Design For EMI‐EMC Introduction to the WORLD OF BLACK MAGICBy Vemana Shankar
EMI‐EMC EMI EMC Overview Of EMI-EMC EMI EMC Design For EMI-EMCEMI-EMC For Testing & Measurement EMI-EMC Standards EMI EMC for Layout EMI-EMC
Agenda• What is EMC ?• Basic EMI-EMC Tests• EMC SSpectrum t• Elements of EMC Situation• Statement of problem St t t f bl• Electronic design in a systems approach• EMI-EMC EMI EMC and Signal Integrity d Si lI i• Board Level Grounding• Ground for Grounding• Board level EMC and mixed signal design
What is EMC ? Electro Magnetic Compatibility From a designer’s point of view, EMC phenomena have to be considered in two different ways: • How the environment may affect the design(susceptibility)EMS. • How the design may affect the environment(interference)EMI. Traditionally, Traditionally the only government regulations have been on the emission side: An electronic device is not allowed to emit more than a certain amount of radio frequency energy to avoid disturbing radio communication or operation of other electronic equipment equipment. Most countries in the world have regulations on this topic EMI + EMS = EMC
EMI‐EMC TESTS EMI‐EMC TESTS• Radiated Emission• Conducted Emission• Radiated Susceptibility• Conducted Susceptibility• Electro Static Discharge• Electrical Fast Transients• Surge Testing• Power Frequency Magnetic Field Testing• Line Voltage Fluctuations• Harmonics and FlickerThe radiated coupling path will be more efficient in the higherfrequencies while a conducted coupling path will be more efficient inthe lower frequencies frequencies.
The EM SpectrumFrequency - 300kHz 300MHz 300GHzWavelength - 1km 1m 1mm 0.7-0.4um 0.03um 0.3nm RadiowaveAudio Microwave Infrared Ultra X Ray Gamma Domain of Violet Vi l t Ray R EMI/EMC Visible 10kHz - 40GHz
Intra System Intra-System and Inter-System EMC Inter System Plug-in card Inter-Systems EMC Motherboard Intra-System EMCEMC Phenomena That affect the electro-magnetic environment can be divided into two characteristic groups1. Transient Interference Sources2. Continuous Interference Sources
Electromagnetic Compatibility & Product Design Approach Electromagnetic Compatibility & Product Design Approach EMC for Board design Board design (Layout) EMC for Test & EMC for Systems & Measurement Installation Circuit Design Signal Integrity DFT D-EMC DFM
Elements of an EMI Situation Elements of an EMI Situation–Source "Culprit" p–Coupling method or Medium "Path"–Sensitive device "Victim" VICTIM SOURCE
Statement of the problem and Design RequirementsTools that help •Question the customer •Differentiate Needs and Wants Needs as reflected to problem statement True needs Analog design octagon
Electronics design in a systems approach lectronics design in a systems approach1. Standards (DO254, FAA, MIL-217, ASTM, CE, FCC, TUV, UL,IEC,CISPR)2. Protocol (CAN, RS-232, ETHERNET, SPI, TTP)3. Topology (Point TO POINT, multi-drop, star, mesh, bus)4. Physical layer (UTP, TP, STP)5. Noise margin (cm, dm, THD, lvc, ttl, cmos, Differentials, Single Ended)6. Voltage levels (rs-232,rs485,lvds,ttl,cmos,can,se, Diff)7.7 Bandwidth (Amplifiers, DAC ADC (Amplifiers DAC, ADC, ETHERNET)8. Data rate (RS232,CAN,RS485, ttp)9. Distance (Trace length, Trace width-1mil,2mils,50mils,100Mils, cable length)10. Analog transmission of analog signals (example ?)11. Analog transmission of Digital signals (example ?)12. Digital transmission of analog signals (example ?)13. Digital transmission of digital signals (example ?)14. Signal integrity (Layer stackup, trace width, spacing, termination….)15. Power integrity (PS Layout, PSTopology, SWINCHING,BODE,LOOP ANALYSIS)16. Data integrity (15V& 3.3VCMOS, PARALLEL,SERIAL,SPEED,CPU,CLOCK ,PWB)17. DATAconversion (ADC,DAC,COMPARATOR)18. Analog signal conditioning (Amplifiers, ADC, DAC, SENSORs, transducers)19. Digital signal processor or Controller (Fixed point, floating point…)20. Grounding (Protection, Shielding, reference, mother earth, zero, AGND, DGND, Chassis, Field, Aruguments, rules, ground bounce, Equippotential, bonding,EMC grounding, plane, neutral, isolated, non isolated, Pulldown, safety, symbols, Return path, differential Gnd, common mode Gnd, pspice ground) , ,p p g )
Mixed signal design in a systems approach Low voltage interfacesL lt i t fGrounding in Mixed signal SystemsDigital and Power Isolating TechniquesPower supply noise reduction & filteringDealing with Logic design and its noise
Differential Mode & Single‐ended Mode Differential Mode & Single ended ModeDM: Interference signal in two lines are oppositely directed and thus no ground current path is requiredCM: Interference signal in two lines are unidirectional and return through ground
Differential Transmission Differential Transmission Below figure shows the electrical schematic diagram of a differential transmission circuit in which noise sources VN and VG add to each signal line and are common to both signals. The differential receiver measures the difference between the two lines and rejects the common voltage of the signals. If used with closely coupled lines, the complementary signals cancel each other’s electromagnetic fields resulting in high immunity and low noise emissions. This immunity to external noise influence and the low radiated emissions make differential signaling a good choice when relatively high signaling rates and long distance are required in electrically noisy, or noise-sensitive applications. Differential signaling comes with the additional cost of the line driver, receiver, and interconnection over the cost of single-ended transmission. , , gSince ground noise is also common to both signals, the receiver rejects this noise as well. The twisted pair cable usedin these interfaces in combination with a correct line termination to avoid line reflections allows very high data rates termination—to reflections—allowsof more than 10 Mbps and a cable length of up to 1200 m. Most recent standards allow up to 2.5 Gbps.
Advantages and Disadvantages Advantages and DisadvantagesAdvantages of Single-Ended TransmissionThe advantages of single-ended transmission are simplicity and l h d f l d d l d low cost of implementation. A single-ended f l l d dsystem requires only one line per signal. It is therefore ideal for cabling, and connector costs are moreimportant than the data transfer rate, e.g. PC, parallel printer port or serial communication with manyhandshaking lines, e.g. EIA-232. Cabling costs can be kept to a minimum with short distancecommunication, depending on data throughput, requiring no more than a low cost ribbon cable. For longer , p g g p , q g gdistances and/or noisy environments, shielding and additional ground lines are essential. Twisted pair cablesare recommended for line lengths of more than 1 meter.Disadvantages of Single-Ended Transmission Single EndedThe main disadvantage of the single-ended solution is its poor noise immunity. Because the ground wireforms part of the system, transient voltages or shifts in voltage potential may be induced (from nearby highfrequency logic or high current power circuits), leading to signal degradation. This may lead to false receivertriggering. For example, a shift in the ground potential at the receiver end of the system can lead to anapparent change in the signal, sufficient to drive the input across the thresholds of the receiver, thusincreasing its susceptibility to electromagnetic fields.Crosstalk is also a major concern especially at high frequencies. Crosstalk is generated from both capacitiveand inductive coupling between signal lines Capacitive coupling tends to be more severe at higher signal lines.frequencies as capacitive reactance decreases. The impedance and termination of the coupled line determineswhether the electric or the magnetic coupling is dominant. If the impedance of the line is high, the capacitivepickup is large. Alternatively, if the line impedance is low, the series impedance as seen by the inducedvoltage is low, allowing large induced currents to flow. Single-ended transmission is much more susceptible toexternal noise and the radiation of EMI is increased compared to differential systems These problems will systems.normally limit the distance and speed of reliable operation for a single-ended link.
Advantages & Disadvantages of Differential Transmission Advantages & Disadvantages of Differential TransmissionAdvantages of Differential Transmission gDifferential data transmission schemes are less susceptible to common-mode noise than single-ended schemes.Because this kind of transmission uses two wires with opposite current and voltage swings compared to only onewire for single-ended any external noise is coupled onto the two wires as a common mode voltage and is single ended,rejected by the receivers. This two-wire approach with opposite current and voltage swings alsoradiates less electro-magnetic interference (EMI) noise than single-ended signals due to thecanceling of magnetic fields.Disadvantages of Differential TransmissionThe Differential data transmission is expensive and the high data-rates that are possible with differential p g ptransmission require a very well-defined line impedance and correct line termination to avoid linereflections. For this method of transmission twisted pair cables instead of less expensive multi-conductor cablesare recommended.
Block Level Representation of EMI‐EMCBlock Level Representation of EMI‐EMC
ESD, Transient & SURGE ! & EMC gap ESD Transient & SURGE ! & EMC gapDon’t get confused by the similarities between 4 kV ESD testing, 4 kV fast transient burst testing and 4kVk surge. The voltages are the same, b the energy b h d them is totally d ff h l h but h behind h ll different. Dropping a smallllrock on your foot may hurt, but you will still be able to walk. Dropping a large rock from the sameheight will most likely cause severe damage to your foot. Doing this 250 times per second will reduceyour shoe size permanently. When the surge boulder falls, youd rather be somewhere else.Surge immunity test is the mother of all transient test, It tries to emulate what happens whenlightning hits (near) the power network, and the energies involved are high
Electrical & Physical Parameters and EMC bridgeElectrical & Physical Parameters and EMC bridge
Board level ground cont… Board level ground cont Single point grounds, with regards to noise, are very undesirable because of the series connection of all the individual circuit grounds. At high-frequencies the inductances of the ground conductors increase the ground impedance. A single-point ground is preferable below 1 MHz. Between 1 and 10 MHz a single point-ground can usually be used, provided the length of the longest ground conductor is less than one twentieth of a wave-length to prevent emissions and to maintain a low impedance.Multi-point grounds, have very low ground impedance and should be used at high frequencies and indigital circuitry. The low impedance is due primarily to the lower inductance of the ground plane. Theconnection between each circuit and the ground plane should be kept as short as possible to minimizetheir impedance. Multipoint grounds should be avoided at low frequencies since ground currents from allcircuits flow through a common ground impedance . the ground plane plane.A hybrid ground, is one in which the system grounding configuration appears differently at differentfrequencies . a single point ground at low frequencies and a multi point ground at high frequencies single-point frequencies, multi-point frequencies.When different types of circuits (low-level analog, digital, noisy, etc.) are used in the same system or onthe same PCB, then each must be grounded in a manner appropriate for that type of circuit. Thedifferent ground circuits should be tied together, usually at a single point.
Ground for Grounding Ground for GroundingThe term “signal grounding” encompasses two primary andcomplementary objectives namely signal voltage reference and signal objectives,current return path. In systems and facilities, a ground structure istypically intended to function as a common signal reference structureand to provide a near perfect voltage reference for the signal. The term “reference” implies voltage consideration rather than current( p(open-circuit voltage can exist, but no current can flow through an open g , g pcircuit); designers thinking in terms of voltage may be tempted to ignorethe need for adequate currentreturn paths. For high-frequency signals, “ground” is a concept that does notexist in reality. Signal ground can better be defined as a low-impedancepath for the signal’s current to return to the source.The concept of equipotential reference defines an ideal objective oftheth grounding system, whereas th concept of current return path di t h the t f t t thcharacterizes what the ground actually is.
Ground for Grounding Cont… Ground for Grounding ContSafety Grounding is intended for preclusion of hazards due to powerfaults or lightning strikes, which could set a facility ablaze and constitute asafety hazard to equipment and personnel. EMI Grounding is intended for controlling common mode EMI currentdrainage from cable shields and suppression devices as well to serve as an“image plane” for conductors routed adjacent to them. Signal Grounding essentially constitutes a “functional” or “technical”ground, intended to provide an equipotential signal voltage referencebetween components of the system and serve as a path for signal currentreturn, particularly in unbalanced or single-ended interfaces. With theexception of electrical safety considerations and certain issues related toelectrostatic shielding, connections of electronic circuits t th ground play l t t ti hi ldi ti f l t i i it to the d lno other role than to provide the signal and EMI current returns (whetherdesired or unintentional)
Ground for Grounding Ground for GroundingReal-world ground structures are non-ideal i nature. SR l ld d t t id l in t Some potential t ti ldifference always exists; thus, all reference conductors should beassumed to carry current, whether intended or unintended. The extentto which potentials in the ground system can be minimized and groundcurrents reduced will determine the effectiveness of the ground
Radiated & Conducted EnergyRadiated & Conducted Energy
Impedance Impedance• Resistance Capacitive reactance and inductive reactance all Resistance, act to oppose the flow of current• R it Resistance R opposes current flow because of resistivity of the t fl b f i ti it f th conductor from which it is made• Capacitive reactance XC opposes current flow because of charge present on the plates of the capacitor• Inductive reactance XL opposes current flow because of electromagnetic field within the inductor• The term that covers all these kinds of opposition to current flow i fl is IMPEDANCE
EMI Sources and Paths in a PWB EMI Sources and Paths in a PWB
Fourier and Frequency Domain of Digital SignalsFourier and Frequency Domain of Digital Signals In Figure above, the bandwidth contains 99% of the spectral energy of the signal. The spectrum of the square wave in Figure is also its Fourier series. Fourier theory states that a periodic signal can be expressed in terms of weighted sum of harmonically related sinusoids.A square wave has an AC component during the transition times and a DC component during the steady state.The AC current contains all of the frequency components of the square wave. In addition to the fundamentalfrequency, a digital signal also contains harmonic frequencies which are integer multiples of the fundamental q y, g g q g pfrequency. For example, a digital signal with a fundamental frequency of 10 MHz has harmonic frequencycomponents at 20, 30, 40, . MHz
Measuring Common Mode Currents Measuring Common Mode CurrentsEquation gives the electric field in dBµV per meter for a short wire (relative to wavelength) in freespace due to the spectral amplitude of current In. Use this equation to estimate the electric fieldemissions due to CM current.
Measuring common mode currents cont.. Measuring common mode currents contSolving Equation above for the current gives Equation below:Table below shows the maximum CM current that can flow on a single wire to just meet the limitfor radiated emissions To find and measure the maximum CM current move the current probe emissions.along the harness length while monitoring the current with a spectrum analyzer.
Radiated Emissions Near field and Far field Radiated Emissions Near field and Far fieldAbove equation predicts the maximum electric field in the far field from a small loop. It is accurate whenthe loop perimeter is less than one-quarter wavelength, and approximate for larger loops. In the near fieldmultiply Equation above by Equation mentioned below py q y q
Radiated emissions design example Radiated emissions design exampleTable below shows the radiated emissions at 1 meter from a PCB circuit with the following values:Area = 5.0x10-4 meter2 (5 cmx1 cm)Fundamental frequency = 10 MHzImax = 10 mAI ARise time = fall time = 5 ns (typical high-speed CMOS)
PWB Design Flow PWB Design Flow EMI Control techniques at source Important techniques to control EMI at source are Proper Grounding: single point, multi point or hybrid grounding depending upon the frequency of operation Shielding: Metal barrier is used to suppress coupling of Radiated EM energy into the equipment. EMI Filtering: used to suppress conducted interference on Power, Power signal and control lines. lines PCB Layout: Proper PCB design from the early design stage is required
Mixed signal design exampleMixed signal design example
EMI‐EMC FOR LAYOUT CONT….PART 2 Thank you Thank you
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