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UNIT -5

LECTURE ON EMI & EMC

By:

AJAY YADAV

ASSOCIATE PROFESSOR

DEPT. OF ECE

AIET, JAIPUR

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BEFORE EMI&EMC

CONCEPT OF RADIATION

Maxwells equations

enclenclE

B

encl

AdEdt

dI

dt

dIsdB

AdBdt

d

dt

dsdE

AdB

QAdE

)()(

0

0000

0

Gausss law

Gausss law formagnetism

Faradys law

Amperes law

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Displacement Current & Maxwells Equations

Maxwells equations: Differential form

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Oscillating electric dipole

First consider static electric field produced by

an electric dipole as shown in Figs.

(a) Positive (negative) charge at the top (bottom)

(b) Negative (positive) charge at the top (bottom)

Now then imagine these two charge are moving

up and down and exchange their position at every

half-period. Then between the two cases there is

a situation like as shown in Fig. below:

What is the electric field

in the blank area?

Maxwells Equations and EM Waves

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Oscillating electric dipole (contd)

Since we dont assume that change propagate instantly once new positionis reached the blank represents what has to happen to the fields in meantime.

We learned that E field lines cant cross and they need to be continuous except

at charges. Therefore a plausible guess is as shown in the right figure.


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What actually happens to the fields based on a precise calculate is shown inFig. Magnetic fields are also formed. When there is electric current, magnetic

field is produced. If the current is in a straight wire circular magnetic field is

generated. Its magnitude is inversely proportional to the distance from the

current.


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What actually happens to the fields based on a precise calculate is shown inFig.


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This is an animation of radiation of EM wave by an oscillating electric dipoleas a function of time.


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At a location far away from the source of the EM wave, the wavebecomes plane wave.

+

+

-

-

-

-

+

+

V(t)=Vocos(t)

time t=0 time t=/one half cycle later

XBB

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+

+

-

-

x

z

y



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A qualitative summary of the observation of this example is:

1) The E and B fields are always at right angles to each other.

2) The propagation of the fields, i.e., their direction of travel away from the

oscillating dipole, is perpendicular to the direction in which the fields

point at any given position in space.3) In a location far from the dipole, the electric field appears to form closed

loops which are not connected to either charge. This is, of course, always

true for any B field. Thus, far from the dipole, we find that the E and B

fields are traveling independent of the charges. They propagate away from

the dipole and spread out through space.


In general it can be proved that accelerating electric charges give rise to

electromagnetic waves.

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What is EMI and EMC ?

An electromagnetic disturbance which may degrade the

performance of an equipment (device, system or sub-system) or

causes malfunction of the equipment, is called electromagnetic

interference (EMI).

Electromagnetic compatibility (EMC) is a near perfect state in

which a receptor ( device , system or subsystem) functions

satisfactorily in common electromagnetic environment, without

introducing intolerable electromagnetic disturbance to any

other devices / equipments / system in that environment.

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Electronic equipment is subjected to a variety of electromagnetic interference sources.

Careful design is required to guarantee compatibility with environment- Intersystem

EMI

Conducted noise

AC power circuitElectric Motors

Power Line

Lightning

Radio & TV

Broadcast

IgnitionMobile

Radio

Ship

Radar

Handy

Talkie

Telecommunications CE RE

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RF

AmplifierMixer IF

Amplifier

Detector Audio

Amplifier

Power

Supply

Oscillator

Speaker

Antenna

Electric field coupling

Conductive coupling

Magnetic field coupling

Common impedance coupling

EXAMPLE - Intersystem

EMI

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Introduction

Elements of an EMI Situation

Source "Culprit"

Coupling method "Path"

Sensitive device "Victim"

SOURCEPATH

VICTIM

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CAUSES OF EMI

Sources Refrigerator, washing machine, electric motors.

Arc welding machine.

Electric shavers, AC, computers.

Fast switching digital devices, ICs.

Power cords of computers, UPS etc. Air craft navigation and military equipments.

Victims Communication receivers.

Microprocessors, computers.

Industrial controls.

Medical devices.

House hold appliances.

Living beings.

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EFFECTS OF EMI

Momentary disturbance in TV and radio reception due tooperation of mixer-grinder / electric shavers / a passingvehicle etc.

Reset of computers and loss of data.

Burn out of sensitive cells / components. Change of setting of status of control equipments.

Failure of pace maker implanted in a patient due to awalkietalkie.

False initiation of electro explosive detonator.

Malfunctioning of flight controlling system due to use oflaptop by passenger.

Biological hazards.

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A BASIC EMI SITUATION

EMI

Source

Coupling

Path

Victim

of EMI(Emitter)

(Media) (Receptor)Impedance? Impedance? Impedance?

Voltage measuring device - high impedance circuit

Voltage generative device - high impedance circuit

E-field source/victim - high impedance circuit

Current measuring device - low impedance

Current generating device - low impedance

H-field source / victim - low impedance

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Interference coupling mechanisms

coupling path

Direct couplingRadiated

coupling

Near field

coupling

source victim

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COUPLING PATH

Direct coupling

Coupling via

power or signal

lines

Common

impedance

coupling

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DIRECT COUPLING

Coupling via

power or signal

lines

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DIRECT COUPLING

Common

impedance

coupling

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COUPLING PATH

Near field coupling

Magnetic or

inductive

coupling

Electric or

capacitive

coupling

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NEAR FIELD COULING

Magnetic or

inductive

coupling

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NEAR FIELD COULING

Electric or

capacitive

coupling

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COUPLING PATH

Radiated coupling

Waveimpedance

Fieldgeneration

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Lets see how this all got started

Dead Smart Guys

First Transmitters: Spark Devices

Heinrich Hertz (1857-1894) clarified andexpanded on

James Clerk Maxwells Electromagnetic

Theory

Marconi: first use & patent

HertzMaxwell

Marconi

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How Does EMI Affect Electronics?

Radiated and conducted interference

Conducted Interference Enters and Exits Equipment through

Wiring and Cabling

Radiated Interference Enters and Exits Equipment through Wiring

and Enclosure Penetration

Radiated Susceptibility Radiated Emissions

Conducted Susceptibility Conducted Emissions

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Interference to TV Reception

Two Interfering Signals Injected into TV

No Interference

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EMI/EMC COUPLING MODES

Coupling modes

Antenna modeCommon modeDifferential

mode

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Common Coupling Modes

Common and Differential Mode

Crosstalk (cabling and conductors)

Field to cable (Antenna)

Conducted (direct)

Field to enclosure

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Crosstalk

(cable-to-cable coupling)

SOURCE

VICTIM

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Radiated Coupling: Field to Cable

Loop Area

Induced Current

Electromagnetic Wave

Coupling proportional to: E/H Field, Loop Area, Frequency

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COMMON and DIFFERENTIAL MODE

COMMON-MODE: Line to Ground DIFFERENTIAL MODE: Line-to-Line (Normal Mode)

VCM

VDM

INoise

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Radiated Coupling: Field to Cable

Patient Monitor

Loop AreaInduced Current

Electromagnetic WaveRadio

VCM

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Instrumentation Interference

Interference Current, If

Ideal Response

Frequency (Hz)

EKG Signal

Real Response

Frequency (MHz)

NOISE

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Effect of Modulation

Interference Current, If

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How Does EMI Affect Electronics?

Electrostatic Discharge & Transient Pulses

ESD can induce glitches in circuits, leading to

false triggering, errors in address & data lines

and latch-up of devices

Upset

Damage Degradation leading to future failure(s)

Gee, the humidity

is low in here.

Whats this for?

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Filtering

Interference Current

EKG Signal

C

C

Interference Current

EKG Signal

Please, Im veryticklish

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Surge Coupling

Lightning and pulse sources cause high-energy transients into

power and data cables

IndirectDirect

Digital Equipment Sources

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Digital Equipment SourcesFourier Analysis

F(t)Log Ff=

1/T

2f 3f

T

A

Spectrum of a Square Wave

T

A

Log F

F(t)f=

1/t

f=1/tr

tr

t

Spectrum of a Trapezoidal Wave

(Characteristic of Digital Devices)

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Equipment Emissions Limits

Emissions Limits @ 3 meters

0

10

20

30

40

50

60

70

10 100 1000 10000

Frequency (MHz)

dBuV/m

FCC BCISPR B

FCC ACISPR A

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The decibel (dB)

The dB is used in Regulatory Limits (FCC, CISPR, etc.)

The dB is a convenient way to express very big and very small numbers

The Bel was named after Alexander Graham Bell

Bel = LOG10(P2/P1)

deciBel provides a more realistic scale:

dB = 10LOG10(P2/P1)

Voltage & Current are expressed as follows:

dB (V or I) = 20LOG10(V2/V1)

20LOG derives from the conversion from Power to Voltage

(ohms Law: P = E2/R)

Named

after me!

dB

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dB

Can have several reference units: Watt: dB above one Watt (dBW)

Milliwatt: dB above one milliwatt (dBm)

Volt: dBV

Microvolt: dBuV

Microamp: dBuA picotesla: dBpT

Electric Field: dBuV/m

Radio Receiver Sensitivity ~ 10 dBuV

E-Field Limit for FCC: ~40-60 dBuV/m Distance to moon: 107dBmile (20LOG2.5E+5miles)

National debt: 128dB$ (10LOG6E+12)

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Broadband Sources

Man-made noise dominates Intended transmissions, switching transients, motors, arcing

Intermittent operation of CW causes transient effects

Digital Switching

Inductive kick Switch bounce

Digital Signaling Broad spectrum based on pulse width & transition time

HDTV

CDMA

UWB Technologies

C bl O i

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Cables - Overview

Major coupling factor in radiating emissions from an equipment and coupling of

emissions from other sources into an equipment Acts as radiating antenna, receiving antenna, and cable-to-cable coupling

mechanism External cables are not typically part of the equipment design but the installation

requirements must be considered during the design Problem is a function of cable length, impedance, geometry, frequency of the

signal and harmonics, current in the line, distance from cable to observation point Frequency Effects: Tied into Cable Wavelength

For example, wavelength at FM Radio Band (100 MHz) is 1 meter Human Body Resonance

= c/f = 3X108/frequency = 300/fMHz

/

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Cables - Length/Impedance

Efficiency as an antenna - function of length compared to wavelength

At typical data transfer rates - length is short

At harmonics or spurs the length may become long

Impedance mismatch creates a high SWR

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How very important

Frequencies of testing from 26 MHz to 1 GHz

Corresponding cable lengths:

L ~ 11 meters @ 26 MHz to 30 cm @ 1 GHz Short cables can be large contributors to Interference

Problems Power cables

Grounding wires

Patient cables Data cables

Control harnesses

Structures!

C bl L

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Cables - Loops

Emissions are a function of 1) Current; 2) Loop Geometry; 3) Return Path of the Current

Current flow creates a magnetic field H=I/2R for a single wire model

Single wire case is not realistic

Loop geometry formed by the current carrying conductor and the return line contribute

to the field strength

Electric field strength:

E f AI

RV m MHz cmamps

meters

( / ) ( ) ( )

( )

( )

. * * 13 2 2

V ~

I

Area

E (& H)

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Filters - Overview

Passband

High pass

Low pass Single component, L, Pi, T

Common mode; differential mode

Placement

Components Lead length

Leakage Limitations

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Low Pass Filter

Noise Current

EKG Signal

C

C

Noise Current

EKG Signal

Frequency (Hz)

Rejection

EKG Signal

Noise

Attenuation of Noise

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Filters - Types

Filt C t

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Filters - Components

Discrete Component Filters Component selection

Lead length considerations

Power Filter Modules Filtered Connectors

Construction

Selective loading Termination (bonding and grounding)

Circuit Design Real Performance

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Circuit Design Real Performance

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Filters

Power Line Filter Typical Schematic

Signal Line Filter(Screw-in Type)

Signal Line Filter

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Filter - Placement Isolate Input & Output

Establish boundaries with filters between

Input or Output interfaces and active circuitry

Digital and Analog

Compartments and Modules Prevent bypass coupling

Control line exposure on line side of filter

Use dog-house compartment

Shielded cables to control exposed cable runs

Terminate - Terminate - Terminate Low impedance to ground termination

Minimize lead length

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Filter Performance

Poor Installation =Poor Performance

Filter

Filter INFilter OUT

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Filter Placement

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EMC DESIGN

There are many design considerations that need to

be taken Cable wiring

Connectors

Grounding

Shielding

The reference for good consideration is standard

h ld

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Shield Concepts

+ -

Field Terminations on Inside

Metal Sphere

Faraday Cage

Ground 0V Potential

V+

V=0

+ -Electric Field Coupling

E-FieldV+

Shield Concepts

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Shield Concepts

Magnetic Field Shielding

Common at powerline and lowfrequencies;

High-current conditions

I

V

>>1

Ferrous Shield

Low residual field

Magnetic Field Coupling

V

I

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Effects of Openings

+ -

Metal Sphere

Faraday Cage V=0

V+

V=?

Cable Leakage

+

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Radio Frequency Effects

VRF~

Shielded Enclosure

RF Source

RF L k

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RF Leakage

VRF ~

Metal Box

RF Source

L

L ~ /2Perfect Transmission

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Shielding

The Business Card Test

Good to about 1 GHz

Shi ldi O i

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Shielding - Overview

Shields - conductive barriers Reflection

Absorption

Materials Electric field - conductivity

Magnetic field - permeability

Discontinuities Windows

Vents

Seams

Panel components

Cable connections

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Shielding Effectiveness

SHIELD

Incident Field E1 Resultant FieldE2

SE = E2

/E1

(dB)

ReflectedER

h ld

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Shielding -Reflection/Absorption

RR fE dB meters Hz

( )

( ) ( )

log(* *

) 322 10 2 3

R f RH dBHz meters

( )

( ) ( ). log( * * ) 14 5 10

2

R fP dB Hz( ) ( )log( * ) 168 10

A k t fdB Hz ( ) ( )* * * *

Plane wave occurs when E to H wave impedance ratio = 1

f RMHz meters( ) ( )> 3002

k = 3.4 for t in inches and k = 134 for t in meters

Shielding Material

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Shielding - Material

Metal Conductivity - Permeability -

Silver 1.05 1

Copper 1 1

Gold 0.7 1

Aluminum 0.61 1Zinc 0.29 1

Brass 0.26 1

Nickel 0.2 1

Iron 0.17 1000

Tin 0.15 1

Steel 0.1 1000Hypernick 0.06 80000

Monel 0.04 1

Mu-Metal 0.03 80000

Stainless Steel 0.02 1000

All are good electric field shields Need high u for Mag Field Shield

Shielding Seams/Gaskets

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Shielding - Seams/Gaskets

Required openings offer no shielding in many applications

Apertures associated with covers tend to be long or require many contact

points (close screw spacing)

Large opening treatment

Screens, ventilation covers, optic window treatments

WBCO formed to effectively close opening

Seam opening treatments

Overlapping flanges

Closely spaces screws or weld

Gasket to provide opening contact Gasketed SE

SE a LdB cm( ) ( ).log( * ) 115 10 1 2 SE a LdB in( ) ( )

.log( * ) 99 10 1 2

Shielding Penetration

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Shielding - Penetration

Conductors penetrating an opening negates the shieldingprovided by absorption and reflection

Cables penetrations require continuation of the shield or

Conductors require filtering at the boundary

Cable shields require termination

Metal control shafts serve as a conductor

Use non-metallic

Terminate shaft (full circle)

Grounding - Overview

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Grounding - Overview

Purpose Safety protection from power faults

Lightning protection

Dissipation of electrostatic charge

Reference point for signals Reference point is prime importance for EMC

Potential problems

Common return path coupling

High common impedance High frequency performance

Grounding Impedance

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Grounding - Impedance

Establish a low impedance return Ground planes

Ground straps for high frequency performance

Establish single point or multipoint ground Single point for low frequency or short distance

Distance(meters) < 15/f(MHz)

Multipoint for high frequency or long distance

Distance(meters) > 15/f(MHz)

Bonding

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Bonding

Bonds should have two basic characteristics Low impedance < 2.5 milliohms

Mechanical & electro-chemical stability

Low impedance

Avoid contamination

Provide for flush junction to maximize surface contact

Use gaskets or fingerstock for seam bonds

Provide a connecting mechanism

Mechanical and electro-chemical stability

Torque to seat for the mechanical connection Lock washers to retain bond

Allow for galvanic activity for dissimilar metals

Galvanic Scale

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Galvanic Scale

Component Selection

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Component Selection

T

A

Log F

F(t)

f=1/T

2f 3f

T

A

Log F

F(t)f=

1/t

f=1/tr

tr

t

Spectrum of a Square Wave

Spectrum of a Trapezoidal Wave

(Characteristic of Digital Devices)

Circuit Design

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Component Selection

Circuits available in an EMI version

Specify logic of necessary speed - not faster than required

EMI performance varies between manufacturers

MAX485 MAX487

EMI V dV

dt *

Switching Power Supplies

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Switching Power Supplies

Two Sources:

Harmonics of switching power supply

Broadband emissions due to ringingwaveforms

&f

f

U d d d (Ri i ) W f

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Underdamped (Ringing) Waveform

Typical in switching circuits

f100 MHz+

100s

Volts

10s kHz

dV/dT = 100sMV/s

Broadband (radiated & conducted)

Circuit Design - Summary

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Circuit Design Summary

Consider EMI at the beginning Understand requirements

Select components

Design in protection

Circuit Design - Layout

Design in ground planes, guards, segregation EMI gains from layout has virtually zero recurring cost

Grounds and Returns

Develop a ground scheme

Consider digital, analog, return, and shield terminations

Design in hooks Provide space for potential fix actions that may be required

Decoupling &

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Power Distribution

Connect all ground pins of high frequency circuitstogether in the same ground structure.

Do not separate, isolate, break or otherwise cutthe ground plane.

Do not separate, isolate, break or otherwise cutthe power plane.

Do not insert impedances into Vcc/power traces.

Isolated Power/Grounding

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Isolated Power/Grounding

Example Trace Layout (Bad Idea!)

Exception: Analog circuit isolation

Top 10 Common Mistakes

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1. Improperly shielded cables: The principalproblem is the cable-to-backshell termination

2. Unfiltered cable penetrations

3. High Frequency sources with poor termination:

High frequency sources: signals and power supplies

4. Case seams and apertures: bad/no gasket, or

improper mating surfaces5. Poor bonding between metal parts of unit


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5. Long ground leads on shields and bondingconductors

6. No high frequency filtering on analog inputs:

Radiated and conducted immunity7. Not accounting for the high frequency effects of

ESD

8. Inadequate filters on I/O cables for emissions

9. Inadequately-installed power line filters

The Ten Steps to

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pAvoiding EMI Problems

1. Signal Termination

2. Layout

3. Decoupling & PowerDistribution

4. Grounding

5. Bonding

6. Filtering

7. Cabling8. Shielding

9. Surge Suppression

10. CHECKLIST

CHECKLIST

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CHECKLIST

Signal Termination

RC Terminations (33 ohms + 27 pF) onper iodic sign als

Group high frequency so urces together;minimize trace runs of high frequency

signals

Dont source/sink I/O (whether internal orexternal) through h igh frequency de vices

Position oscillators and crystals away fromI/O and openings in the chassis

Snub sw itching power supp ly waveforms tominimize HF energy

Decoupling & Power D istribution Connect all ground pins of high frequency

circuits togethe r

0V reference (bond 0V to chassis)

Solid power and Ground planes

No impedances in Vcc/power traces.

Bonding C hecklist Bond 0V to chassis ground

Bond 0V to connector frames and shells

Bond conn ector frames to chassis

Bond m etal frames together

Filtering

Filters are installed at enc losure wall LC filter on unshielded cables

Plan for capacitor on shielded lines

Cabling Route cables to avoid coupling

Use onlyfully-shielded cables

Fully-terminate shield grounds tometal/metalized connector shels

Terminate shells to chassis

Shielding The Business Card Test

Use correctly-rated suppressor line-to-lineand line-to-ground

Gas Tubes

Varistors

SAD (Silicon Avalanche Diodes)

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