1988, Flomerics has pioneered a different approach to numerical analysis and simulation.  Flomerics' software tools and services help bridge the gap between science and industry by embedding complex analysis and optimization software deeply into the design process, in a way which enhances productivity in engineering design.  This approach is called "Design-Class Analysis.

 

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