Design For EMI‐EMC 

Introduction to the WORLD OF BLACK MAGIC

By Vemana Shankar 

EMI EMCEMI‐EMC  Overview Of EMI-EMC Overview Of EMI EMC

Design For EMI-EMC

EMI-EMC For Testing & Measurement

EMI-EMC Standards

EMI EMC for LayoutEMI-EMC for Layout

AgendaAgenda

What is EMC ? • What is EMC ? • Basic EMI-EMC Tests

EMC S t• EMC Spectrum• Elements of EMC Situation

St t t f bl • Statement of problem • Electronic design in a systems approach

EMI EMC d Si l I i • EMI-EMC and Signal Integrity • Board Level Grounding • Ground for Grounding • Board level EMC and mixed signal design

What is EMC ?

Electro Magnetic Compatibility

What is EMC ?

From a designer’s point of view, EMC phenomena have to be considered in two different ways: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 the only government regulations have been on theTraditionally, the only government regulations have been on theemission side: An electronic device is not allowed to emit more thana certain amount of radio frequency energy to avoid disturbing radiocommunication or operation of other electronic equipmentcommunication or operation of other electronic equipment.

Most countries in the world have regulations on this topic

E M I + E M S = E M C

EMI‐EMC TESTSEMI‐EMC TESTS  • Radiated Emission

Conducted Emission • Conducted Emission

• Radiated Susceptibility

• Conducted SusceptibilityConducted Susceptibility

• Electro Static Discharge

• Electrical Fast Transients

• Surge Testing

• Power Frequency Magnetic Field Testing

• Line Voltage Fluctuations

• Harmonics and Flicker

The radiated coupling path will be more efficient in the higherfrequencies while a conducted coupling path will be more efficient inthe lower frequenciesthe lower frequencies.

The EM SpectrumThe EM Spectrum

Frequency - 300kHzWavelength - 1km

300MHz1m

300GHz1mm 0.7-0.4um 0.03um 0.3nm

Audio Microwave InfraredRadiowave

Audio Microwave Infrared

UltraVi l t

X Ray GammaR

Visible

Violet RayDomain ofEMI/EMC10kHz 10kHz -40GHz

Intra-System and Inter-System EMCIntra System and Inter System EMC

Plug-in card

Intra-System EMCMotherboard

Inter-Systems EMC

Intra-System EMC

EMC Phenomena That affect the electro-magnetic environment can be divided into two characteristic groups

1. Transient Interference Sources1. Transient Interference Sources2. Continuous Interference Sources

Electromagnetic Compatibility & Product Design ApproachElectromagnetic Compatibility & Product Design Approach 

EMC for Board design

Board design(Layout)

EMC for Test & Measurement

EMC for Systems &Installation Circuit Design Signal Integrity

DFT

DFM D-EMC

Elements of an EMI SituationElements of an EMI Situation–Source "Culprit"p–Coupling method or Medium "Path"

"–Sensitive device "Victim"VICTIM

SOURCE

Statement of the problem and Design Requirements

Tools 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 approachlectronics 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 Bandwidth (Amplifiers DAC ADC ETHERNET)7. Bandwidth (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 

L lt i t fLow voltage interfacesGrounding in Mixed signal SystemsDigital and Power Isolating Techniques Power supply noise reduction & filteringPower supply noise reduction & filteringDealing with Logic design and its noise

Single Ended Transmission‐Switching Levels

Differential Transmission‐Switching Levels

Differential Mode & Single ended ModeDifferential Mode & Single‐ended Mode

DM: Interference signal in two lines are oppositely directed and thus no ground current path is requiredDM: Interference signal in two lines are oppositely directed and thus no ground current path is required

CM: Interference signal in two lines are unidirectional and return through ground

Differential TransmissionDifferential TransmissionBelow figure shows the electrical schematic diagram of a differential transmission circuit in which noise sources VNand VG add to each signal line and are common to both signals. The differential receiver measures thedifference between the two lines and rejects the common voltage of the signals. If used with closelycoupled lines, the complementary signals cancel each other’s electromagnetic fields resulting in highimmunity and low noise emissions. This immunity to external noise influence and the low radiated emissionsmake differential signaling a good choice when relatively high signaling rates and long distance are required inelectrically noisy, or noise-sensitive applications. Differential signaling comes with the additional cost of the linedriver, receiver, and interconnection over the cost of single-ended transmission., , g

Since 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 ratesin these interfaces in combination with a correct line termination—to avoid line reflections—allows very high data ratesof more than 10 Mbps and a cable length of up to 1200 m. Most recent standards allow up to 2.5 Gbps.

Advantages and DisadvantagesAdvantages and DisadvantagesAdvantages of Single-Ended Transmission

h d f l d d l d l f l l d dThe advantages of single-ended transmission are simplicity and low cost of implementation. A single-endedsystem 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 TransmissionDisadvantages of Single Ended Transmission

The main disadvantage of the single-ended solution is its poor noise immunity. Because the ground wire forms part of the system, transient voltages or shifts in voltage potential may be induced (from nearby high frequency logic or high current power circuits), leading to signal degradation. This may lead to false receiver triggering. For example, a shift in the ground potential at the receiver end of the system can lead to an apparent change in the signal, sufficient to drive the input across the thresholds of the receiver, thus increasing its susceptibility to electromagnetic fields.Crosstalk is also a major concern especially at high frequencies. Crosstalk is generated from both capacitive and inductive coupling between signal lines Capacitive coupling tends to be more severe at higher signal and inductive coupling between signal lines. Capacitive coupling tends to be more severe at higher signal frequencies as capacitive reactance decreases. The impedance and termination of the coupled line determines whether the electric or the magnetic coupling is dominant. If the impedance of the line is high, the capacitive pickup is large. Alternatively, if the line impedance is low, the series impedance as seen by the induced voltage is low, allowing large induced currents to flow. Single-ended transmission is much more susceptible to external noise and the radiation of EMI is increased compared to differential systems These problems will external noise and the radiation of EMI is increased compared to differential systems. These problems will normally limit the distance and speed of reliable operation for a single-ended link.

Advantages & Disadvantages of Differential TransmissionAdvantages & Disadvantages of Differential Transmission

Advantages of Differential Transmissiong

Differential 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 iswire for single ended, any external noise is coupled onto the two wires as a common mode voltage and isrejected 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 Transmission

The Differential data transmission is expensive and the high data-rates that are possible with differentialp 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 gapESD, Transient & SURGE ! & EMC gapDon’t get confused by the similarities between 4 kV ESD testing, 4 kV fast transient burst testing and 4k h l h b h b h d h ll d ff llkV surge. The voltages are the same, but the energy behind them is totally different. Dropping a smallrock 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, you'd 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 groundBoard level ground

Board level ground contBoard level ground cont…

Board level ground contBoard level ground cont…Single point grounds, with regards to noise, are very undesirable because of the series connection of allthe individual circuit grounds. At high-frequencies the inductances of the ground conductors increase theground impedance. A single-point ground is preferable below 1 MHz. Between 1 and 10 MHz a singlepoint-ground can usually be used, provided the length of the longest ground conductor is less than onetwentieth 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 planecircuits flow through a common ground impedance . the ground 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 frequenciesfrequencies . a single-point ground at low frequencies, and a multi-point ground at high 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 GroundingGround for Grounding The term “signal grounding” encompasses two primary andcomplementary objectives namely signal voltage reference and signalcomplementary objectives, namely signal voltage reference and signalcurrent 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.and to provide a near perfect voltage reference for the signal.

The term “reference” implies voltage consideration rather than current(open-circuit voltage can exist, but no current can flow through an open( p 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 ofth di t h th t f t t ththe grounding system, whereas the concept of current return pathcharacterizes what the ground actually is.

Ground for Grounding ContGround for Grounding Cont…Safety Grounding is intended for preclusion of hazards due to powerSafety 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 tol t t ti hi ldi ti f l t i i it t th d lelectrostatic shielding, connections of electronic circuits to the ground play

no other role than to provide the signal and EMI current returns (whetherdesired or unintentional)

Ground for GroundingGround for Grounding 

R l ld d t t id l i t S t ti lReal-world ground structures are non-ideal in nature. Some potentialdifference 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 groundto which potentials in the ground system can be minimized and groundcurrents reduced will determine the effectiveness of the ground

GroundingGrounding 

Radiated & Conducted EnergyRadiated & Conducted Energy

ImpedanceImpedance 

• Resistance Capacitive reactance and inductive reactance allResistance, Capacitive reactance and inductive reactance all act to oppose the flow of current

R i t R t fl b f i ti it f th• Resistance R opposes current flow because of resistivity of the 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 inductorelectromagnetic field within the inductor

• The term that covers all these kinds of opposition to current fl i IMPEDANCEflow is IMPEDANCE

EMI Sources and Paths in a PWBEMI Sources and Paths in a PWB 

Transmission LinesTransmission Lines

Differential Mode Radiation & Controlling 

Common mode radiation and groundCommon mode radiation and ground  

Digital Logic Current FlowDigital Logic Current Flow 

Positive Pulse WaveformPositive Pulse Waveform 

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 squarewave in Figure is also its Fourier series. Fourier theory states that a periodic signal can be expressed interms 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 fundamentalq 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 CurrentsMeasuring Common Mode Currents  

Equation 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.emissions due to CM current.

Measuring common mode currents contMeasuring common mode currents cont..Solving 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 probefor radiated emissions. To find and measure the maximum CM current move the current probealong the harness length while monitoring the current with a spectrum analyzer.

Radiated Emissions Near field and Far fieldRadiated Emissions Near field and Far field 

Above equation predicts the maximum electric field in the far field from a small loop. It is accurate when the loop perimeter is less than one-quarter wavelength, and approximate for larger loops. In the near field multiply Equation above by Equation mentioned belowp y q y q

Radiated emissions design exampleRadiated 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 MHzI 10 AImax = 10 mARise time = fall time = 5 ns (typical high-speed CMOS)

PWB Design FlowPWB Design Flow 

EMI Control techniques at sourceImportant 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 ofRadiated EM energy into the equipment.

EMI Filtering: used to suppress conducted interference on Power signal and control linesPower, signal and control 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 youThank you