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Designing for EMC Early |
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Addressing electromagnetic compatibility (EMC) late in the design cycle is becoming less and less tenable as product complexity and densities increase while design cycles continue to shrink. The rules of thumb commonly used to calculate EMC are breaking down at higher frequencies and can easily be misapplied. The result is that 70% to 90% of new designs fail first-time EMC testing, resulting in high late-stage redesign costs and often even higher lost sales costs if the product ship date is delayed. Designers should consider the use of collaborative, conceptual analysis-based EMC simulation early in the design process to identify and fix problems at a much lower cost. Faster clock speeds lead to increased challenge in meeting EMC requirements. In the Gigahertz world, enclosure resonances enhance emissions, making apertures and seams problematic, ASIC heat sinks can exacerbate radiated emissions. In addition, regulations are being evolved by the regulatory agencies in order to insure compliance at higher and higher frequencies. To top it off, the trend towards integrating wireless capabilities, such as Wi-Fi, Bluetooth, WiMax, and UWB, presents further challenges as intentional radiators are designed into systems. Traditional approach to EMC designNormally, EMC design is considered by both the electrical hardware designers and mechanical designers in parallel with little if any communications between the two groups. Rules of thumb are often used during the design process with the hope they will be sufficient for the device being designed. Many of these EMC rules are becoming obsolete at higher frequencies leading to failure during testing. After the design stage, a prototype is built and tested for EMC compliance. This all too often results in EMC problems being identified at a point when it is too late to design in EMC compliance. Often expensive fixes on the existing design are the only options available. Design changes generally increase by an order of magnitude or more as the design moves from conceptual to detailed to validation. So a change that would only cost $100 at the conceptual level might exceed tens thousands of dollars at the testing stage, not to mention the negative impact on time-to-market. Challenges of EMC simulationIt has become essential to include EMC design as an integrated part of the product cycle to obtain first-pass compliance in the test chamber in order to ensure on-time delivery within budget. This can be achieved simply with a 3D solution of Maxwell's equations, which provide an elegant mathematical representation of electromagnetic interactions. But EMC simulation presents specific challenges not always encountered in other areas of computational electromagnetics. A typical EMC problem involves an enclosure that is very large relative to the features, such as slots, holes and cables, that are important to EMC performance. Accurate modeling requires that the large and small details be included in the model. This results in large aspect ratios, which means the ratio of the largest to smallest feature, in turn requiring very fine grids to resolve the finest details. Compact model technology can allow large and small structures to be included in a simulation without prohibitive simulation times. Another challenge is that the EMC characterization must be performed over a very wide frequency range. The time required to calculate electromagnetic fields at each sample frequency would be prohibitive. 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. Space is divided into cells modeled at the intersection of orthogonal transmission lines. Voltage pulses are transmitted and scattered at each cell. Electric and magnetic fields are calculated from voltages and currents on the lines at each time step.
With same source and input power in all three cases, emissions increase solely due to physical configuration. EMC simulation yields quite accurate results. The illustration above compares the computed radiated power in dBuV/m (blue) to the measured radiated power (blue). The small discrepancies in resonant peak location for the multimodule cases can be attributed to difficulties in obtaining precision alignment of the modules in the measurements. It is interesting to note that the differences in resonant peaks and amplitude of radiated power is due solely to the layout of the system, as the input power is the same in all cases. Wide range of potential applicationsEMC simulation is applicable for examining components and subsystems such as radiation profile versus frequency in heatsink grounding as well as assessing different grounding techniques, the impact of heatsink shape, etc. The shield effectiveness of different airvent hole sizes and shapes and metal thicknesses can also be compared. Recent applications in this areas have included a study to evaluate the use of large-hole airvents to allow for air-flow while controlling EMC by placing two such panels back to back. EMC simulation is also well-suited to EMC design and optimization at the system level to compute broadband shielding effectiveness, broadband radiated emissions, 3-D far-field radiation patterns, cylindrical near-field radiated emissions to mimic a turn-table type measurement scenario, as well as current and E and H field distributions for visualizations that help to locate EMC hot spots. Typical system level EMC applications include: designing enclosures to ensure maximum shielding effectiveness; assessing the EMC ramifications of component location within an enclosure; computing cabling coupling both internal and external to the system; and examining the effects of radiation from the cables. EMC simulation also helps identify specific mechanisms for unwanted electromagnetic transmissions through chassis and subsystems such as cavity resonances, radiation through holes, slots, seams, vents and other chassis openings, 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 chassis mounted components. EMC impact of different types of joints
Simple, quick running enclosure models can be used to perform design tradeoffs of different seam configurations. Here the radiation from a butted vs. an overlapping enclosure seam is evaluated. By comparing the relative shielding levels provided, the engineer can make an intelligent design decision based on the EMC budget for the enclosure and the cost of implementing a particular design configuration. Adding additional internal components to the simulation has only a small effect on simulation time so the designer can easily assess the shielding of the seam in a very realistic environment which accounts for coupling between slot resonances, cavity modes, and interactions with internal structures, all of which are not taken into account by design rules for slot leakages and which can lead to costly over- or under-designing.
Evaluating different thicknesses and hole shapes of panels. The graphs show the shielding provided by the panels for thicknesses (left) and hole shapes (right). A typical application of EMC simulation is to evaluate the shielding of different ventilation panels. While there are rules for designing air vent panels for EMC leakage, EMC simulation can accurately predict more exotic configurations, such as back-to-back panels with large holes, waveguide arrays, etc., while keeping thermal and cost constraints in mind. The application above shows the computation of shielding for panels with round and square hole geometries and different thicknesses. The graphs show the shielding provided by the panels for thicknesses (left) and hole shapes (right). Evaluating radiation from a heat sink
This EMC simulation application examines the radiation from a heatsink. In this simple model, the heatsink is excited by a broadband signal source located directly underneath it, representing electromagnetic coupling to the heatsink from an IC to which it is bonded. The plot shows the radiated power spectrum for three different configurations. Clearly, the radiation level depends on the geometry and frequency. While grounding the smaller heatsink provides and improvement at lower frequencies, the radiation is increased in the middle part of the frequency range. Solving a cable coupling problem
This example shows the use of EMC simulation to examine system-level cable coupling. The geometry consists of three network hubs in a 19-inch rack. A four-wire ribbon cable connects the PCBs in the top and bottom hubs to the middle hubs. The center hub has the only EMC source in the model. EMC simulation computed the currents coupled from the center hub to the connection at a PCB in the upper hub. The coupled current displayed two strong resonances at 600 MHz and 800 MHz. A common approach for dealing with this sort of problem is to add filtering to the affected cable and then gauge the impact with simulation. The lower plot shows that adding a low pass filter reduces, but does not eliminate, the magnitude of the coupled current at the resonant frequencies. This is a band-aid fix because it does not address the problem at its source.
EMC simulation visualized the internal physics of the cable coupling application in order to find the root cause of the problem. Examining the electric field distribution inside the center hub at 600 MHz made it possible to identify electric field hotspots that identified a cavity resonance that generated high field levels near the cables. By adding a metal partition to the hub, the resonant cavity mode was suppressed and the coupling was eliminated.
As complex high-frequency interactions degrade shielding performance, ensuring adequate shielding at the enclosure level becomes more important. The illustration above shows the desired shielding effectiveness (SE) for a typical equipment rack (red curve) and the actual measured SE (blue curve). While the enclosure provides adequate shielding in the sub-GHz range, it falls far short of meeting the EMC budget for SE across much of the band above 1-GHz. EMC impact of a thermal design change
This example uses EMC simulation to identify and solve a problem that arose from a thermally driven design change. The example is based on a model of a controller node, essentially a dual-processor Pentium computer, for an enterprise storage system. After this design was committed to hardware, the standard Pentium heatsinks were replaced with heat-pipes which occupied the same footprint as the heatsinks but were taller and whose fins were oriented horizontally instead of vertically.
A broadband simulation was performed to compute the radiated emissions of the system. Engineers were specifically interested in isolating the emissions due to a 120-MHz oscillator signal present in the system because they had measurements indicating a problem. Therefore, after computing the broadband response, an indirect excitation was used in post-processing to extract the response to the desired source signal. It is very clear that the radiation increases significantly (~40-dB) at the fundamental harmonic of the oscillator frequency (120-MHz). It's remarkable that such an innocuous thermal design change has such a major and alarming impact on EMC compliance.
This slide shows the visualization of the electric field in a plane through the center of the controller node (side view). Note that the controller node can be seen in outline in the figure and the visualization plane extends beyond the enclosure in order to see radiated field outside of the box. The figure on the left shows the design with heatsinks and the one of the right shows the heat-pipes. It is obvious that the field levels both inside and outside the controller node with the heat-pipes are significantly higher than for the baseline case. Examining field levels for various visualization planes and locations, it quickly became apparent that the close proximity of the horizontal fins to the lid of the enclosure created a large capacitive coupling path leading to a resonance with high field levels, generating high levels of radiation.
Having identified the root cause, cost-effective solutions were explored. In this case, eliminating the capacitive coupling path by making a ground connection between the top of the heat-pipes and the enclosure lid provided an excellent, low-cost solution. This was achieved by placing a small section of EMI gasket with a conductive adhesive on the top fin of the heat pipes such that the contact with the lid compresses the gasket and forms an electrical ground connection. This eliminates the resonant phenomena seen in the previous illustration. The illustration above shows the radiated emissions plot with the results for the grounded heat pipe included. The fix resulted in emissions that are virtually identical to the baseline case, improving thermal performance without having a negative impact on emissions. In conclusion, using simulation early in the design process makes it
possible to investigate and predict key EMC phenomena, and hence optimize
electronic product design in terms of EMC requirements and shielding effectiveness
before building a prototype. Modern simulation tools enable designers
to evaluate more designs than it's practical to prototype, and optimize
products from an EMC perspective to a level that wasn't possible in the
past. It's also important to note that EMC design cannot be done in isolation,
because design changes made for EMC reasons frequently impact other design
issues such as thermal management. That's why it's significant that some
EMC simulation tools enable designers to consider EMC in conjunction with
other important design constraints, in order to optimize overall system
cost and performance. |
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