electromagnetic compatibility “on tour” · pdf fileelectromagnetic compatibility...

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in cooperation with

Frits J.K. Buesink, Senior Researcher [email protected]

Picture or Drawing 20.7 x 8.6 cm

UNIVERSITY OF TWENTE.TELECOMMUNICATION ENGINEERING.

Electromagnetic Compatibility “On Tour”

Engineering Compatible Equipment and Systems

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in cooperation withEngineering Electromagnetic Compatibility

Definition of EMC

“The ability of the System to Operate according to its Specificationsin its Intended Electromagnetic Environment”

“Without generating Unacceptable Electromagnetic Emissionsinto that Environment”

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Three Criteria for EMC

1. No (intolerable) emissions into the environment

2. Operate satisfactorily in its EM environment

3. Not cause interference with itself

3



Performance Criteria

what happens when immunity threshold levels are approached?

ASystem continues to work according to specificationDegradation not acceptableGenerally applies to all interference with a continuous nature

BTemporary degradation acceptable, auto recovery.Usually applies to sporadic interferenceto a non-critical function.

C Degradation acceptable. Recovery after manual RESET.e.g. at mains interruptions. Only for non-critical functions. A

n U

NS

AF

Esi

tuat

ion

is n

ever

acc

epta

ble

!

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3



The necessary elements for an interference situation

EMI, ElectroMagnetic Interference model: source – victim and coupling path

Source Victimcoupling pathcoupling path

Coupling path: always electrical interconnections

to tiny.

EmissionSusceptibility

(Immunity)

Very large….

this can be demonstrated using: a noise generator a radio receiver and some cables

Effects appearat any scale

(relative to wavelength)

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Wheelchair Problems.. (SBS6, 14 June 2007)

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4



Two days later..

recall of all vehicles

Tubantia,16 June 2007

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Whether Pacemakers are Indeed this Susceptible?

fields up to 30 kV/m and not just 50 Hz!

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5



All Currents Run in Loops

Kirchhoffs Current Law: basic for the design of component networks

I1 I2

I3

Ia Ib

Kirchhoff’s electrical current law

213 III

ba II

Every current musthave a return path!

As a Designer,ask yourself:

Where does myReturn Current

Flow?

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Common-mode currents dominate the EMC arena

currents, generated by cables’ “desired currents” into CM or ground-loop

Source Load

“Ground”

“Differential-mode” currentIdm

Icm

“Common-mode” current

CM: 98%of all EMIproblems!

Common-mode current is thatpart of the return current which

follows a different path thanthe designers intended route

CM-currentscan becreated

“elsewhere”

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6



Interference: Current in Circuit 1 influences Circuit 2

the coupling path is usually through common-mode currents (or fields)

signal 1

return 1

signal 2

return 2

coupling e.g. through “ground” connections

Circuit 1

Circuit 2

intended (DM) current of circuit 1

intended (DM) current of circuit 2

unintended (CM) current from circuit 1 to circuit 2 (and vice versa)

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Kirchhoff’s Voltage Law

Us

R1

R2

UR1

UR

2

021 RRS UUU

Inductive EMI Effects: Faraday’s Law

Completes Kirchhoff’s Voltage Law (to suit Maxwell’s equations)

loop 1flux flux

Faraday’s Law:

?

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7



The phenomenon “Self-induction”

current in a conductor is only possible when magnetic field exists

50

coax cable

single wire

1. Waveform for fast edge

A B

A

B

signal integrity =no distortion onthe signal line

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50

coax cable

1a. Waveform for fast edge@ reduced loop area

A B

A

B

Reduce loop area:less time and energy

needed to build H-field

single wire

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50

coax cable

single wire

2. Waveform for slow edge

A B

A

B

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Mutual induction: coupling of circuits (loops)

Field loop 1 induces voltage in loop 2 (“Crosstalk”- or: transformer)

I1

loop 1

loop 2

MModel:

flux 1

1

212

loop

loop

IM

1

11 I

L

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Mutual induction in practice

two circuits with a common return (“ground”) conductor

50

50

50

source (50)

scope

“ground” litz wire

single wire (source)

single wire (passive)

Only a change in current produces crosstalk!

dt

dIVnoise ~

“Common Impedance” crosstalkPhenomenon is referred to as

“Common-Impedance” crosstalk

Note that a slower rise timeproduces less or no crosstalk at all!

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50

50

Ground Plane

Wide ground plane is preferredreturn path for current!

Wide metal reduces common impedance efficiently

the effects of Lenz Law

50

source (50)

scope

single wire (source)

single wire (passive)

“ground” litz wire

g“Common-Impedance” of wireis high compared to wide plate

Icm “squeezes”under cable

(proximityeffect)

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Wide metal also features: the Skin Effect

Lenz’Law and the basis for shielding effects

CurrentSource

0 d

Edd

y cu

rren

ts

curr

ent d

ensi

ty

0 d

J0

e

J0d

d eJJ

0

Induced Eddy currents opposedirection of external current

(Lenz’ Law)

f

1 f = frequency [Hz] = conductivity [S/m] = permeability [H/m]

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Proximity effect

current concentrates under conductor, minimizing loop inductance

Current concentrates underconductor (proximity effect)

R=50

Field distribution can bemeasured with small

sniffer probe

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Cables are used to keep Signal and Return together

field of the return conductor is identical but opposite (if geometry is identical)

H = Magnetic Field [A/m] - H

Ir

rH

2

Ampere’s Law:

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Current carrying conductor always exhibits H-field

minimize fields by locating the return conductor concentric

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external noise source

Inoise

Unoise

1. Coupling into external noise

cable length D

Properties of cables: Transfer Impedance ZT

cable may produce or pick up common mode currents

Idesired

return current flows where?

?

2. Generation of noise in other conductors(e.g. “ground”)

DI

UZ

noise

noiseT

[Ohm per meter]

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“Pig-tails”

effect of geometry changes: fields outside interconnections; CM currents

whencompared…

“Coax is betterthan

twin wires”Coax

Twinwires

Pig-tail destroys cable symmetry

Fields aregeneratedZT goes UP

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13



Icm

Use Current Boundary to protect existing “pig-tail”

pig-tails can be acceptable as long as CM currents are kept away from it

H-fieldlines

Wide metal plate

(Current Boundary)

EMC glands

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The Current Boundary

a provision to split loops (and shut out noise sources)

“Ground 1” “Ground 2”

Icm (noise current)

loop closes through “ground”

“Ground 2”“Ground 1”Icm

loop closes through “ground”

Create one or more “inner-loops”Short

circuit(s)

reduce looparea

check

Unit 1

Unit 2

(Mains cord 1)(Mains cord 2)(I/O cable 1-2)

Situation in practicedetector:AM-radio

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Necessity of current boundaries in practice

equipment interconnected by cables is always part of a “ground loop”

Power PowerI/O

check

Don’t forget lightning as external source!

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Install current boundaries at natural interfaces

edge of PCB, cabinet wall, basement of a building; one boundary per unit!

Right Wrong

Icm Icm

Drawbacks:

Current follows longpath over equipment

Loop area cannot easilybe minimized

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15



Examples of current boundaries on equipment

wide conductors and low-resistance transitions (be careful with paint) !

protect all units with a current boundary!(and check any conductor that passes it)

Short

Wid

e

check DC resistance witha milli- meter: < 1 m!

NoPaint!

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What goes wrong on current boundaries…

mix of plastic and metal EMC glands signals errors

Metal gland onplastic cable sheath

No cable shield…no connection

either ??

Paint….?

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Separating cables with current boundaries

classify cables into categories

Model

ICM

Category

red = “source” =“Emission”

1. Noisy (E)

green =“sensitive” =“Immunity”

2. Sensitive (I)

blue =“indifferent” =

“Neutral”

3. Indifferent (N)

E

I N

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1. herken kringN

Steps:


use Neutral conductor to reduce loop area; then insert current boundary

1. recognize loop

E

I2. reduce looparea

N

3. add boundary

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neutral conductor in practical cases: never a “wire”, always a structure part

cm

cross section:twin wires!

Emission Neutral

(CM-) Transfer impedance of combinationof two relatively thin conductors

is too high (radiates fields)(does not work for high frequencies)

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wide metal reduces fields i.e. the transfer-impedance of the cm-current loop

cm

advantage:proximity & skin

effects

Emission

Neutral

Wide sheet metal (“cable guide”) is far superiorto the previous situation. The common-mode

transfer impedance is much lower. Skin effect helps.

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“Plane” could be metal mesh


use (Ground-) Plane to reduce loop area; then insert current boundaries

NSteps:1. recognize loop

E

I

3. connect current boundaries to plane

2. cover loop with metal (ground-)plane

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Special form of current boundary: a cable guide

Neutral conductor, usually structural part of existing installation

50

50

Cable 1 (source)

Cable 2 (passive)

Tracking-generator

U

f

spectrumanalyser

I CM

(=

no

ise

)

ICM (=noise)

Icm “squeezes”under cable

(proximity effect)36

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in cooperation with

Cables without Cable-Tray

crosstalk through transfer-impedance

37Engineering Electromagnetic Compatibility


in cooperation with

Cables over Cable-Tray, Large Separation


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in cooperation with

Cable Against Cable-Tray, Pig-Tail Ignored….



in cooperation with

Cable Against Cable-Tray, Pig-Tail Treated Properly


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Use available metal to “short-out” CM-currents

ship’s deck and walls can be used as groundplane(s)

Important: keep cables near metal over their full length!(unless cables have sufficient shielding to go “unprotected”)

UNPROTECTED

PROTECTEDGood contact

(< 1 m)

Goodcontact

?!

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in cooperation with

“Grounded Cabinet on Shock-Absorbers” (Litz Wire)


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in cooperation with

Cabinet on Wide Grounding Bracket, Cables Float



in cooperation with

Cabinet on Wide Grounding Bracket, Cables Improved


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Either filter or shield entire cable

when passing through shielding wall

EMC Gland

C L C

EMC Filter

O.K.

O.K.

Not O.K.

cm

E, H fields E, H fieldsreradiate

1 V/m

10 mA(3 - 5 A = RE limit)

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Shielding Experiment

Shielding a noisy interconnection using a metal tube (wave guide)

Wire carryingmodulated RF signal Battery

DC cable

Generator

Radio tuned to RFharmonic frequency

ModulationDetected

cm

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24




Entering a conductor into tube couples out the noise again


DC cable

Generator


ModulationDetected

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Insulating generator case: battery cable now reradiates noise (antenna)


DC cable

Generator


ModulationDetected

cm

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25




Entering a shielded conductor into tube also couples out the noise again


DC cable

Generator


ModulationDetected

grounding wire

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Grounding the shield with the wire does not solve the interference problem!


DC cable

Generator


ModulationDetected

50

26




Cable shield must be grounded directly to the metal shield to stop the noise


DC cable

Generator


ModulationDetected

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Bad “Grounding” habits

it is Inductance, not the milliOhms that count!

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27



Front Door / Back Door EMI

Front Door: Via Intended Coupling; Back Door: Via Unintended Coupling

Receiver87 - 108 [MHz]

Mains Cord

100

[MH

z] in

-ban

din

terf

eren

ce

Front DoorCM

9 [MHz] out-of-bandinterference

Back Door

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A filter in the inserted wire does not help if only grounded with a wire


DC cable

Generator


ModulationDetected

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only when the wide metal filter plate touches, the shielding works


DC cable

Generator


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Filter Installation

installation at least as important as the correct filter choice

A. Filter grounded with long wire

B. Filter mounted on wide metal strip

C. Filter mounted “Plumbers Delight” fashion

(input and output completely separated by shielding)

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Filter Grounded with Long Wire

some effect in the low frequency range; hardly any in higher frequencies

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Filter Mounted on Wide Metal Strip

better than the long wire; sensitive to cable routing (crosstalk)

Wires pressed against ground plane

Wires far from ground plane

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Filter In and Output Separated by Shielding

construction often called “Plumbers Delight” by radio amateurs

Filter performance as specified by manufacturer(noise floor on analyser)

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Lightning and Buildings

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31



Insulation in Lightning Protection

keep lightning current on the outside of the buildingE

xter

nal “

Shi

eld”

Inte

rnal

wiri

ng

surgeprotect

cabinet

1

2

3

4

5

power

data

RV

Ext

erna

l con

duct

ing

stru

ctur

e

CurrentBoundary

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Lightning and Airplanes

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ElectroStatic Discharge

charges built on persons or equipment cause electric sparks (and currents)

63



Electric charging by induction

direct contact not necessary!

Teflon

Wool

Printed Circuit Board

- - - - - - - - - - - - - - - - -+ + + + + + + + + + + + + + + +

1. Charging of an insulator

2. Insulated PCB on charged surface

- - - - - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - -

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33



Electric charging by induction

direct contact not necessary!

+ + + + + + + + + + + + + + + +

3.Touch or Ground PCB:

negative charge disappears (spark)(PCB possibly damaged)

4. Lift PCB: voltage increases! Sparks fly!

- - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - -

+ + + + + + + + + + + + + + + +

PCB

PCBPCB C

QV (CPCB decreases)

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Ground self-induction and a fast discharge edge

“Grounding” or “short-circuit” of an ESD source is difficult (better avoid!)

“long” grounding path

neon lamp flasheson discharge over

“long” grounding path

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34



Inductive load switching

Relays, Valves, and PWM motor control systems

V0

SW

L

C(parasitic)

R(parasitic)

Basic model

0

2

2

1 LEnergy

par

2

2

1VCEnergy

22

2

1

2

1VCL

L= 0.1 HC= 100 pF = 1 A

V = 32 kV (!)

Analysis:

Source: Jasper J. Goedbloed, “EMC”Prentice Hall/Kluwer 1992

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High Voltage in Motor arcs and creates Spikes

reason for Electrical Fast Transients (EFT) tests on equipment

ampl

itude

time (s scale)

breakdown (ns scale)

from EN 61000-4-4

5/50 ns pulses

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35



Protection against Fast Transients: the Source

use of snubber (AC case) or diode protection (DC case)

C(parasitic)

R(parasitic)V0

SW

L

Basic model

0

2

2

1 LEnergy

snub

2

2

1VCEnergy

22

2

1

2

1VCL

L= 0.1 HCsnubber= 1 F = 1 A

V = 320 [V]

Analysis:

Source: Jasper J. Goedbloed, “EMC”Prentice Hall/Kluwer 1992

C(snubber)

R(snubber)

e.g. 1 F

e.g. 10

Alternativesolution

(DC only)

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Non-linear Effects

out of band interference

Voltage

Cur

rent

DC offset

frequency

ampl

itude

100 MHz

0 Hz

How to avoid?

Prevent HF signals fromreaching semiconductors

Do not amplify HF signalsthat did get in

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Example “Non-Linearity Disaster” Pictures

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