essentials of equipment design for electromagnetic compatibility (emc) compliance - 2010

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Essentials of Equipment Design for EMC Compliance

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Essentials of Equipment Design for Electromagnetic Compatibility (EMC)

Compliance

Elya B. JoffeEMC/E3 Engineering SpecialistK.T.M. Project Engineering

e-mail: [email protected]

Instructor

All Rights Reserved

Sponsored by

Who...?Me???

©Copyright 2010

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About the Instructor: Elya B. JoffeJOFFE, Elya B., K.T.M. Project Engineering, Hod-Hasharon, Israel, and Senior EMC engineering Specialist and consultant.

Mr. Joffe has over 25 years of experience in government and industry, in EMC/E3, Electromagnetic Compatibility/Electromagnetic Environmental Effects, for electronic systems and platforms, in particular aircraft and aerospace. He is actively involved in the EMC design of commercial and defense systems, from circuits to full platforms.

His work covers various fields in the discipline of EMC, such as NEMP and Lightning Protection design, as well as numerical modeling for solution of EMC Problems. Mr. Joffe has authored and co-authored over 30 papers in the IEEE Transactions on EMC and Broadcasting, as well as in the proceedings of International EMC Symposia. He is Senior Member of IEEE, Immediate Past President of the IEEE EMC Society, Member of the BoD and President-Elect of the IEEE Product Safety Engineering Society, and Chairs several Committees. He is also the Immediate Past Chairman of the Israel IEEE EMC Chapter and has served as a "Distinguished Lecturer" of the IEEE EMC Society.

Mr. Joffe has received several awards and recognitions from the IEEE and EMC Society for his activities. In particular, he is a recipient of the prestigious "Lawrence G. Cumming Award of the IEEE EMC Society for outstanding service", 2002, the "Honorary Life Member Award" of the IEEE EMC Society, 2004, and the IEEE EMC Society "Technical Achievement Award". He is also a recipient of the IEEE "Third Millennium Medal". He was recently awarded the very prestigious “IEEE Larry K Wilson Transanational Award”.

Mr. Joffe is also a member of the "dB Society". Mr. Joffe has been a member of the CEI-Europe Faculty since 2004.

The biography of Elya Joffe has been published numerous times in the Marquis “Who’s Who In The World” .

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• Module 1: Fundamental EMC Concepts• Module 2: Signals and Coupling Modes• Module 3: Field and Cable Interaction• Module 4: Enclosure Shielding• Module 5: Grounding and Bonding• Module 6: Filtering and Terminal Protection• Module 7: Summary and Wrap-Up

Seminar Outline

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In the (Very) Beginning…• In the beginning, God created the Heaven and the

Earth …• … and God Said, Let…:

0

D

D

t

H Jt

B

BE

ρ∇⋅ =

∇⋅ =

∂∇× = −∂∂∇× = +∂

And there

was light!

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Module 1Introduction - Fundamental Concepts

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The U.S.S. Forrestal IncidentJuly 29, 1967

On July 29, 1967, the US Aircraft Carrier “Forrestal” cruised of the coast of North Vietnam. Its jets had already flown more than 700 sorties and there was no reason to expect this day to be any different. Not threatened by enemy aircraft, the A4 “Skyhawk”s on the deck were loaded with two 1000 lb. bombs, air to ground and air to air missiles. Fully fueled, they were ready for takeoff. Somewhere on the deck of that carrier, attached to the wing of an aircraft, was an improperly mounted shielded connector. As the RADAR swept around, RF voltages generated on that cable ignited a missile which streaked across the deck, striking an aircraft and blowing its fuel tanks apart. Its two 1000 lb. bombs rolled to the deck and exploded. Wing-tip to wing-tip, the planes burned and the bombs exploded. Fire spread below deck, and before it was extinguished, 134 men were dead or missing.

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The Basic Rules in EMC

There are no rules!!! EMI does -

what it wants! where it wants! when it wants!

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The Basic Rules in EMC

1. You can’t win them all...2. You can’t even break even...3. If you think you can...

Go to rule no. 1 !!!

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Electromagnetic CompatibilityDefinition

• The ability of the a device, unit of equipment or system to:-

– function satisfactorilysatisfactorily in its intendedintended electromagnetic environment

– without introducingintroducing intolerable electromagnetic disturbance to to anythinganything in that environment

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Three Aspects of an EMI Problem• Generation of Electromagnetic Energy• Transmission of Electromagnetic Energy• Reception of Electromagnetic Energy

The Solution of any EMI Problem Requires the Removal (or Neutralization) of At Least One of the Components

Source[Emitter][Culprit]

MediumCoupling Path

Victim[Receptor][Receiver]

I = Immunity

E = EMI

A potential EMI Problem exists when

I<E

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Five Dimensions of an EMI “Situation”

Amplitude (A)

EMI “Situation”fF, A, T, (I, D)

Frequency (F)

Time (T)

Key Parameters to an EMC Problem

FATFAT--IDIDFFrequencyrequencyAAmplitudemplitude

TTimeimeIImpedancempedanceDDimensionsimensions

Impedance (I)

Dimension (D)

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The Last Rule in EMC...Murphy is the Patron Saint of

EMC Engineers...

But remember…

Murphy was an Murphy was an O P T I M I S T!!!O P T I M I S T!!!

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Module 2Signals and Coupling Modes

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Spectral Content of Pulsed WaveformsTime vs. Frequency Domain

Reconstruction using 7, 15, 27 Harmonics

• Radiation efficiency proportional to“electrical length” of conductors

max 1E fl l

I r rλ ∝ ∝ ×

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Spectral Content of Pulsed Signals

Effect of Wave-Shape on Spectral Content

1

1f

dπ=⋅

2

1

r

ftπ

=⋅

( )log f

( )e f

20 dB/dec

40 dB/dec

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“Real World” Circuit Elements•• Nothing is like it seemsNothing is like it seems

At high frequencies, where the performance of reactive componentAt high frequencies, where the performance of reactive components is most s is most needed (e.g., for filters) needed (e.g., for filters) -- they may not perform as anticipatedthey may not perform as anticipated

The INVISIBLE CIRCUIT must be considered in hiThe INVISIBLE CIRCUIT must be considered in hi--speed circuit designspeed circuit design

Nothing is like it seemsNothing is like it seemsNothing is like it seemsNothing is like it seemsNothing is like it seemsNothing is like it seemsNothing is like it seemsNothing is like it seems……

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Path of Least Impedance” PrincipleVisualize Return Currents

• Currents always return… To ground??

To battery negative??

• Where are they?

They are all here… flowing to their source!!

“All the rivers flow to the sea, but the sea is not full”

(Ecclesiastes 1:7)

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“Path of Least Impedance” PrincipleWhich Path will the Return Current follow?

• Currents always take the path of least … Distance? Resistance? Impedance!!!

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Equivalent Circuit

“Path of Least Impedance” Principle Which Path will the Return Current follow?

or:-

1( ) 0S S SI R j L I j Mω ω⋅ + − ⋅ =

SL M=

1

S S

S S

I j L

I R j L

ωω

=+

1 1, Sg S

S

RI I I I

Lω<< → ⇔ >>

SS g

S

RI I

Lω>> ⇔ >>

In In ””tightly tightly coupledcoupled””conductors:conductors:

1C

21B

2S

d s

1I11B

2C

12 12

dIV L

dt=

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“Path of Least Impedance” Principle Which Path will the Return Current follow?

2 1

1 2

VsI Z

Z Z= ⋅+1 2

1 2

VsI Z

Z Z= ⋅+

1 2 2 1

1 1

1 1 1 1 1 1

1 1 1 1

If Z >>Z I >>I (Ohm's Law)

min min

If Z , minZ minR +jX

If R << X minZ min X

I Z

R jX

→

→

= + →

→ ↔

second law that refers to entropy directly is as follows:In a system, a process that occurs will tend to increase the total entropy of the universe.

The Second law of Thermodynamics: In a system, a process that occurs will tend to increase the total entropy

of the universe.

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• At LOW FREQUENCIESLOW FREQUENCIES, the current will follow the path of LEAST LEAST RESISTANCERESISTANCE, via ground (IG)

1 /

S

S S

jI I

R L j

ωω

= ⋅+

0

| | @

| | @ S S S

S

S S S

Z R R jZ R j M

Z L L R

Lω

ωω

ωω

→ = + ⋅ =

≈ ⋅ ⋅ >>

≈ >> ⋅

M

Source Cable Load

RS

LS RL

Ig

I1

IS

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• At HIGH FREQUENCIESHIGH FREQUENCIES, the current will follow the path of LEAST INDUCTANCELEAST INDUCTANCE, via the return conductor (IS)

| | @

|

| @ S S S

S S

S

S

Z R R j

Z L LM

R

LZ R j

ω

ω ωω

ω→∞

≈ ⋅ ⋅ >>

≈ >> ⋅= + ⋅ =


1 /

S

S S

jI I

R L j

ωω

= ⋅+

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“Path of Least Impedance” Principle Experiment Set-Up

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“Path of Least Impedance” PrincipleWhen is Inductance Minimized?

• Definition of Total Loop Inductance

• For I=constant, F min implies A min

( ) min min min, ...

A

B d a

LI I

B B I thus L A

φ

φ

⋅

= ≈

= ⇒ ⇒

∫

,B Φ

Current I

Magnetic Flux

X X X X X

X X X X X

X X X X X

LI

Φ=

Loop Area, A

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• High frequencies are “well behaved”; Low frequencies are the “bad boys”

“Path of Least Impedance” Principle Implications of the Rule…

• The principle of “Path of Least Impedance” apply in EMC design in:

Grounding design and topologies Filtering and Terminal Protection Schemes Transmission line (cable) design and shielding Etc…

Few principles in EMC are as important as Few principles in EMC are as important as this onethis one……

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EMI Control Design Techniques

EMC design incorporates efforts, techniques and know-how

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Module 3Field and Cable Interactions

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Common- & Differential-Mode Signals

II I

C =+1 2

2I

I ID =

−1 2

2ID

IDd

IC

ICd

ID -Differential ModeCurrent

IC -Common ModeCurrent

Excellent flux cancellation

No flux cancellation

“Contradictions do not exist. Whenever you think you are facing acontradiction, check your premises. You will find that one of them is wrong”

“Atlas Shrugged”

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Some Sources of Common-Mode Signals

“Ground Loops” External Radiated Field Or Capacitive Crosstalk

Electric Flux

D

Vin

-2 ICM

VG

+IDM

+ICM

-IDM

+ICMA

I1

I2

I3

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• Differential-mode radiation efficiency:

214max 2.632 10 V/m

E f A

I r

− = ×

Common & Differential Mode SignalsSource: Ott, H., Noise Reduction Techniques in Electronic Systems, 1988

Current [mA]Frequency [MHz]

1000100101

1,32013213.21.3210

11,9001,19011911.930

13,2001,32013213.2100

Computed E[µV/m]for r=1 m, A = 10 cm 2

Φ

Θ

I

r

A

X

Y

ZrH

H Θ

EΦ

rP E HΦ Θ= ×

,A rπ λ<<

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• Common-mode radiation efficiency:

Current [mA]Frequency [MHz]

1000100101

630k63k6.3k63010

1,890k189k18.9k1.89k30

6,300k630k63k6.3k100

Computed E[µV/m]for r=1 m, ℓ = 10 cm

6max 1.26 10 V/mE fl

I r

− = ×

Common & Differential Mode Signals

Φ

Θ

I

r

X

Y

ZrE

H Φ

EΘ

rP E HΘ Φ= ×

,L rλ<<

L

Source: Ott, H., Noise Reduction Techniques in Electronic Systems, 1988

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Common Mode Current Field Strength: An Illustration

• Radiation efficiency at a distance R from two close wires carrying Common Mode Current, IC:

• At f=30MHz, MIL-STD-461F, the RE102 requirements for Aircraft (AF) Internal Equipment is 34dBmV/m, or 50mV/m at r=1 meter.

• For L=2 meter, the above formula yields that...

A common current as low as IA common current as low as ICC=656nA is sufficient to =656nA is sufficient to exceed the above MILexceed the above MIL--STDSTD--461E, RE102 Method461E, RE102 Method

E f L Ir

C= × ⋅ ⋅ ⋅ ⋅ ⋅−2 6 28 1017. ( ) ( ), V/m

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Low Frequency Radiated Emissions from Cables

B

r

I

0

2

IB

r

µπ

→

=⋅

Single Wire

( )0

2

I dB

r r d

µπ

→ ⋅=

⋅ ⋅ +

BY

I

Parallel Pair

Id

rx

BX

Twisted Pair ( )

2

00 ;

r

pI d dB q I q e q

p r p

πµ π

− ⋅→ ⋅ ⋅= ⋅ ⋅ ⋅ =

⋅

p (pitch of twist)r

B

d (separation of wires)

I

I

I0(q)=0th order modified Bessel Function of 1st

kindCorrection to B for parallel wire line of same spacing to obtain twisted pair B

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Reduction of Low FrequencyMagnetic Field Emissions from Cables

• Twisting of the cable pair provides two contributions: Reduction of loop area between conductors Effective cancellation of magnetic flux from adjacent “mini-loops”

Twist Factor

Source Load

I+

I-

Loop j Loop j+1

dlj

dlj+1

rj

rj+1

Observation point,

O

1jB +

jB

s

~~ ~~

d

p1

20 1 2 sin ; 2 1

60 @ 100 for 30 40 Twists/m

T

T

R Log nl dBnl n

R dB f kHz

πλ = − ⋅ ⋅ + ⋅ +

≤ ≤ ÷

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Reduction of Low FrequencyMagnetic Field Emissions from Cables

• Common errors in twisted circuits resulting in no magnetic flux cancellation

• Twisting is only effective in differential, balanced pairsdifferential, balanced pairs

Unbalanced Circuit: Part of the Signal Current Returns through the Signal Reference Structure (IG)

Twisting Separate Single-Ended Signal Wiring: Return Currents of Both Circuits (IG(1+2)) Return through the Signal Reference Structure

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Cable 1 meter long/0.1 meter high Cable 10 meters long/1 meter high Cable resonance frequency proportional to cable Dimensions Max. induced current proportional to cable height above ground plane Interaction depends on circuit topology

Electromagnetic Fields Coupling into Cables

20 dB

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Coupling of Low Frequency Magnetic Fields onto Cables

• The most efficient method for controlling magnetic field coupling is reduction of loop areareduction of loop area:-

between wires in balanced loopsbalanced loops

between wires to groundto ground

Induced EMF Into

Loop

Loop Area FrequencyMagnetic

Flux Density

2V A f Bπ= ⋅ ⋅ ⋅C A

BE dl d a

t

∂• = − •∂∫ ∫

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Why Shield Cables?

• Shielding reduces coupling of external EMI to the cable• Shielding reduces radiated EMI from the cable• In coaxial cables onlycoaxial cables only - the shield also serves as the

return path for the signal

"The mathematical theory of wave propagation along a conductor with an external coaxial return is very old, going back to the work of Rayleigh, Heaviside and J. J. Thomson"

(S. A. Schelkunoff, 1934)

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How Does the Shield Work ?

• In a nonnon-shielded cable:-

ZS

ZL

Signal Reference

StructureRet

urn Cu

rrent Pa

th

E-field

H-field

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How Does the Shield Work ?• In a shielded cable, grounded at one one

endend:- E-field terminates at the shield (@ Low f) H-field penetrates the shield

ZS

ZL

Signal Reference

Structure

Return

Curren

t Path

E-field terminated

on Shield

H-field penetrating

the shield

Shield

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How Does the Shield Work ?In a shielded cable, grounded at both both

endsends:- E-field terminates at the shield (@ Low f) H-field cancelled by opposite shield currents

ZS

ZL

Signal Reference

Structure

High F

requnc

y Retu

rn

Curren

t Path

E-field terminated

on Shield

H-field confined

in the shield

Shield

Low Fre

quncy R

eturn

Curren

t Path

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• Design goal:- Cancellation of magnetic flux emerging from

oInternal conductor current and…

oOpposite shield current Magnetic flux from both currents cancels out

• Goal achieved by: Limiting ground current

How Does the Shield PreventMagnetic Field Emissions?

Shield Grounded One End, at Most; Circuit is Still Balanced

Shield Grounded Both Ends; Unbalanced Circuit

“Current, if not obstructed, will always follow the path

of least impedance”

In Balanced Shielded Cables, the Shield Does not Carry Intended Signal Current

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Shield Surface Transfer ImpedanceDefinition

iCMS EfII ==

1. An external incident field external incident field (EI) induces CM currents CM currents on the shield (IS)

2.2. CM voltage, CM voltage, dVdV, due to I, due to ICMCM is is induced between the shield and the induced between the shield and the inner conductor (per dl of cable)inner conductor (per dl of cable)

3.3. Shield Surface Transfer Shield Surface Transfer Impedance, ZImpedance, ZTT , is the transfer , is the transfer function between the twofunction between the two

Signal ReferenceStructure

dx

0(0)SI I=

0( )S

II x I dx

x

∂= +

∂

(0)iV

(0)i

I

( )iI dx

x

( )iV dx

0(0)SV V=

0( )

S

VV x V dx

x

∂= +

∂

S TI Z dx Zdx

TY dx

iI ii

II dx

x

∂+∂

ii

VV dx

x

∂+ ⋅∂

S TV Y dx−iV

0

1; /T

Si

i

I

dVZ m

I dl =

= ⋅ Ω

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Shield Surface Transfer ImpedanceEffect Of Frequency On Transfer Impedance

• The Transfer Impedance, ZT, consists of two components:-

Resistive component, RT

Inductive component, LT

Z R j L mt t t= + ω , /Ω

ℓ

[ ]TZ mΩ

10

λ4

λ2

λ

2 T

cLπ

λ ⋅

2Tc Lπ

⋅ ⋅

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Shield Surface Transfer Impedance Effect of Shield Configuration

• Shield Transfer Impedance for Various Shield Configurations Adding a second shield layer adds ~6 dB of attenuation

• For most cable shields, the Surface Transfer Impedance is inductive at F>1MHz, approximately

• RT becomes negligible• LT dominates

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Shield Termination and Grounding Effect Of Shield Termination: “Pigtails”

• An improperly-terminated shield will often be a significant cause of EMI problems, emission and coupling

It performs almost like no shield is present

0

10

20

30

40

50

1

2.5

4.0

5.5

7.0

8.5 10

F requency [G Hz ]

Shielding Effectiveness [dB]

3600 Shield Termination

2” (*) Pigtail

(*)(*) 22”” Pigtail is the Pigtail is the absolute maximum absolute maximum length length recommendedrecommended

TerminalStrip

CenterConductor

Dielectric

BraidedShield

OuterSheath

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Shield TerminationProper Outer Shield Termination

Conductive Shrinkable Boot Termination

• The shield must be terminated at both ends, at least!• Use a peripheral shield termination (EMI Backshell) in

order to ensure acceptable shielding effectiveness

EMI Backshell Termination BackshellGround Hooks

AdapterStrain Relief

Individual Wire

Shields

Plug

Individual Wire Shield Termination

D-Type Shield Termination

Source: Glenair

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Shield TerminationGrounding of Peripheral Shield

3600 Shield Termination

External Pigtail

Internal Pigtail

Pigtail to Signal Ground

Best

Worst

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Module 4Enclosure Shielding

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Objectives of ShieldingThe technique may be

oldbut it provides me

full protection from EMI...

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A (Absorption) R (Reflection)

γτ

Γ Reflection Coefficient

Propagation CoefficientTransmission Coefficient

EY

HZ

EY

HZ

EY

HZ

EY

EY

HZ

ReflectedWave

AttenuatedTransmittedWave

PropagatingWave

InternallyReflected

Wave

IncidentWave

l

HZ

Shielding Mechanisms of Metallic Enclosures

B (Secondary Reflections)

( ) ( )2120 120 20l l

dB LogLog e L eSE ogγ γ

τ⋅ ⋅ ⋅ ⋅ − Γ ⋅

+

⋅= +

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The Wave ImpedanceAt Near & Far Field Regions

• The electromagnetic field impedance:

• In the vicinity of the source, the wave impedance depends on the source characteristics:

High-Z source: ZW > 120p W

Low-Z source: ZW < 120p W

• In the “far field”: ZW = 120p W

ZE V m

H A mW [ ]

[ / ]

[ / ]Ω =

Distance, normalized to l/2pr

Wave Impedance, W

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Reflection Losses (R) And When The Wave Hits The Surface...

• In a perfectly conducting plane, reflection would have been complete

Induced surface currents neutralize the incident field

• In a practical conducting plane, conductivity is finite

Induced surface currents penetrate the shield and can induce internal fields Reflection loss for an EM wave will Reflection loss for an EM wave will

depend on the ratio of the free depend on the ratio of the free space impedance Zspace impedance ZWW to the surface to the surface

impedance impedance \\of the shield Zof the shield Zss

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Reflection Losses (R)

Plane WaveLow Z WavesHigh Z Waves

3 2[ ] 322 10

f r

re

r

R dB Logσµ

= + ⋅ ⋅ ⋅

[ ] 168 10f

rp

r

R dB Logσµ

= + ⋅ ⋅

2f r[ ] 14.6 10 r

h

r

R dB Logσ

µ ⋅ ⋅

= + ⋅

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Absorption Losses (A)• An EM wave penetrating through a

metallic medium is attenuated due to Ohmic losses

• Absorption loss in a screen decreases exponentially and is reduced by 1/e at a distance d equal to the penetration depth δ

Skin-Depth: The depth where the field/surface current is attenuated to e-1 (37%, approx.) of its value on the surface

δω µ σ

=⋅ ⋅2

( ) 0

t

J t J e δ−

= ⋅

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Absorption Loss in a shield 1Absorption Loss in a shield 1δδ (one skin depth) thick is 9dB (one skin depth) thick is 9dB Absorption Losses are independent of the field characteristics aAbsorption Losses are independent of the field characteristics and nd

are dependant on thickness of the shield onlyare dependant on thickness of the shield only

Absorption Losses (A)

Or:-

20 log 8.69 ,t

tA e dBδ

δ

− = ⋅ = ⋅

131.4 ,Hz r rA t f dBµ σ= ⋅ ⋅ ⋅ ⋅Absorption Loss for 1mm Shield

of Steel & Copper

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Reflection & Absorption CombinedIron Metal Sheet

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58

Apertures: The Inevitable Necessities Violating Shielding Integrity

Knobs

Lamp Holes

Ventilation

Openings

Lamps

Slot

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Shield Compromises• Previously we assumed a perfect, infinitely large & planar shield• in practice, shielding is compromised:

Holes & apertures for: Holes & apertures for: Connectors, components, cable entries, ventilation, displays

SeamsSeams, e.g., Mating members (screwed, riveted, welded, etc.)

Doors and access coversDoors and access covers

VentsVents, e.g., Ventilation, heating

NonNon--homogenous areashomogenous areas, e.g., Screens, meshes

• Shielding performance is typically dominated by aperture leakage

As those are, usually, inevitable necessities, the As those are, usually, inevitable necessities, the enclosure design will focus on aperture controlenclosure design will focus on aperture control

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60

Leakage From A Single ApertureAperture Reflection Losses vs. Shield Attenuation

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61

Leakage From A Single Aperture• Induced currents flow as long as there

are no obstacles in their path• Any and all apertures must be arranged

in such a way as to minimize their effect on the currents

o An H-field, which is predominantly tangential close to a metallic screen, may penetrate through an opening and introduce an induced current into an underlying cable or circuit

o An E-field, which hits a metallic screen at right angles, may penetrate through an opening and enable an induced voltage to run along an underlying cable or circuit

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Leakage From A Single ApertureAperture Reflection Losses

From Babinet’s Slot-Dipole Reciprocity Theorem:

and:

we derive the following expression for Aperture Reflection Loss:

2

4

WSlot Dipole

ZZ Z× =

2120Dipole

W WZ j Ln Cot

S S

π = − ⋅ ⋅

(*) For a circular aperturecircular aperture, add 2 dB add 2 dB [~~20log(π/4)]

( )( )[ ] 97 20log 20log 1 ln , W2

mmmm MHz

mma

WR dB W f

S

λ∗ = − ⋅ + + ≤

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63

Leakage From Multiple Apertures Effect of Shield Discontinuity on

Magnetically- Induced Shield Current• Multiple small openings are preferable to

a single large oneo Note that multiple small holes may be very

effective in stopping fields at higher frequencies

• Multiple Apertureso Small Holeso High Cutoff

Frequencyo __?_ Couplingo __?_ Shielding

• Single Apertureo Large Holeo Low Cutoff

Frequencyo Large Couplingo Little Shielding

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64

Leakage From Multiple Apertures:• Many, smaller apertures are preferable, compared to a

single, large aperture

Higher cutoff frequency

Higher attenuation at the same frequency

Area of each hole, a

Number of holesper unit square, N

Total Number

of holes, n

KK11 should be considered should be considered only if the source is far only if the source is far from the aperture, i.e.,from the aperture, i.e.,r >> d, Wr >> d, W

1 10 log(1 ) 10log( ) 10log( ),K a / a / n dB= ⋅ − ⋅ ≅ − ⋅ = −

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65

What If The Shield Is Deep ?

• A “deep” aperture: t/W>>1, t/d >>1 acts like a waveguide below cutoff (WGBC) waveguide below cutoff (WGBC)

• Effect considered as Aperture Absorption LossesAperture Absorption Losses

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66

Leakage From A Single ApertureAperture Absorption Losses - WGBC

Ape

rtur

e Abs

orpt

ion

Loss

es A

a, d

BFor t/W > 3, For t/W > 3, AaAa>100 dB>100 dB

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67

The most common application of The most common application of honeycomb panels is for ventilation and honeycomb panels is for ventilation and cooling air entry, without compromising cooling air entry, without compromising the shielding integrity of the enclosurethe shielding integrity of the enclosure

Applications of Waveguide Below Cutoff (WGBC)

Honeycomb Panels & Cooling Vents

ShieldedEnclosure

Honeycomb

Honeycomb

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68

Equipment and System Shield DesignThin Film Coating

• The primary shielding mechanism: Reflection (R)

Shielding Effectiveness of Conductive Glass to High-Z

Fields

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69

Equipment and System Shield DesignScreen Mesh: More (Shielding) for less (Visibility)

Application of wire mesh shield for displays

Shielding mesh placedin front of the displayin front of the display

EMI ProofMetal Case

Conductive Glassor Wire Mesh

FeedthroughFilter

Panel

Wire meshGasket

Page


70

Equipment and System Shield Design Slots and Seams

• The first law in shielding practices:

There is no perfect bondThere is no perfect bond• If not properly closed, EMI leakage will occur

through seams & slots in the enclosure Attributing for emissions and coupling at

frequencies above 300MHz, typically

• Proper treatment must be implementedto slots & seams for maintaining shielding integrity

Use of overlapping seams

Use of multiple screws

Use of conductive gaskets

And And …… Surface TreatmentSurface Treatment

Page


71

• Overlapping seams:- Increase the capacitance between the conductors

Reduces seam impedance (increases reflection losses)

Increases the effective depth of the waveguide between the conductors, improving absorption losses

Slots and SeamsUse of Overlapping Seams

Page


72

Slots and SeamsUse of Screws To Fasten Mating Panels At

Seams

W

S

Gap

GapDimension

• This solution is good, but:- Minimum Seam Width = 5 x Gap Dimension Fastener spacing - W, not greater than

l/50, @ military systems

l/20, @ commercial systems

Page


73

Solutions For Increasing Mating Member SE

SE RequirementsSE Requirements CommentsComments

SE SE ≤≤≤≤≤≤≤≤ 30 dB 30 dB andand f f ≤≤≤≤≤≤≤≤ 1 GHz1 GHz Stiffen cover, avoid EMI gasketsStiffen cover, avoid EMI gaskets

30 < SE 30 < SE ≤≤≤≤≤≤≤≤ 50 dB 50 dB & & f f ≤≤≤≤≤≤≤≤ 1 GHz1 GHz Twilight region, stiffen and lap Twilight region, stiffen and lap over flangesover flanges

50 < SE 50 < SE ≤≤≤≤≤≤≤≤ 60 dB 60 dB & & f f ≤≤≤≤≤≤≤≤ 1 GHz1 GHz Same as 30Same as 30--50 dB, but excessive 50 dB, but excessive number of screw required and number of screw required and extremely rigid coverextremely rigid cover

SE > 60 dB SE > 60 dB oror f > 1 GHzf > 1 GHz Use EMI gasketUse EMI gasket ?

Page


74

Conductive EMI Gaskets• Conductive (EMI) gaskets are

used for: Filling the aperture with

conductive material obtaining electrical bonding

between the mating members

• Gaskets must be used when... Excessive SE requirements

(SE >40dB @ 1GHz) arespecified

Aperture sized cannotbe reduced significantly from l/2

Emission or interferencefrequencies exceed 100 MHz

Mating members are of dissimilar metals

Gasket

Page


75

Maintaining Shield IntegrityPenetrating Objects

GroundingGroundingConductorsConductors

““GroundableGroundableConductorsConductors””::Pipe, Cable Pipe, Cable Shield orShield orWaveguideWaveguide

Insulated Insulated ConductorsConductors

ProperProper CompromisingCompromising Serious ViolationSerious Violation

Filter/Surge Arrester

Page


76

Module 5

Grounding and Bonding

"Ground is where potatoes and carrots thrive"

(Dr. Bruce Archambeault)

Page


77

Purposes for Grounding• Safety: Prevention of shock hazard to personnel

Due to lightning strokes or power line short circuits to enclosure

Traditionally

• Path for return current in particular vehicles (e.g., aircraft)

Vehicle serves as return conductor

Page


78

• The voltage across the finite ground impedance, ZG due to noisy circuit (Circuit #2) is:

Ground Related InterferenceCommon Impedance Interference Coupling (CIIC)

2

2 2

22 2

2 2

;

G S/G

S L G

G SG S L

S L

Z EE

Z Z Z

Z EZ Z Z

Z Z

⋅=

+ +

⋅≅ << +

+

• The interference voltage coupled across the load ZL1 of the sensitive circuit (Circuit #1) is:

Thus11 1

1 1

;L /Gi G S L

S L

Z EV Z Z Z

Z Z

⋅≅ << +

+ ( ) ( )1 1 2

1 1 1 1 2 2

L /G L G Si

S L S L S L

Z E Z Z EV

Z Z Z Z Z Z

⋅ ⋅ ⋅≅ =

+ + ⋅ +

( ) ( )1

1 1 2 2

20 L GdB

S L S L

Z ZK Log

Z Z Z Z

⋅=

+ ⋅ +

Page


79

Objectives of Practical GroundingObjectives of Practical GroundingObjectives of Practical GroundingObjectives of Practical Grounding• Grounding may not be the Solution; rather it could be

Part of the Problem• The objective of grounding system design could be

stated as follows:

• "Design the system such that in spite of the need for in spite of the need for grounding, system performance will not be degraded grounding, system performance will not be degraded due to ground-coupled interference".

"Grounding Systems are "Grounding Systems are Interference Interference

Distribution Devices"Distribution Devices"

(Dr. Carl E. Baum)

Page


80

• Lower the impedance of the common return path (Bonding) Reduces the ground voltage drop below the sensitivity levels of the victim

circuits

• Limit other currents I ≠≠≠≠ IX circulating in the return path used for circuit X

Isolating currents from difference circuits, reducing coupling between currents flowing in the same path

So, We Have A “Practical” Ground...What Do We Do???

ΩΩ ΩΩ ΩΩ ΩΩ ΩΩ ΩΩ......

• Design a noise tolerant system Using differential circuits with high common

mode rejection, for instance

• The choice of each technique (or their combination) depends on feasibility, system/circuit size, cost, frequency and safety aspects

Page


81

Optimizing Ground System DesignGoals of Equipment and System Level

Grounding System• The grounding scheme inside a system must accomplish the

following goals: Analog, low level circuits must have extremely noiseless dedicated

returns; typically wires are used, dictating a single point, “star” grounding scheme

Analog, high frequency circuits (RF, video, etc.) must have low impedance, noise free return circuits, generally in form of planes or their extensions (e.g., coaxial cables)

Digital, logic circuit returns, especially high speed digital circuit returns, must have low impedance over the entire bandwidth (determined by the “edge rates” ), as power and signal returns share the same paths

Returns of powerful loads (e.g., solenoids, motors, relays, lamps, etc.) should be separated from all the above, even if they end up at the same power supply output terminal

For signals that communicate between parts of the equipment or system, the grounding scheme must provide a common reference with minimum ground shift (low common mode noise) between system parts

Page


82

Ground System Topologies A “Floating” System

Page


83

Ground System Topologies Single Point Ground (SPG)

“Daisy Chain” Single Point Ground

ℓℓℓℓ

Page


84

Ground System Topologies Single Point Ground (SPG)Single Point (“Star”) Ground

ℓℓℓℓ

Page


85

• At higher frequencies, where the length of the ground conductors approaches l/4, the SPG is ineffective

Distance along GND Conductor

λ/4

ZS

0

This circuit should ideally be grounded This circuit should ideally be grounded every every λλλλλλλλ/10/10÷÷ λλλλλλλλ/20!/20!

Ground System Topologies Single Point Ground (SPG)

SSignal Reference

Structure

GRP="0V"

Vx

Ix

x

Vx

, Ix

inZ →∞x=888888888888

x=888888888888

A standing wave (black) depicted as the sum of two propagating waves traveling in opposite directions (red and

blue).

Page


86

Ground System Topologies Multi-Point Ground (MPG)

Page


87

Video Processor

Main CPU

I/O Circuit

Aux . Rx

“ActiveAntenna”

Tx/Rx

Power Supply

5VD

15VA

5/3.3VD

15VA

5VA

5VD

5VD/RF

15VA/RF

15VA/RF

28VA/RF

CGP

DC/DC Module

5VD

3.3VD

15VA

5VA

5VD/RF

15VA/RF

5VD/RF

15VA/RF

15VA/RF

28VA/RF

Equipment-Level “Ground Tree”Design Process

CGP

3.3V /5 VD15VA

15VA/RF

5VD/RF

15VA/RF

28VA/RF

5VD

???

Aux . Rx

Tx/Rx

Main CPUVideo

Processor

5VA

5VDI/O Circuit

Enclosure Chassis

15VA

5VD

Page


88

A model for illustrating the effect of grounding topology on system performance

CA

d= ⋅ε ε π0

91 36 10= × F m/C=Capacitance of PCB to Ground

“Ground Loops”SPG vs. MPG

Circuit #1 Circuit #2

ICM #1

ICM #2

VSRS=VCM

TransmissionLine

C d

A

C

A

d

VS

ZS

Z2Z1

ZCM

R1

R2

ZL

h

S

Page


89


Longitudinal Conversion Loss factor, LCLLongitudinal Conversion Loss factor, LCL:Constant

20'

CMdB

DMVo

VLCL Log

V=

= ⋅

Page


90


• At Low Frequencies Capacitances, C, are dominant Circuit impedance reduces with

Frequency (f) CM current increases with f DM voltage increases with f

• At High Frequencies Low Pass Characteristics of the

transmission line are dominant Circuit impedance increases with f Termination impedance limits line

currents

Both sides floated

Floated Both Ends

Frequency [Hz]

Load DM Voltage

Page


91


• At Low Frequencies Circuit series impedance, due

to the capacitances, C, is reduced

CM current (and DM voltage) increases

• At High Frequencies No change from previous case

One side grounded

Floated One End

Frequency [Hz]

Load DM Voltage

Page


92


• At Low Frequencies Circuit series impedance, is

independent of capacitances, C Circuit impedance determined

by wiring & Load resistance (R) CM current (and DM voltage)

independent of f

• At High Frequencies No change from previous

cases

Both sides groundedGrounded Both Ends

Frequency [Hz]

Load DM Voltage

Page


93

Ground System Topologies SPG vs. MPG

• Low frequency circuits Single point grounding only Floating provides marginal improvement and increased risk Low frequency performance is strongly dependent on the circuit grounding

topology Low frequency performance significantly degraded with multipoint grounding

• High frequency circuits Multipoint grounding only High frequency performance independent of grounding topology

“Ground Loop”

Page


94

“Ground Loops”Techniques for Opening “Ground Loops”

Isolation Transformer

• Signal is coupled magnetically, thus the transformer inserts a high longitudinal impedance in series with the CM current path

•• Common Mode decoupling of 100Common Mode decoupling of 100--140 dB can be achieved @ f=1kHz140 dB can be achieved @ f=1kHz•• Expensive, frequency limited, and not always practical for signaExpensive, frequency limited, and not always practical for signal circuitsl circuits

Page


95


BALUNs (Common Mode Chokes)

Alternative Symbols

SSignal Reference Structure

EGZGS ZGL

VN

, V

L

ZS ZLB

ZG

ES ZLA

CP

IS

ICM2ICM1

L2

L1

M

CM

Current

Signal DM

Current

Core

Hi µ−

CM-Generated

Flux

DM-Generated

Flux

• Inserts high-Z for CM signals, while passed “unnoticed” by DM currents - A “mode-selective filter”

•• CM rejection > 80CM rejection > 80--100 dB can be achieved @ high100 dB can be achieved @ high--ff’’ss•• Bulky; can be implemented by Ferrite beadsBulky; can be implemented by Ferrite beads

Page


96


Optical Isolator/Optocoupler

• Signal is coupled optically, thus the opto-isolator inserts a high longitudinal impedance in series with the CM current path

•• Common Mode decoupling of 60Common Mode decoupling of 60--80 dB can be achieved80 dB can be achieved• For digital circuits only

Page


97


Isolation Amplifier

• Grounds isolation within the two stages of the buffer amplifier• Each stage referenced to its associated ground•• Common Mode decoupling of 60Common Mode decoupling of 60--80 dB (*) can be achieved80 dB (*) can be achieved

(*) 120 dB in special applications(*) 120 dB in special applications

Page


98


Circuit Bypassing

• Basically a HF filtering mechanism, shunting CM noise to ground• Care to be paid not to “kill” the intended signal•• Performance depends on value of capacitors, often requiring Performance depends on value of capacitors, often requiring

combination of several approachescombination of several approaches

Page


99


Example: 10/100BaseT Interface

Typical 10/100BaseT Receive and Transmit Interfaces Circuit Consists of Balancing Magnetics and Bypass Capacitors

TransmitInterface

Receive Interface

Page


100

“Bonding”: Definition•• BondingBonding: The establishment of a low impedance low impedance path between two metal

surfaces, e.g., Between two points on a ground reference plane

Between the ground reference plane and a component, circuit or structural element

etc.

• Purposes of bonding:- Avoid development of electric potentials between metallic parts, which could

produce interference

• A good bond:- Enables the design objectives of other EMI control methods, e.g.,

grounding, filtering, shielding, etc.

Minimizes RF voltage differences and ground current loops

Deters the electrostatic charge buildup

Protects from lightning & shock hazards

Page


101

Implementation of Direct Bonds

Bonding Area(Clean both members over entire mating surface + 1/8” perimeter

Bolted MembersBonding of Connector

Bracket Installation: Rivet or Weld

Page


102

Direct and Indirect Bonds

Direct (Hard) Bond

Indirect (Jumper) Bond

Bonding Area(Clean both members over

entire mating surface + 1/8”perimeter

Page


103

Indirect Bond Impedance

Bonding Impedance of a Bonding Strap

1

2 S C

rfL Cπ

=

S

S C

r

LZ

R C=

Page


104

• In DC, resistance is given by:

• Resistance increases with frequency increase due to skin effect

•

d

Ar

L=length of conductor

2, 0 ( ), rDC

Lf z D

r

LR

AH Cρρ δ

π=

⋅⋅ ⋅ ≈ ≥=

Lowering Bond ImpedanceResistance of Conductors

( )

( )

21

42 1

4

2 1 , 0, 4

DCr r

D

DC

C

CA

Rr f

Rr f f r

R rR π µ σ

π

δ

δ

≅ ⋅ ⋅ ⋅ ⋅ ⋅ + ≅

≅ ⋅ ⋅ ⋅

= ⋅ +

>

>

+ <<

Page


105

• Reduces with frequency• BUT… Reactance INCREASES with frequency

A grounding conductor as atransmission line with a ground

plane

Ground Plane

Equipment

Case

Grounding

Conductor

Zin Z

0=(L/C)1/2 Z

L=0

Ground

Plane

Cable

Ground

Log |Z|

Log f

Series Resonance

ParallelResonance

Lowering Bond ImpedanceIntrinsic Inductance of Conductors

2 2

; P

AC

P

AC

LZ Q L Q

R

LZ

R

ωω

ω

= =

=

0

1

2

tan( )in

fLC

Z jZ x LC x

π

ω

=

= ⋅ ⋅ ⋅

( )

; S

AC

S AC

AC

L LZ Q

Q R

LZ R

L R

ω ω

ωω

= =

= =

S ACZ R=

Lω

ACR

X

Page


106

Surface Treatment

• Surfaces must be maintained as smooth as possible

• Remove dirt, paints and non-conductive protective coatings from the bond area

• Select the mating metals according to the electrochemical chart (“dissimilar metals”considerations)

• Apply conductive protective coatings

Page


107

•• The rate of (galvanic) corrosion depends on the separation betweThe rate of (galvanic) corrosion depends on the separation between en the mating metals in the electromotive Force (EMF) Seriesthe mating metals in the electromotive Force (EMF) Series

•• Corrosion is minimized if the combined potential difference doesCorrosion is minimized if the combined potential difference does not not exceed:exceed:

•• 0.3V: in harsh environments0.3V: in harsh environments Exposure to salt spray/weatheringExposure to salt spray/weathering

•• 0.5V: in benign environments0.5V: in benign environments Interior, salt free condensation onlyInterior, salt free condensation only

Corrosion results from a compatible conductive elastomer and a pure silver-filled elastomer

mated with aluminum, after 168 hours of salt-fog exposure

Compatible gasket

Silver-filled gasket

Electromotive Force Series & Corrosion Control

Page


108

Corrosion Control

Page


109©Copyright 2008

Anodic

CathodicCathodic

Anodic

Cathodic

Anodic

POORPOOR BETTERBETTER BESTBEST

• Surface treatment is the only assurance of a long lasting (and effective) bond

Select the mating metals according to the electrochemical chart (“dissimilar metals” considerations)

Apply conductive protective coatings

Steel: Plate with tin, zinc or conductive cadmiumCopper, bronze: Plate with tinAluminum: Alodine or Irridite(*) conversion treatment

(*) Irridite #14 is the best selection

• With dissimilar metal contacts, coating just one of the “electrodes” is insufficient

Complete coating, or at least edge sealing is requiredComplete coating, or at least edge sealing is required

Application Of Protective Coatings

Page


110

Module 6Filtering and Terminal Protection

Page


111

Sources & Types of Conducted EMI• Conducted EMI can be generated within a system (CE)

Switch-mode power supply emissions• Low Frequency (due to power line harmonics)

– Typically Differential mode

• High Frequency (due to switching and rectification)

– Common and Differential mode

• Transient emissions due to the switching if Inductive loads

•• The The ““threatthreat””: Interference to sensitive loads sharing the power : Interference to sensitive loads sharing the power systemsystem

Signal line emissions• Mostly high frequency common mode emissions

– Due to coupling (crosstalk and radiated EMI coupling to I/O lines)

•• The The ““threatthreat””: Radiated EMI and crosstalk: Radiated EMI and crosstalk

Page


112

Sources & Types of Conducted EMI• Conducted EMI can be coupled into the system (CS)

Power and signal line EMI and transients• Low Frequency

– Due to magnetic induction

– Due to power line harmonics, and voltage variations/fluctuations(power lines only)

– Typically Differential mode

• High Frequency EMI

– Due to radiated fields pickup

– Typically Common mode

• Transients and surges

– Due to switching of Inductive loads

– Due to lightning induced surges and transients

Page


113

EMI Filters: Definition

• A filter is a simple method for attenuating conducted (and subsequently - radiated) emissions and

IL LogE

ELog

E

E

L

L

L

L

( )( )

( )

( )

( )f

f

f

f w / Filter Inserted

f w / o Filter Inserted= ⋅ = ⋅20 201

2

A filter is simply a two-port device, with the following transfer function, H(f):

Page


114

Low-Pass Filters (LPF)• Low pass filters are the most commonly used filters for

EMC Applications Power Line Filters

Low Frequency Signal Line Filters

• Filters typically consist of reactive elements, for loss reductionDiscrete FiltersDiscrete Filters Symmetrical Filters Symmetrical Filters Shunt Capacitor p (Pi)-FilterSeries Inductor T-Filter

AA--Symmetrical FiltersSymmetrical FiltersL-Filter

IL dB Log k i

i

/i[ ] [ ( ) ]= ⋅ + ⋅

=

⋅∑10 11

2f

f 0

Page


115

Passive EMI FiltersShunt Capacitor

0

10

20

30

40

50

0.0001 0.001 0.01 0.1 1 10

Frequency [MHz]

Insertion Loss [dB]

Insertion Loss CurveZS=ZL=50Ω, C=0.1µF

( ) 20 log ;

1 << ,

S L

S L

S L

Z ZIL f C dB

Z Z

C Z Z

ω

ω

⋅≅ ⋅ ⋅ +

A current divider!!!

Page


116

Passive EMI FiltersSeries Inductor

( ) 20 log ;

>> ,

S L

S L

LIL f dB

Z Z

L Z Z

ω

ω

≅ ⋅ +

0

10

20

30

40

0.0001 0.001 0.01 0.1 1 10

Frequency [MHz]

Insertion Loss [dB]

Insertion Loss CurveZS=ZL=50Ω, L=100µH

A voltage divider!!!

Page


117

Passive EMI FiltersSymmetrical Filters: “π-Filter”

p

( ) 2 2 4 6

1

3

02 23

10 log 1 2 ;

1 1 2; ; ;

2 2

IL f f D f D f dB

d LD d damping factor f Hz

CR RLCd π

≅ ⋅ + − +

− = = = ≅

0

10

20

30

40

50

60

70

0.1 1 10 100

Frequency [MHz]

Insertion Loss [dB]

Insertion Loss Curve

ZS=ZL=50Ω, C=0.5nF, L=2µH

Both current & Voltage divider!!!

Page


118

Passive EMI FiltersSymmetrical Filters: “T-Filter”

T

( ) 2 2 4 6

12

3

0 23

10 log 1 2 ;

1 1 2; ; ;

2 2

IL f f D f D f dB

d CR RD d damping factor f Hz

L L Cd π

≅ ⋅ + − +

− = = = ≅

0

10

20

30

40

50

60

70

80

0.01 0.1 1 10

Frequency [MHz]

Insertion Loss [dB]


ZS=ZL=50Ω, C=100nF, L=2µH

Ditto!!!

Page


119

Passive EMI FiltersA-Symmetrical Filters: “L-Filter”

L

L

0

10

20

30

40

50

0.01 0.1 1 10

Frequency [MHz]

Insertion Loss [dB]

( ) 2

20 log ;

S L

L

Z Z

LIL f LC dB

Z

ωω

≅ ⋅ +

<<

( ) 2

20 log ;

L S

S

Z Z

LIL f LC dB

Z

ωω

≅ ⋅ +

<<


ZS=5Ω, ZL=50Ω ,C=3nF, L=10µHZL=5Ω, ZS=50Ω ,C=3nF, L=10µH

( )2 2

4

1

2

02

10 log 1 ; 2

1 1 2; ; ;

2

f DIL f f dB

d LD d damping factor f Hz

CR LCd π

≅ ⋅ + +

− = = = ≅

Ditto!!!

Page


120

Common- & Differential-Mode Filtering

ZS

ZL

e(t)

ZS

ZL

e(t)

ZS

ZL

e(t)

ZS

ZL

e(t)

Differential-ModeTopology

Common-Mode Topology

p-Filter

T-Filter

Page


121

Common-Mode FilteringCommon Mode Chokes

Circuit #1 Circuit #2

CM Current

CM Current

DM

Current

• Common Mode Chokes:... Provide high CM losses, compensating for smaller capacitors

Affect CM signals only with virtually no effect on DM signals

Have high-m (m =2,500- 10,000) cores (high inductance, e.g., 1- 2mHy), without saturation

(Almost) zero inductance for power line (net) current

CM

Current

Signal DM

Current

Core

Hi µ−

CM-Generated

Flux

DM-Generated

Flux

Page


122

Power Line Filters• Power line filters are intended to suppress EMI emissions and EMI

interference, coupling via power lines• Power line filters contain:

Common and Differential mode filters

Often - a series inductor on the Protective earth line to eliminate chassis noise emissions

• ZP.E. must be < 0.1ΩΩΩΩ @ fPWR (safety)

• Select the filter according to required suppression, current and voltage rating, safety criteria and space available

• Available for: DC power

AC 1-phase power

AC 3-phase power

Page


123

Ferrite Beads• Losses in inductors is a disadvantage

Power dissipation at in-band frequencies

Reduction of filter’s Q

• In Ferrite-based filters, these parameters are an... advantageAlso...

Ferrites are a simple, easy to implement and cost-effective high frequency filtering solution

Ferrites are inert ceramics containing granulated iron compounds

Ferrites are free of organic matter and are not degraded by most environments

Page


124

At low frequencies, the inductor shorts out the resistorAt high frequencies, the inductor represents a high

impedance, thus EMI flows through the resistance, R and is dissipated as heat

Symbol

Equivalent Circuit

R

L

Ferrite Beads

A dB LogZ Z Z

Z Z

S SB L

S L

[ ] = ⋅+ ++

20

Page


125

Application of Ferrite Beads

• Ferrites are easily installed on cables (“snap-on”) which makes them ideal for troubleshooting and “EMC fixing”

VDM

VCM

eS

ZS

ZL

VDM

VCM

eS

ZS

ZL

Differential-Mode Suppression

Common-Mode Suppression

Page


126

L: Lead Inductance

R: Lead to Foil Contact Resistance

R1: Resistance of Metalized Foil

C: Capacitance

L1: Foil Inductance

RS: Shunt Resistance

Effect of Capacitor’s Lead Inductance

ES

ZS=50Ω

1V ZL=50Ω V

L

L=2µH RL=1mΩ

RC=3mΩ

C=0.5nF

π-Section Low Pass Filter

Poor Bond Impedance, ZB

ZB

LB=0.5nH

RB=1mΩ

Intended Path Unintended

Path

1 2

0

10

20

30

40

50

60

70

80

90

0.01 0.1 1 10 100 1000

Frequency [MHz]

Insertion Loss [dB]

Effect of bad filter grounding on

Insertion Loss

Page


127

Effective Filtering - How ?• Standard capacitors suffer from disadvantages for EMC

(high frequency) applications Series inductance of the capacitor’s leads Parallel capacitance between the runs of an inductor

The “Rabbit out of the hat”...:Feedthrough Filters

Page


128

Feed-Through Filters

““FeedthroughFeedthrough”” devices devices are also available in are also available in filter configurations, filter configurations,

e.g., L, T, e.g., L, T, ππ

Page


129

Transient Suppression DevicesThe Transient Phenomena

• Transients are special phenomena of EMI Very high levels (kVolts, kAmps) Very short durations (nSecs to µµµµSecs) Very short rise times (nSecs to µµµµSecs) Transients may damage the equipment!

Page


130

Max V (Into Large Z) Max I (Into Small Z)

ESD 15 kV + 10's to 100's of Amp's

EFT kV's 10's of Amp's

Surge kV's kAmp's

These levels are “slightly” higher than, say, TTL levels...

Duration:

pSec

nSec

mSec

(10’s - 100’s)

(1 - 10’s)

(0.1 - 10)

Surge

(10’s - 100’s) mSec

EFT

(10’s) nSec

ESD

(10’s - 100’s) nSec

1/RisetimesJ

mJ(10’s - 100’s)

(10’s - 100’s)

(1 - 10’s)

ESDEFTSurge

mJ

EnergyContent [J]

Large V’sin cks

Fields from Switch Arcs

to 4 kV/m15 A/m, @ 10 cm

Characterization of Transients

Page


131

• Typically, Open Circuit Output VoltageOpen Circuit Output Voltage from the generator, and sometimes the impedance, are specified

• Thevenine-Norton conversion cannot be used, due to the non-linear characteristics of the loads/protection devices (e.g., spark-gaps, avalanche- diodes, MOVs, etc.), which typically clamp to a constant voltage, independent of current

• Therefore, Short Circuit Output CurrentShort Circuit Output Current of the generator should also be specified

Transients and EOS Waveform Characteristics: Current vs. Voltage

Open Circuit Voltage (VOC)

ZSource

+

V

-

VOpen Circuit

Short Circuit Current (ISC)

ZSource

I IShort Circuit

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132

Transient Waveform Characteristicsa/b mSec Notation

α µSec Front Time (tf)

β µSec Time to Half Value (td)

t t tfront ≡ × −1 25 2 1. ( )

t t trise ≡ −3 0

0

100

200

300

400

500

600

700

800

900

1000

0.00 10.00 20.00 30.00 40.00 50.00

Time [uSec]

Value [V/A]

90%

10% t1 =t10%

t2 =t90%

50%‾50%-

Time to Half Value

- td

8/20 µSec per IEC-61000-4-51.2/50 µSec per IEC-61000-4-5

Examples

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0

100

200

300

400

500

600

700

800

900

1000

0.00 10.00 20.00 30.00 40.00 50.00

Time [uSec]

S.C. Current [A]

Standard Transient Waveforms8/20 µSec Unidirectional Current Surge

Front Time T Sec

Time to Ha Sec

f r: T

lf Value

= ⋅ = ±

= ±

125 8 20%

20 20%

. µ

µ• An approximate expression

t=3.911 mSec

A=0.01243 (mSec)-3

IP=Peak Current (from standard)

I t A I t ep

t

( ) ≈ ⋅ ⋅ ⋅−

3 τ

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•• Blocking Blocking the current surge current surge by a series highseries high--impedance impedance devicedevice

Series resistors & inductors Limiting Limiting tthe voltage surge voltage surge by a nonnon--linear protection devicelinear protection device

Varistors Avalanche Diodes (“Tranzorbs”TM)

•• DivertingDiverting the surge current surge current by a shunt lowshunt low--impedance impedance devicedevice

Spark GapsOr

Or

Transient Protection Principles

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Transient Protection Principles

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Stand-Off Voltage (VR) Highest reverse voltage at which the Device will be non-conducting.Should be greater than circuit’s max. operating voltage

Min. Breakdown Voltage (BVMIN) Reverse voltage at which the Device conducts 1mA. This is the point where the Device becomes a low impedance path for the transient.Should be lower than circuit’s min. vulnerability level

Max. Clamping Voltage (VC MAX) Maximum voltage drop across the Device while it is subject to the Peak Pulse Current, usually for 1mSecDetermines Device’s voltage overshootShould be lower than circuit’s min. vulnerability level

Transient Suppression DevicesPrimary Parameters of Non-Linear TSDs

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Peak Pulse Current (IP) Maximum allowable pulse current which does not modify performance parameters of the device by more than ± 10%Device should be able to handle the Peak Pulse Current

Peak Pulse Power (PP) Clamping Voltage × Peak Pulse CurrentDevice should be able to handle the Peak Pulse Power

Shunt Resistance(RS) Resistance of the Device while not conducting.In most Devices, excluding Varistors, RS ≈1010ΩShould be as high as possible

Shunt Capacitance (CS) Capacitance between the electrodes of the Device @ 1 kHzDetermines bandwidth of the Device, and maximum usable frequencyShould be as low as possible

Transient Suppression DevicesPrimary Parameters of Non-Linear TSDs

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Spark Gap/Gas Tube DevicesActive, non-linear device, constructed of 2-3 electrodes typically encased in a ceramic case

filled by an inert gas (Ar, Ne)

Or

Three-ElectrodeSpark Gaps

Two-ElectrodeSpark Gaps

Or

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Spark Gap/Gas Tube Devices Breakdown Voltage

DC Sparkover Voltage

dV

dtV Sec≈ 100 /

Impulse SparkoverVoltage

dV

dtkV Sec≈ 1 /µ

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• Primary advantages Low voltage when conducting Can conduct high currents (5 to 20 kAmp

for 10mSec) Low shunt capacitance (< 2 pF) Negligible leakage current during normal

operation

• Primary disadvantages Relatively slow response Ignition (conduction) voltage varies Follow current during discharge May not extinguish in DC power circuits

Spark Gap Devices

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Metal Oxide Varistor (MOV)Passive, non-linear device, acting

as a non-linear resistor: V=I××××R(I or V)

I K V= ⋅ αNon-linearity Factor

Geometry Constant of the Device

Varistor: α >> (25 - 60)

Resistor: α = 1

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Metal Oxide Varistor (MOV)

Line to Line & Line to Ground

Applications

• Primary advantages Relatively fast response High energy absorption Can conduct wide range of currents (up to 20 kAmp) Wide selection

• Primary disadvantages Large parasitic capacitance (1 to 10 nF)

Limits signal’s bandwidth to 1 MHz

small leakage current during normal operation Degrade when exposed to current surges

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• Primary consideration: Energy dissipation in the device• Energy in the surge:

Metal Oxide Varistor (MOV)Selection of Device

5µS 50µS

50A

100A

t

I

E K V I JC P= ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ × =−τ 0 5 500 100 5 10 0 136. .

E K V I JC P= ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ − × =−τ 1 4 500 100 50 5 10 3 156. ( ) .

Total Energy3.28J

E K V IC P= ⋅ ⋅ ⋅τ

ClampingVoltage

Peak Current

τ

k=0.637I P

τ t

k=0.5

τ

k=1.4

τ

k=1.0

I P / 2

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Avalanche/TVS Diodes

Or

Passive, silicone diode, with high doping

Capable of very low clamping levels

min dV

dI

• Avalanche Diodes are especially fit for board level protection

Available in a 3V to 400V range, so they can protect semiconductors or other sensitive components

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• Primary advantages Relatively fast response (1 pSec) Unidirectional or bidirectional devices Wide range of reverse standoff voltage (5.5

V to 700 V+ ) Wide maximum clamping voltage ranges(7 V

to 500 V) Capable of handling surge currents levels of

0.6 to 0.9 kA

• Primary disadvantages Large shunt capacitance (50 to 1000 pF)

Limits signal’s bandwidth

Handles relatively small transient currents (< 500 A)

Avalanche Diodes/TVS Diodes

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Such devices areSuch devices arecommercially availablecommercially available

Avalanche Diodes/TVS DiodesAvalanche Diodes/TVS DiodesAvalanche Diodes/TVS DiodesAvalanche Diodes/TVS DiodesReduction ofReduction ofReduction ofReduction of DiodeDiodeDiodeDiode’’’’s Capacitances Capacitances Capacitances Capacitance

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Composite TVS Circuits

• Often, discrete devices cannot provide acceptable protection

One protection stage is insufficient Surge level differs significantly from circuit signal level

• In those cases, composite circuits may be used

Series

Control

Element

Filter

Network

High Energy

Dissipator

Series

Control

Element

Series

Control

Element

Filter

Network

Low Energy

Dissipator

Hazard

Input

Protected

Equipment

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• The spark gap will divert most of the surge current, after its ignitionafter its ignition

• The “fast” suppression devices will respond “immediately” to the fast front time of the transient

• The series element will limit the incident surge current and ensure the ignition of the spark gap

Composite TVS Circuits

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Installation of Filters and Transient Protection Devices

• The installation of filters has utmost importance for ensuring their performance

• Filters & TSDs must be installed with maximum separation between input to output leads, preferably - in a Feedthrough manner

Input to output coupling may dissolve the filter’s suppression effectiveness

““IRON RULEIRON RULE””SEPARATE PROTECTED AND NONSEPARATE PROTECTED AND NON--PROTECTED LINESPROTECTED LINES

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Filter/TSD Connectors

"RF Dirty Area"

"Clean" Input

Noise

sources

When multiple lines must be filtered, a filter connector offers a cost effective, compact and efficient solution

Filter connectors are available both as plugs and receptacles

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Installation of Filters and Transient Protection Devices

Effects of Non-Ideal Properties of Y Capacitors

Adverse Effect of Lead Inductance on Protection Level Provided by a MOV TSD

0

10

20

30

40

50

60

70

80

90

0.01 0.1 1 10 100 1000

Frequency [MHz]

Insertion Loss [dB]

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Installation of TSDsInadvertent Transformer Effect

• The spark gap fires and generates a fast transient current in the loop A-B-C-D

• This current will induce a magnetic field into the loop E-F-G-H which will induce a voltage source in that loop

• This is a “parasitic transformer which should be avoided

Vd

dt

dB

dtdsEFGH = − = − ⋅∫

Φ

S

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Module 7Summary and Wrap-Up

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Just tell me what rules I need to follow to ensure that I don’t have

EMC-related problems.

Just tell me what rules I need to follow to ensure that I don’t have health-related problems with my

brain surgery.

Courtesy: Prof. T. HubingUniversity of Missouri-Rolla

What are the Most Important EMC Design Guidelines?

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155

Thank you for your

attention!!!

Summary and Wrap-Up

essentials of equipment design for electromagnetic compatibility (emc) compliance - 2010

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