Ten Tips For Successfully Designing With Automotive EMC .
Analog Applications JournalAutomotiveTen tips for successfully designing withautomotive EMC/EMI requirementsBy Mark SauerwaldApplications Engineer, Automotive Connectivity and EthernetIntroductionFigure 1. Typical testing chamber with specialconical tiles to stop reflectionsThe automotive industry and individual automobile manufacturers must meet a variety of electromagnetic compatibility (EMC) requirements. For example, two requirementsare to ensure that electronic systems do not emit excessive electromagnetic interference (EMI) or noise, and tobe immune to the noise emitted by other systems. Thisarticle explores some of these requirements and offerssome tips and techniques that can be used to ensure thatequipment designs are compliant with these requirements.Overview of the requirements for EMCCISPR 25 is a standard that presents several test methodswith suggested limits to evaluate the level of radiatedemissions from a component to be installed in a vehicle.[1, 2]In addition to the guidance that CISPR 25 provides tomanufacturers, most manufacturers have their own set ofstandards to augment the CISPR 25 guidelines. Theprimary purpose of CISPR 25 testing is to ensure that thecomponent to be installed in the automobile will not interfere with other systems within the vehicle,CISPR 25 requires that the electromagnetic noise levelin the room where the test is performed must be at least6 dB lower than the lowest levels being measured. SinceCISPR 25 has places where it looks for levels as low as18 dB (µV/m), an ambient level of less than 12 dB (µV/m)is needed. As reference, this is approximately the fieldstrength for a typical AM radio station, 1 km from theantenna.[3]In today’s environment, the only way to meet thisrequirement is to perform testing in a special chamberthat is designed and built to shield the testing environment from outside fields. Additionally, since normalbudgets require that the chamber be of finite size, it isimportant to protect the testing environment from reflections of signals generated within the room. Therefore,test-chamber walls must be lined with a material that willnot reflect electromagnetic (EM) waves (Figure 1). Testchambers are expensive and typically rented by the hour.To save costs, it is a good idea to evaluate EMC/EMI issuesduring the design phase to achieve first-time success inthe chamber.Another testing standard is the ISO 11452-4 BulkCurrent Injection (BCI) suite of tests that are used toverify if a component is adversely affected by narrow-bandelectromagnetic fields. Testing is done by inducing disturbance signals directly into the wiring harnesses with acurrent probe.Texas Instruments10 tips for successful EMC testing1. Keep loops smallWhen a magnetic field is present, a loop of conductivematerial acts as an antenna and converts the magneticfield into a current flowing around the loop. The strengthof the current is proportional to the area of the enclosedloop. Therefore, as much as possible, keep loops fromexisting, and keep any required enclosed areas as small aspossible. An example of a loop that might exist is whenthere is a differential data signal. A loop can form betweenthe transmitter and the receiver with the differential lines.Another common loop is when two subsystems share acircuit, perhaps a display and an engine control unit(ECU) that drives the display. There is a common ground(GND) connection in the chassis of the vehicle—a connection to this GND at the display end and at the ECU end ofthe system. When the video signal is connected to thedisplay with its own ground wire, it can create one hugeloop within the ground plane. In some cases, a loop likethis is unavoidable. However, by introducing an inductoror a ferrite bead in the connection to ground, a DC loopcan still exist, but from an RF emissions standpoint, theloop is broken.Also, a loop is formed by every differential driver/receiverpair when a signal is sent over the twisted-pair cable.Generally, this loop has a small area for the cable portionof the link because the twisted-pair is tightly coupled.However, once the signal gets to the board, close couplingshould be maintained to avoid opening up the loop area.4AAJ 3Q 2015
Analog Applications JournalAutomotive2. Bypass capacitors are essentialportion of the circuit. It may still emit energy, but goodshielding can capture the emissions and send them toground before they escape from the system. Figure 2 illustrates how shielding can control EMI.Shielding can take a variety of forms. It might be assimple as enclosing a system in a conductive case, or itcould involve fashioning small custom metal enclosuresthat are soldered over emission sources.CMOS circuits are very popular, in part, because of theirhigh speed and very-low power dissipation. An ideal CMOScircuit only dissipates power when it is changing statesand when the node capacitances need to be charged ordischarged. From a power-supply standpoint, a CMOScircuit that requires 10 mA on average may be drawingmany times that during clock transitions, then little or nocurrent between cycles. Therefore, emission-limiting techniques are focused on peak voltage and current valuesrather than average.Current surging from the power supply to the power pinon a chip during clock transition is a prime source foremissions. By placing a bypass capacitor close to eachpower pin, the current required to supply the chip duringthe clock edge comes directly from the capacitor. Thenthe charge on the cap builds up with a lower, steadiercurrent between cycles. Larger capacitors are good forsupplying large surges of current, but tend to react poorlyto very high-speed demands. Very small capacitors canreact quickly to demand, but their total charge capacity islimited and can quickly become exhausted. The best solution for most circuits is to use a mix of different-sizedcapacitors in parallel, perhaps 1-µF and 0.01-µF capacitorsin parallel. Place smaller size capacitors very close to thechip’s power pins, while larger-sized capacitors can beplaced further away.Figure 2. Example of shielding100 V –(a) Typical EMI problem3. Good impedance matching minimizes EMI–When a high-speed signal is sent through a transmissionline and it encounters a change in the characteristicimpedance on that line, part of the signal is reflected backto the source of the signal and part continues along in theoriginal direction. Invariably, the reflection leads to emissions. For low EMI, good high-speed design practice is anecessity. There are a plethora of good sources for transmission-line design information.[4, 5] Here are somesuggested precautions when designing transmission lines: Remember that the signal exists between the groundplane and the signal trace. Emissions can be caused byan interruption in either the signal trace or the groundplane, so pay attention to ground plane cutouts or discontinuities beneath the signal trace. Try to avoid sharp angles on the signal trace. Nicelycurved corners are much better than right-angle turns. Often times, an FPD-Link signal will have componentstapped off of it; such as power over coaxial cable, powerconnections, AC-coupling caps, and many others. Tominimize the reflections at the components, try to usesmall components such as 0402 size and set the width ofthe trace to be the same as the width of the 0402 component pad. Also, be sure to set the characteristicimpedance of the trace by controlling the dielectricthickness in the stackup.–––––– – – ––(a)––––––100 V – E 0V (b) EMI controlled with shielding5. Short ground connectionsEvery bit of current that flows into a chip flows back outagain. Several tips in this article discuss having shortconnections to the chip—bypass capacitors close to theIC, keeping loops small, etc. However, often forgotten isthe path that the ground current has to take to get back toits source. In an ideal situation, a layer of the board isdedicated to ground and the path to GND is not muchlonger than a via. However, some board layouts havecutouts in ground planes that can force ground currents totake a long path from the chip back to the power source.While the GND current is taking this path, it is acting asan antenna to transmit or receive noise.4. ShieldingDon’t shortcut good shielding techniques. When designingto minimize emissions, put a shield around the offendingTexas InstrumentsE 10 V5AAJ 3Q 2015
Analog Applications JournalAutomotive6. No faster than needed10. Spread-spectrum clocking reduces peak emissionsThere is a tendency to worry about timing margins and touse the fastest logic possible to provide the best timingmargins. Unfortunately, very fast logic has sharp edgeswith very high-frequency content that tends to produceEMI. One way to reduce the amount of system EMI is touse the slowest logic possible that will still meet timingrequirements. Many FPGAs allow programming the drivestrength at lower levels, which is one way to slow the edgerates. In some cases, series resistors on logic lines can beused to decrease the slew rates of signals in the system.With components such as FPD-Link serializers and deserializers (SerDes), there is often a data bus and clock thathave the option of spread-spectrum clocking. In spreadspectrum clocking, the clock signal is modulated. Theresult is that energy generated by the edges of the clockand data signals is spread across a wider frequency bandthan it would otherwise occupy. Since EMI specificationsare set to limit peak emissions at any frequency within aband, spreading noise across a wider band can help tominimize the noise peaks .A good example of a deserializer is the DS90UB914A-Q1,which is often used in conjunction with the DS90UB913A-Q1serializer. These devices are used to provide a video linkbetween a camera in an advanced driver assistance system(ADAS) and the processor. The deserializer recovers theclock that the image sensor in the camera provided to theserializer and outputs this clock along with the data foruse by the processor. Ten or 12 high-speed data lines thattransition concurrently with a high-speed clock are aprime source of EMI. To mitigate this EMI, the DS90UB914Ahas an option to use a spread-spectrum clock with theoutput data, rather than the lower-jitter clock that theimage sensor provides. The spread-spectrum clock iscontrolled through registers in the deserializer.7. Supply line inductorsTip #2 discussed bypass capacitors as a way to decreasethe impact of current surges. Inductors on the supply linesare another side of the same coin. By placing an inductoror ferrite bead on a power-supply line, it forces the circuitsconnected to that supply to draw their dynamic powerrequirements from the bypass capacitors, rather than allthe way back from the power source.8. Caps at inputs to switching suppliesOne recurring theme when looking to solve EMI issues isto reduce dv/dt and/or di/dt wherever possible. In thiscontext, DC/DC converters may seem completely harmlessuntil it is realized that they don’t convert directly from DCto DC. Rather, they go from DC to AC to DC. Hence, theAC in the middle has the potential to cause EMI problems.One area where automotive designers are concernedabout creating interference is in the AM radio band. Mostevery automobile is equipped with an AM radio, which hasa very sensitive, high-gain amplifier tunable from 500 kHzto 1.5 MHz. If a component is emitting a signal within thisband, it will probably be audible on the AM radio. Manyswitching power supplies use switching frequencies withinthis same band, which leads to issues in automotive applications. As a result, most automotive-switching suppliesuse switching frequencies that are above this band—oftenat 2 MHz or higher. If there is insufficient filtering either atthe input or the output of a switching power supply, someof this switching noise may find its way into other subsystems that may be sensitive to the root or subharmonicfrequencies.ConclusionAs automobiles rely more on electronics for critical vehicleoperation in addition to entertainment and comfort functions, there is a growing need to operate without error inthe presence of interference and to not provide interference to other systems within the vehicle. By following thetips and techniques outlined in this article, and throughselection of appropriate components, engineers are able todesign robust systems that enable automotive systems tooperate reliability without EMI problems.References1. CISPR 25 specification, ANSI eStandards Store2. Vincente Rodriguez, “Automotive Component EMCTesting: CISPR 25, ISO 11452-2 and equivalentStandards,” Safety & EMC 20113. AM Broadcast Groundwave Field Strength Graphs, FCCEncyclopedia4. Brian C. Wadell, “Transmission Line Design Handbook,”Artech House, Jan 1, 19915. Howard W Johnson and Martin Graham, “High SpeedSignal Propagation: Advanced Black Magic,” PrenticeHall Professional, 20039. Watch for resonancesFor various sources of interference, inductors and capacitors have been prescribed to tame the dv/dt and di/dt evilsthat can lead to EMI. However, inductors and/or capacitorscan have undesirable characteristics related to self resonance. This problem can often be rectified by adding aresistor in parallel to the inductor to absorb the energy ofthe oscillation before it becomes big enough to causeissues. Another potential issue is when there is a seriesinductor, either a discrete component or a parasitic inductance from a power line, that leads to a component with abypass capacitor. The resulting L-C circuit has the potential to oscillate at the resonant frequency. Once again, thiscan be tamed with a resistor, often placed in parallel withthe inductor.Texas InstrumentsRelated Web sitesProduct information:DS90UB914A-Q1DS90UB913A-Q1Subscribe to the AAJ:www.ti.com/subscribe-aaj6AAJ 3Q 2015
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automotive EMC/EMI requirements Introduction The automotive industry and individual automobile manu-facturers must meet a variety of electromagnetic compati-bility (EMC) requirements. For example, two requirements are to ensure that electronic systems do not emit exces-sive electromagnetic interference (EMI) or noise, and to be immune to the noise emitted by other systems. This article .
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