essentials of equipment design for electromagnetic compatibility (emc) compliance - 2010
TRANSCRIPT
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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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“Path of Least Impedance” PrincipleWhich Path will the Return Current follow?
• 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→∞
≈ ⋅ ⋅ >>
≈ >> ⋅= + ⋅ =
“Path of Least Impedance” PrincipleWhich Path will the Return Current follow?
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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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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Leakage From A Single ApertureAperture Reflection Losses vs. Shield Attenuation
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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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Essentials of Equipment Design for EMC Compliance
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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Essentials of Equipment Design for EMC Compliance
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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Essentials of Equipment Design for EMC Compliance
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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Essentials of Equipment Design for EMC Compliance
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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Essentials of Equipment Design for EMC Compliance
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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Essentials of Equipment Design for EMC Compliance
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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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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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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 ?
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Essentials of Equipment Design for EMC Compliance
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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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
76
Module 5
Grounding and Bonding
"Ground is where potatoes and carrots thrive"
(Dr. Bruce Archambeault)
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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• 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
⋅=
+ ⋅ +
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Essentials of Equipment Design for EMC Compliance
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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)
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
82
Ground System Topologies A “Floating” System
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Essentials of Equipment Design for EMC Compliance
83
Ground System Topologies Single Point Ground (SPG)
“Daisy Chain” Single Point Ground
ℓℓℓℓ
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Essentials of Equipment Design for EMC Compliance
84
Ground System Topologies Single Point Ground (SPG)Single Point (“Star”) Ground
ℓℓℓℓ
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Essentials of Equipment Design for EMC Compliance
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).
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Essentials of Equipment Design for EMC Compliance
86
Ground System Topologies Multi-Point Ground (MPG)
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
89
“Ground Loops”SPG vs. MPG
Longitudinal Conversion Loss factor, LCLLongitudinal Conversion Loss factor, LCL:Constant
20'
CMdB
DMVo
VLCL Log
V=
= ⋅
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Essentials of Equipment Design for EMC Compliance
90
“Ground Loops”SPG vs. MPG
• 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
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Essentials of Equipment Design for EMC Compliance
91
“Ground Loops”SPG vs. MPG
• 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
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Essentials of Equipment Design for EMC Compliance
92
“Ground Loops”SPG vs. MPG
• 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
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Essentials of Equipment Design for EMC Compliance
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”
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
95
“Ground Loops”Techniques for Opening “Ground Loops”
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
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Essentials of Equipment Design for EMC Compliance
96
“Ground Loops”Techniques for Opening “Ground Loops”
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
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Essentials of Equipment Design for EMC Compliance
97
“Ground Loops”Techniques for Opening “Ground Loops”
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
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Essentials of Equipment Design for EMC Compliance
98
“Ground Loops”Techniques for Opening “Ground Loops”
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
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Essentials of Equipment Design for EMC Compliance
99
“Ground Loops”Techniques for Opening “Ground Loops”
Example: 10/100BaseT Interface
Typical 10/100BaseT Receive and Transmit Interfaces Circuit Consists of Balancing Magnetics and Bypass Capacitors
TransmitInterface
Receive Interface
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Essentials of Equipment Design for EMC Compliance
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“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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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Direct and Indirect Bonds
Direct (Hard) Bond
Indirect (Jumper) Bond
Bonding Area(Clean both members over
entire mating surface + 1/8”perimeter
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Essentials of Equipment Design for EMC Compliance
103
Indirect Bond Impedance
Bonding Impedance of a Bonding Strap
1
2 S C
rfL Cπ
=
S
S C
r
LZ
R C=
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Essentials of Equipment Design for EMC Compliance
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
≅ ⋅ ⋅ ⋅ ⋅ ⋅ + ≅
≅ ⋅ ⋅ ⋅
= ⋅ +
>
>
+ <<
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
108
Corrosion Control
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
110
Module 6Filtering and Terminal Protection
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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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
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Essentials of Equipment Design for EMC Compliance
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):
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Essentials of Equipment Design for EMC Compliance
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
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Essentials of Equipment Design for EMC Compliance
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!!!
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Essentials of Equipment Design for EMC Compliance
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!!!
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Essentials of Equipment Design for EMC Compliance
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!!!
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Essentials of Equipment Design for EMC Compliance
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]
Insertion Loss Curve
ZS=ZL=50Ω, C=100nF, L=2µH
Ditto!!!
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Essentials of Equipment Design for EMC Compliance
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
≅ ⋅ +
<<
Insertion Loss Curve
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!!!
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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!
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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
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• 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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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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146
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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Thank you for your
attention!!!
Summary and Wrap-Up