Requirements for thermal and electromagnetic compatibility often butt heads. Here's how to reconciles the differences.
Cooling analysis for electronic enclosures is well accepted. It's used by most OEMs of electronic equipment and component suppliers in early design stages to identify thermal-management issues and correct them at relatively low cost.
Electromagnetic compatibility, on the other hand, is still usually addressed with easily misapplied rules of thumb that break down as designs move to higher frequencies. The result is that 70 to 90% of new designs fail first-time electromagnetic-compatibility (EMC) testing, producing high late-stage design costs and often even higher lost revenues when the product ships late.
Software capable of simulating EMC effects early in a design has been available but is rarely used due to the time and effort required to develop the required electronic system model. But a new approach uses the same thermalanalysis model to evaluate EMC at early design stages. The approach begins with rough models to identify problem areas and progresses to increasingly more-detailed models used to make design decisions.
DO IT EARLIER
Most OEMs of electronic equipment and component suppliers recognize the need to resolve thermal issues in early design stages. Many have adopted software that performs component and system-level analysis to address thermal management prior to physical testing. Their goal: Avoid additional design iterations. On the other hand, a common approach used to address EMC issues prior to prototyping has been to use rules of thumb. But they are fast becoming obsolete as higher frequencies become commonplace.
At gigahertz frequencies, enclosure resonances turn emissions and ASIC heat sinks into efficient antennas. Integrating wireless capabilities also presents challenges as intentional radiators are designed into systems. The result is an increasingly high failure rate during EMC compliance testing.
The cost of design changes typically increases by an order of magnitude as the design advances from concept to prototype to test and validation. Expensive fixes on existing designs are often the only alternatives when problems are discovered late.
Oddly, the increasing emphasis on early thermal design has inadvertently increased EMC problems. Thermal design often conflicts with EMC design so fixes that are implemented to address thermal concerns often exacerbate or create EMC problems. The obvious example is that thermal design requires large holes for adequate airflow while EMC design requires small holes to reduce emissions. A hole will pass electromagnetic fields in and out of the enclosure if one or more of its dimensions are equal to or larger than the wavelength of the field.
Other examples are thermal connections added to conduct heat from a hot component. In a small module where forced cooling is not available, it may be necessary to conduct the heat to the enclosure. Switching currents can couple to the stub and cause it to radiate like an antenna.
In thermal design, a large surface area is often used to increase convective heat flow. The same large surface areas may also provide unintended capacitive current paths. Capacitive coupled current can flow from a heat sink to the chassis and then onto cables connected to the chassis.
There's nothing new about simulating electromagnetic interactions by a 3D solution of Maxwell's equations. The transmission-line method performs the field solution in the time domain using broadband excitation, yielding data over an entire band in a single simulation run. Typical system-level EMC applications include designing enclosures for maximum shield effectiveness, assessing EMC ramifications of component location within an enclosure, and computing cable coupling both internal and external to the system.
EMC simulation also identifies specific mechanisms for unwanted electromagnetic transmissions through chassis and subsystems such as cavity resonances, radiation through-holes, slots, seams, vents and other chassis openings. It also uncovers conducted emissions through cables, coupling to and from heat sinks and other components, and unintentional wave guides inherent to optical components, displays, LEDs, and other chassismounted components.
Up to now, EMC simulation required a separate detailed model of the electronic system, which takes a considerable amount of time to produce. So EMC simulation has been used relatively infrequently and mostly in later stages of design to diagnose and identify solutions for problems identified during compliance testing.
More recent methods use the same thermal model that most companies already build early in the design cycle for EMC analysis.
Here's how thermal and EMC designs can be addressed together early in design. A 2U server provides an example. An important aspect of the approach is that it begins with a coarse model of the entire system to identify critical aspects of the design from both a thermal and EMC standpoint, and moves to more-detailed analysis of critical areas.
First develop a thermal budget. This is common practice in most electronicdesign teams. Quick calculations give a rough idea of the difficulty of cooling the enclosure. The analysts create a bill of materials, typically on a spreadsheet, to track total power dissipation. Then ruleof-thumb calculations roughly determine chassis size, airflow, and heat-sink requirements for critical components. Thermal simulation is not normally used at this stage because rules of thumb provide the rough estimates needed in less time.
The approach departs from common practice in using analogous methods to simultaneously develop an EMC budget to determine the approximate shielding effectiveness (SE) required for the enclosure. This budget takes into account ambient threats such as electromagnetic pulses, lighting, intentional transmitters, and the electromagnetic environment as well as the sensitivity of internal components.
From here, thermal engineers usually begin using the thermal budget to determine fan sizing and bulk airflow required to cool the chassis. They need more-detailed information at this stage so they usually create a relatively rough thermal model of the entire system.
To reduce solution time, the model contains minimal detail. For example, two-resistor compact models of individual components are often used. At this stage, thermal analysts pay special attention to the airflow over the motherboard, something unavailable from rule-of-thumb calculations. The analysis usually performed at this stage is illustrated in a fan-sizing chart.
The new approach departs from normal practice in using the thermal model to perform EMC simulation of the enclosure design to evaluate shielding effectiveness. The purpose of this simulation is not to calculate the overall shielding effectiveness, which probably came from physical testing, but rather to diagnose the enclosure and identify which areas are sensitive to EMC outputs.
The simulation typically mimics a shielding effectiveness test in a lab by measuring radiated emissions at a cylinder located around the box in a 3 or 10-m chamber. Simulation's advantage is that it determines the amount of radiation and a path by which it escapes from the enclosure. In this example, the simulation showed that most emissions escape through long seams. Gaskets along these seams increased the shielding effectiveness to well above that required by the EMC budget at a much lower cost than gasketing the entire enclosure.
The next stage of thermal management involves refining the simulation model to take advantage of a sufficiently detailed parts list is to provide information. More components are added to the simulation and compact or lumped models are often replaced by more-detailed and accurate versions. For instance, refined heat sinks and a daughter card are added to the 2U server. The additional detail make it possible to accurately predict junction operating temperatures of critical components. Engineers change the physical layout of the board to minimize hot spots.
REFINING THE EMD MODEL
The same refinements in the thermal model make it possible to start looking at the effect physical layout has on radiated emissions. A more-detailed system level model helps engineers quickly zero in on issues such as a heat sink that acts as an antenna and how the box structure contributes to electromagnetic interference.
Just as with the thermal model, morerefined models help identify problem areas that require deeper investigation in subsequent analysis. At the same time, system-level analysis will typically clear most design from an EMC standpoint to minimize additional time spent on it.
As design work continues, a detailed thermal analysis of more-detailed components identifies potential problems by simulating the refined model. In this case, simulation of the refined model shows that a custom heat sink is needed for a high-speed ASIC.
The refined-model generates boundary conditions for a detailed model consisting only of the heat sink, component, and thermal interface. Engineers adjusted the fin size, thickness and spacing, heat-sink material, and interface material, and rerun the simulation to optimize thermal conditions. At the same time, the team simulates alternative heat-sink designs from an EMC standpoint, viewing the surface currents and electric field distribution.
Most board radiations were coming from heat sinks, which were acting as nonintentional antennas. To minimize radiation from heat sinks, different grounding solutions were simulated and evaluated. The optimal number of grounding points and their location were found by analyzing the surfacecurrent distribution. Engineers quickly corrected the problem by adding three additional grounding points.
This simulation process combines thermal and EMC design in a manner that generates rough analysis results in the early stages to identify areas of concern. Increasingly more-detailed and accurate results are produced as design continues, providing information needed to make decisions with confidence prior to prototyping and testing. The result is that mechanical engineers can identify thermal and EMC issues in the early stages of design, long before prototypes are available, and perform studies to resolve them. The step-bystep approach lets ME and EMC engineers consider EMC requirements as part of the thermal/mechanical design process. This can significantly reduce the risk of costly redesign that comes when considering strongly coupled design requirements sequentially.
The integrated environment ensures the transmission of accurate information and provides immediate notification of design changes. Being able to address thermal-management and EMC issues within a single environment also lets mechanical engineers get a head start on the difficult design trade-offs frequently required between these two disciplines.
FAN SIZE COMPARISON
Mass flow (kg/sec)
Temperature rise (°C)
|Low-capacity fans, small vent holes|| |
|Low-capacity fans, large vent holes|| |
|Large-capacity fans, small vent holes|| |
|Engineers considered three vent and fan combinations and selected the middle one because it met thermal and cost considerations.|