by dr. d. p. johns#, a. gupta#, f. forghan*, r. kiesling*, k. le page*

# flomerics inc
* avici systems inc.

introduction

as the electronics industry relentlessly demands faster digital devices and higher power densities, managing thermal performance and complying with electromagnetic compatibility (emc) regulations is becoming a huge challenge. this problem is often exasperated because the thermal and em designs have conflicting requirements.

thermal analysis has evolved from simple analytical models coupled with trial and error kind of experimentation to rigorous numerical models that embody the physics of the problem comprehensively. as a consequence, computer simulation has started to play a central role in product design and development.

similarly, emc analysis is rapidly taking the same direction. the traditional approach is to apply rules of thumb derived from practical experience and equivalent circuit models. such design guidelines are approximate, especially with increasing digital speed, so it is often necessary to take remedial action at the end of the design cycle. this introduces a significant delay in introducing the product into the market and corresponding escalating costs.

3-d electromagnetic field simulation offers a new approach to emc design. simulation tools have matured through their application to high frequency microwave and antenna design. therefore, they are already in a good position to accurately represent the physics of electromagnetic interactions at high-speed-digital frequencies. the time-domain transmission-line modeling method (tlm) 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.

this paper reports computational modeling work done by flomerics and avici, to help design a thermally and electromagnetically robust tera-bit router system. 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.

the goal of the design engineers was to achieve neb thermal compliancy and fcc electromagnetic compliancy. thermal and electromagnetic modeling enabled possible problem areas to be identified and visualized. this helped guide the design process by eliminating some of the non-effective designs, and costly experimentation to prove so, thus reducing the overall product cycle time and engineering cost.

the simulation was divided into three stages. the first stage considered modeling at the component level; air vents, heat sinks, seams and wires. the second stage involved characterizing one module for thermal and pressure drop and resonant frequencies. the following pictures show 1(a) temperature, 1(b) surface current distribution over the module. the temperature profile is for a velocity of 400 ft/min. the surface current varies with frequency. in this case, it is plotted at 400 mhz.

fig 1 (a) surface temperature distribution on the module

fig 1 (b) surface current distribution at 400 mhz

the next pictures show 2(a) air velocity and 2(b) electric field distribution over the module, for the same set of parameters.

fig 2 (a) velocity contour plot at 400 ft/min of airflow

fig 2 (b) electric field distribution at 400 mhz

the third stage considered performance at the bay level. the top shelf, being the hottest, became the focus of attention. a baseline model was created, consisting of a shelf containing ten modules seated into a backplane. in the electromagnetic case, conducted emissions via a cable feeding power supply to the modules was a primary concern. the following pictures show 3(a) temperature distribution in the bay and 3(b) surface current distribution around the top shelf. current flows out of the system along the power cable and couples to the exterior of the back-plane/modules.

fig 3 (a) bay level temperature plot

fig 3 (b) conducted emissions at 1 ghz

the surface current induced on the bay gives rise to electromagnetic radiation. the radiation may be displayed as a 3d pattern as shown in 4(a), where the distance from the origin (or center of the shelf) to any point on the pattern represents the radiated power in that direction. we can see that the radiation is mainly in the z direction, or outwards from the front of the bay. this effect was also noticed during emc testing.

fig 4 radiated power at 1 ghz

the goal from an emc perspective is to reduce the electromagnetic emissions, particularly at frequencies generated by the asic waveforms i.e. the fundamental clock frequencies and harmonics. the picture below shows the simulated reduction in emissions achieved in part by filtering the power cable, partitioning the module and backplane and introducing absorbing material.

fig 5 reduction in emissions