grounding: the grounds for emc design - emc chapter 27emc27.elfak.ni.ac.rs/downloads/grounding...

Grounding: The Grounds for EMC Design

1


Elya B. JoffePresident, IEEE PSE SocietyPast President, IEEE EMC Society


"Ground is where potatoes and carrots thrive"

(Dr. Bruce Archambeault)


2

Grounding in EMC Engineering

• “Grounding” is probably of electrical/electronic systemdesign, often considered as "black magic“

Not easy to understand intuitively

No straightforward definition, modeling or analysis

Many uncontrolled factors affect its performance

• Grounding forms an inseparable part of all electronic andelectrical designs, from circuit through system up toinstallation design

Implemented for EMC and ESD protection, for safety purposes, for lightning and surge protection, etc.


3

Parasitics in Passive Circuit Elements

Parasite – An organism that grows, feeds, andis sheltered on or in a different organismwhile contributing nothing to the survival of itshost


4

“Real World” Resistors

RLS

CP

1

1R S

P

Z j L

j CR

ωω

= + +

Frequency response of "Real World" (non-ideal) Resistors, (R=10ΩΩΩΩ, LS=50nH and CP=1nF)

0

2

4

6

8

10

12

1 10 100

Frequency [MHz]

Imp

ed

an

ce [

Oh

m]


5

“Real World” Capacitors

C RS

(ESR)LS

(ESL)

Rp

1

1C S S

P

Z j L R

j CR

ωω

= + +

+

Frequency response of "Real World" (non-ideal) Capacitors, (C=10nF, LS=5nH and RS=2mΩΩΩΩ)

0.01

0.1

1

10

100

1 10 100

Frequency [MHz]

Imp

ed

an

ce [

Oh

ms]


6

“Real World” Inductors

RSL

Rp

Cp

1

1 1L

P

P S

Z

j CR j L R

ωω

=+ +

+

Frequency response of "Real World" (non-ideal) Inductors, (L=1µµµµH, R=10mΩΩΩΩ and CP=10pF)

10

100

1000

10000

10 100Frequency [MHz]

Imp

ed

an

ce [

Oh

ms

]


7

Conclusions: “The Invisible Circuit”• Nothing is like it seems at first…

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

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


8

Common Mode and Differential Mode Signals


9

Common- & Differential-Mode Signals

II I

C=

+1 2

2I

I I

D=

−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 a contradiction, check your premises. You will find that one of them is wrong”

“Atlas Shrugged”


10

Sources of Common-Mode Signals

“Ground Loops” External Radiated Field

Or Capacitive Crosstalk

Electric Flux

Dr

Vin

-2 ICM

VG

+IDM

+ICM

-IDM

+ICMA

I1

I2

I3


11

Signal Propagation:“Path of Least Impedance” Principle

Fermat's principle leads to Snell's law; when the sines of the angles in the different media are in the same proportion as the propagation velocities, the time to

get from P to Q is minimized.

Hikers choose the easy way to cross hills

In thermodynamics, the enthalpy or heat content is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure and entropy.


12

One Rule to bring them alland in the darkness bind them....

One Ring to rule them all,One Ring to find them,

One Ring to bring them alland in the darkness bind them....


13

Path of Least Impedance” Principle Visualize 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)


14

“Path of Least Impedance” PrincipleElectrical Energy and Electrical Potential

• In order to bring two like charges near each other or to separate two opposite charges work must be done

• Whenever work gets done, energy changes form

• As the monkey does work on the positive charge, he increases the energy of that charge

• The closer he brings it, the more electrical potential energy it has

• When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy

• Does the path the monkey travels with the charge make a difference?


15

• Work done against an electric force in carrying a charge along a path from point a to b

• Electric Potential between with the points a and b:

b b b

a a a

W F dl qE dl q E dl= − • = − • = − •∫ ∫ ∫

( )work,or Δelectric-energy

charge moved

b

E a b

a

WV E dl

q→∆ = − • = =∫

Fur

dluur

dluur

a b b aW W→ →= −


• Does the path the monkey travels with the charge make any difference? Around a closed loop?

( ) ( ) 0

b

b aE a b E a b a

a

V E dl V V V→ → →∆ = − • = − ⇒ ∆ ≡∫ Right?

In a lossless medium…


16

• When time-varying magnetic fields are present it is not possible to describe the electric field simply in terms of a scalar potential V because the electric field is no longer conservative

• is path-dependent because: ∇∇∇∇×E≠0 (Faraday's law of induction)


• In such cases we must consider the magnetic (vector) potential as well, and:

• Which remains conservative

C

E dl•∫

Wrong!!!

AE V

t

∂= −∇ −

∂

ur

In a lossless medium…

EFur

dluur

dluur

a b b aW W→ →= −

MFur

b

b a

a

E dl V V− • ≠ −∫


17

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

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


18

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 ”tightly coupled” conductors:

1C

21Br

2S

d sr

1I11Br

2C

12 12

dIV L

dt=


19


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

→

→

= + →

→ ↔

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

of the universe.


20


• At LOW FREQUENCIES, the current will follow the path of LEASTRESISTANCE, 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 FREQUENCIES, the current will follow the path ofLEAST 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

= ⋅+


22

“Path of Least Impedance” Principle Experiment Set-Up


23

“Path of Least Impedance” PrincipleWhen is Inductance Minimized?

• Where is Inductance minimized?

• Same circumference, differentareas…

A

B da

LI I

φ⋅

= ≈∫ur r

,B Φ

Current I

X X X X X

X X X X X

X X X X X


24

“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 da

LI I

B B I thus L A

φ

φ

⋅

= ≈

= ⇒ ⇒

∫ur r

ur ur

,B Φ

Current I

LI

Φ=


25


Frequency of 1 kHz

Frequency of 1 MHz

(Simulation run on Agilent Technologies "Momentum" 3D Planar EM Simulator; Courtesy of

Alexander Perez, Agilent Technologies)

Frequency of 1 GHz

Frequency of 10 GHz


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

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

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

Grounding architectures Cable shield termination methods Power decoupling and filtering Transmission line layout and routing Etc…

Few principles in EMC are as important as this one…


27

Grounding Essentials, or… The Grounds for Grounding


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Purposes for Grounding

• Safety: Prevention of shock hazard to personnel Due to lightning strokes or power line short circuits to enclosure

• Path for return current (e.g., aircraft) Vehicle/platform/structure serves as return conductor

•EMC


29

• AC power distribution is governed by national codes• One requirement: With each outgoing phase and neutral wire there

must be a safety ground

Rational for GroundingElectrical Shock Hazards

230V

Phase

0V

Neutral

to Return Ground

230V

Phase

0V

Neutral

to Safety GND

Equipment Enclosure Equipment EnclosureAccidental

Short

Accidental

Short

GND @

Service

EntryGND @

Service

Entry

No Safety Ground: Hazard Safety Ground Protection: SafeThe safety ground shunts the fault currents to the power return,

bypassing (and saving) the person

230230

1,0!!!

00L

B

V VI mA

R≈ ≅ =

Ω


30

Ground Coupled interference

"Grounding Systems are Interference

Distribution Devices"

(Dr. Carl E. Baum)


31

The Grounding ProblemLightning Hazards

Control CenterFuel Tank

External Lightning Protection System

Good Grounding

Surge propagating on Data Lines

Ungrounded cable penetration the

facility50 kV

The ungrounded cable penetrating the fuel tank, caused a potentialdifference between the cable and the facility’s structure, and thus- caused its explosion

50 kA

Good Grounding


32

• The voltage across thefinite groundimpedance, ZG due tonoisy circuit (Circuit#2) is:

Ground Coupled InterferenceCommon Impedance Interference Coupling (CIIC)

2

2 2

22 2

2 2

;

G SNG

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 sensitivecircuit (Circuit #1) is:

Thus11 1

1 1

;L NGi G S L

S L

Z EV Z Z Z

Z Z

⋅≅ << +

+ ( ) ( )1 1 2

1 1 1 1 2 2

L NG 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

⋅=

+ ⋅ +


33

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

Part of the Problem

• The objective of grounding system design could bestated as follows:

• "Design the system such that in spite of the need forgrounding, system performance will not be degradeddue to ground-coupled interference".


34

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

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

• Design a noise tolerant system Using differential circuits with high common mode rejection, for instance

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

circuits

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

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


35

Reducing ground noise voltage drop, VNminimizes coupling between the circuits

Optimizing Ground System DesignReducing Common Path Impedance

• By providing a wider signal returnpath for the amplifier signal, thevoltage drop of the (lowfrequency) motor current createsa lower voltage drop across thecommon return path

• Valid as long as the interferingcurrent is of lower frequency, andspreads across the plane


36

Avoiding IM Circulation through Signal Return Path Impedance

Sensor

IM

M

IM

BA

Optimizing Ground System DesignAvoiding A Common Impedance Path

Sensor

IM

M

IM

A,B

• By providing a dedicated signalreturn path for the amplifiersignal, all the way to the amplifiercommon (point B), the motorcurrent does not flow through thecommon impedance, and nointeraction occurs

• Valid as long as the sensor itselfis not grounded!


37

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


38

Ground System Topologies• Ground system topology is determined by several factors:

Signal Characteristics

System dimensions

System-specific separation & isolation requirements

Safety requirements

• The main ground system topologies are: A “floating” system

Single-point ground

Multi-point ground

POWER

ANALOG

AUDIO

DIGITAL

PWM

SYSTEM GROUNDING

It isNO MAGIC!!!


39

Ground System Topologies A “Floating” System


40

Ground System Topologies Single Point Ground (SPG)

“Daisy Chain” Single Point Ground

SSignal Reference

Structure

Safety

Signal

Power

Ground BusZ3Z2Z1

I1 I2 I3I2+I3I1+I2+I3 I3

A B C

GRP

Signal Source

ES1

llll

System #1 System #2 System #3


41

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

S

ES1

Signal Reference

Structure

Safety

Signal

Power

VNG

ZGIN

Z3Z2Z1

I3I2

I1

Signal Source

GRPllll

System #1 System #2 System #3


42

• At higher frequencies, where the length of the groundconductors approaches λλλλ/4, the SPG is ineffective

Distance along GND Conductor

λ/4

ZS

0

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

Ground System Topologies Single Point Ground (SPG)

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

(red and blue).

inZ →∞


43

Ground System Topologies Multi-Point Ground (MPG)


44

• But… It is not a cure for all• Unlike the Holy Grail,,,

• Careful design is still required

Ground System Topologies Multi-Point Ground (MPG)


45

• When a system comprises of several types of circuits,a composite grounding topology may be used

Single point grounding, for low frequencies (d ≤ λ/20 MHz)

Multi-point grounding for high frequencies (d > λ/20 MHz)

Ground System Topologies Hybrid Grounding


46

Equipment-Level “Ground Tree” Design Process

• Identify circuits• Define Chassis Ground connections at the circuit level (heat-sink

and RF Ground)• Define PCB-level signal returns (ground) requirements• Identify isolation requirements• Define local ground connections• Define CGP/SPG location• Connect GNDs from circuits and Power Supply to CGP• Identify “special cases” (GND System Violations) and potential

ground loops• Incorporate “isolation measures” (transformers, optocouplers,

balanced interfaces, e.g.RS-422 and Isolated Ground Connections)• Define special power supply outputs and connect returns to the

CGP when applicable; define special isolated outputs


47

/

.

/

5

15

5/3.3

15

5

5

5 /

15 /

15 /

28 /

DC/DC Module

5

3.3

15

5

5 /

15 /

5 /

15 /

15 /

28 /

Equipment-Level “Ground Tree” Design Process

CGP

V VDVA

VA RF

VD RF

VA RF

VA RF

VD

LOOP ???

.

/

VA

VD/

Enclosure Chassis

VA

VD


48

• No Isolation: PossibleSystem-Wide GroundLoops

• Isolated Power Supplies:Ground Loops throughPrimary Circuit Eliminated

Role of Switch-Mode Power Supplies in Grounding System Design


49

Understanding and Precluding“Ground Loops”


50

“Ground Loops”• Ground loops occur when ground potential between any two pieces of

equipment differs• A potential difference in the grounds may cause a current flow in

the interconnects Modulating the input of the circuitry thus treated as a desired signal

appearing at its inputs

• Can be a significant source of EMI when circuits are extremelysensitive (e.g., analog circuits)

“Ground Loop”


51

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

CA

d= ⋅ε ε π

0

91 36 10= × F m/

C=Capacitance of PCB to Ground

“Ground Loops”

Circuit #1 Circuit #2

ICM#1

ICM#2

VSRS

=VCM

Transmission Line

VL

C d

A

C

A

d

VS

ZS

Z2

Z1

ZCM

R1

R2

ZL

h

S


52

“Ground Loops”

Longitudinal Conversion Loss factor, LCL:

Constant

20'

CMdB

DMVo

VLCL Log

V=

= ⋅


53

“Ground Loops”

• 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]

Lo

ad

DM

Vo

lta

ge


54

“Ground Loops”

• 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]

Lo

ad

DM

Vo

lta

ge


55

“Ground Loops”

• 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 grounded

Grounded Both Ends

Frequency [Hz]

Lo

ad

DM

Vo

lta

ge


56

Ground System Topologies

• 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


57

“Ground Loops” Techniques for Opening “Ground Loops”

Isolation Transformer

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

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


58


BALUNs (Common Mode Chokes)

CM

Current

Signal DM

Current

Core

Hi µ−

CM-Generated

Flux

DM-Generated

Flux

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

• CM rejection > 80-100 dB can be achieved @ high-f’s• Bulky; can be implemented by Ferrite beads


59


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 60-80 dB can be achieved• Mainly for digital circuits


60


Isolation Amplifier

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

(*) 120 dB in special applications


61


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

combination of several approaches


62


Example: 10/100BaseT Interface

Typical 10/100BaseT Receive andTransmit Interfaces CircuitConsists of Balancing Magnetics andBypass Capacitors

TransmitInterface

Receive Interface


63

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

EMC-related problems with my Grounding design.

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. Hubing

Clemson University

Summary: Grounding Design RulesWhat are the most important Grounding & Shielding design

guidelines?


64

for your Attention!!!


65

Questions?

Comments?

Snide Remarks?


66

For Further information:

Elya B. Joffe

[email protected]: -E

grounding: the grounds for emc design - emc chapter 27emc27.elfak.ni.ac.rs/downloads/grounding...

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