EMC Components and Filters
When Capacitors aren’t ……..
Rationale Many techniques for controlling EMI rely on
some type of filtering Filters involve inductors, capacitors and
resistors These components have strays associated with
them, which alter their behaviour. See Shortcomings of Simple EMC Filters
http://64.70.157.146/archive/old_archive/040126.htm
Topics
ComponentsCapacitors InductorsResistors
Decoupling Filters
Capacitors – Approx Frequency Ranges.
Al Electrolytic 1 F to 1F
Tantalum Electrolytic 0.001 F to 10 F
Paper and MetallisedPaper. 1 F to 1mF
Mylar. 0.01 to 10 F
Polystyrene and Polycarbonate. 25pF to 0.25 F
Polypropylene. 47pF to 0.15 F
Mica and Glass. 1pF to 0.01 F
Low Loss Ceramic. 1000pF to 1 F
0.001 0.01 0.1 1 10 100 1 10 100 1000
MhzkHz
20 – 25nH
About 1.4nH
Capacitors
Have Equivalent Series Resistance (ESR) and ESL.
Electrolytics require correct DC polarityBest capacitance to volume ratioHigh ESR (>0.1Ω)ESR increases with frequencyHigh ESL
Capacitors
Electrolytics cont.Limited reliability and lifeLow frequency devicesRipple current limitationsParallel inductor improves high frequency (up
to 25kHz) response
Capacitors
Paper and MylarLower ESRHigher ESLUses
Filtering Bypassing Coupling and noise suppression
Capacitors
Mica and CeramicsLow ESL and ESRKeep leads shortUses
High frequency filtering Bypassing decoupling
Capacitors
Polystyrene and PolypropyleneLow ESRVery stable C – f characteristicMylar is a metalised plastic
Polyethelyne terephthlalate DuPont trade name
Capacitors
Equivalent CircuitR C L
Capacitors
Effect of equivalent Circuit
100 1 103
1 104
1 105
1 106
1 103
0.01
0.1
1
10
100
Capacitive ReactanceEquivalent Circuit Impedance
Frequency (MHz)
Mag
nit
ud
e of
Rea
ctan
ce &
Im
ped
ance
C 0.1 106
R 0.02
L 1.5 109
Inductors
Equivalent Circuit Now a parallel resonance R will be low
Winding resistance C will be low
Inter – winding capacitance
Inductors Effect of equivalent circuit
1 10 100 1 103
1 104
1 103
1 104
1 105
1 106
1 107
1 108
Inductive ReactanceEquivalent Circuit Impedance
Frequency (kHz)
Mag
nitu
de o
f R
eact
ance
& I
mpe
danc
e
C 1001012
L 50 103
R 0.02
Inductors
Strays give a resonance that is quite sharp.R and C are low
Above resonance inductor looks capacitive Air cored coils are large
Produce unconfined fieldsSusceptible to external fieldsSolenoid has infinite area return path
Inductors
Ferromagnetic coilsalso sensitive to external fieldsown field largely confined to coreSmaller than air cored devices
Permeabiity increase by factors > 10000
Saturate if a DC is presentAir gap reduces this effect
Inductance lowered
Inductors
Ferromagnetic coilsCore material depends on frequency
LF – Iron Nickel Alloys HF – Ferrites
Can be noisy caused by magnetostriction in laminations of core
RF chokes tend to radiateShielding becomes necessary
Resistors
Equivalent Circuit Parallel RC
Resonance C will generally be low L comes from leads
and constructionwirewound
Resistors
Effect of Equivalent Circuit
1 10 100 1 103
1 104
1 105
0.1
1
10
100
1 103
Equivalent Circuit Impedance
Frequency (kHz)
Mag
nitu
de o
f R
eact
ance
& I
mpe
danc
e
C 0.001106
R 1000
L 1 106
Resistors
As frequency increases resistor begins to look inductive
WirewoundHighest inductanceHigher power ratingsUse for low frequencies
Resistors
Film TypeCarbon or Metal Oxide filmsLower inductance
Still appreciable because of meander line construction
Lower power ratings
Resistors Composition
Usually CarbonLowest Inductance
Mainly LeadsLow power capabilityC around 0.1 to 0.5pFSignificant for High values of R
Normally neglect L and C except for wirewound
Decoupling
Power rails are susceptible to noiseParticularly to low power and digital devicesCaused by common impedance, inductive or
capacitive coupling Decouple load to ground
Use HF capacitorClose to load terminals
Decoupling
Circuit Diagram
Rs
Source
LT RT Noise Voltage
CT
DecouplingCapacitor
Load
Distribution System Load
Decoupling
Components of Transmission System form a Transmission Line System
This has a characteristic impedanceNeglect resistance term
Transient current ΔIL gives a voltageT
T
C
LZ 0
0ZIV LL
Decoupling
Z0 should be as low as possible (a few Ω) Difficult with spaced round conductors
Typically Z0 = 60 - 120 Ω
Separation/diameter ratio > 3 Two flat conductors
6.4mm wide. 0.127mm apart give 3.4 Ω
Filtering
Not covering design in this module Effectiveness quantified by Insertion
Loss
1
2
EfilterwithVoltageoutput
EfilterwithoutVoltageoutputIL
dBE
EIL
1
2log20
Filtering
Impedance Levels Insertion loss depends on source and load
impedanceDesign performance achieved if system is
matchedL and C are reflective componentsR is Lossy, or absorptive
Reflective Filters Generally, filters consist of alternating
series and shunt elementsL
L /2 L /2
C
C
L
L
C
C /2 C /2
Rs High
Rs High
Rs Low
RL Low
RL Low
RL High
RL High
Rs Low
Reflective Filters
Any power not transmitted is reflected. Series Elements
Low impedance over passbandHigh impedance over stopband
Shunt ElementsHigh impedance over passbandLow impedance over stopband
Generally use Lowpass filters for EMC
Reflective Filters
Filter ArrangementsShunt CSeries LL-C combinations
Classic filter designs
T and Pi Sections
Reflective Filters - Capacitive
Shunt Capacitor Low Pass Source and Load Resistances Equal
CR
R
Vs
Vo
RCjV
V
s
o
2
1
fRCFwhereVF
V so
212
1
Reflective Filters - Example Derived Transfer
Function
C = 0.1μF and R = 50Ω
221
2 1log101log20 FFIL
0.1 1 10 1000
20
40
60
80
Derived Characteristic
Frequency (MHz)
Inse
rtio
n L
oss
(d
B)
Reflective Filters - Example
Effect of strays in Capacitor
Short Leads
Long Leads0.1 1 10 100
0
20
40
60
80
Long LeadsShort Leads
Frequency (MHz)
Inse
rtio
n L
oss
(dB
)
C 1 107
L 1 108
R 50Rc 0.01
C 1 107
L1 1.25 109
R 50Rc 0.01
Reflective Filters - Inductive
Series Inductor
R
R
Vs
Vo
L
RL
jV
V
s
o
1
1
R
LfFwhereV
FV so
212
1
Reflective Filters - Inductive Derived Characteristic
same as for Capacitive Strays Effect
0.1 1 10 1000
20
40
60
80
With Strays
Frequency (MHz)
Inse
rtio
n L
oss
(dB
)
L 2.5 104
Rc 0.2
C 5 1011
R 50
Reflective Filters
Cut-off frequency Insertion loss rises to 3dB
Implies F = 1 or This gives us fc = 63.7kHz
Based on values given earlier
21log103 F
fRC1
C 1 107
R 50
Lossy Filters
Mismatches between filters and line impedances can cause EMI problems
Noise voltage appears across the inductorRadiates
Interference is not dissipated but “moved around” between L and C.
Add a resistor to cause “decay”
Lossy Filters
Neglect source and load resistors Transfer Response
CVo
L
R
Vs
1
11
1
2
CRjLCCjLjR
CjV
V
s
o
Lossy Filters Natural Resonant Frequency
Damping Factor
Transfer Function becomes
LC
10
CL
R2
12
1
0
2
0
jV
V
s
o
Lossy Filters
Transfer Characteristic
Critically damped for minimum amplification
Best EMI Performance
0.01 0.1 1 1060
40
20
0
20
OverdampedCritically DampedUnderdamped
Normalised Frequency
Inse
rtio
n L
oss
(dB
)
0.1
0.5
10
Ferrite Beads
Very simple component
Equivalent Circuit
Impedance
Conductor
Ferrite Bead
L
R
222 LRZ
Ferrite Beads
Frequency Response
Cascade of beads forms lossy noise filter
1 10 100 1 1030
50
100
150
High LHigh R
Frequency (MHz)
Bea
d Im
peda
nce
(Ohm
s)
Ferrite Beads
Noise suppression effective above 1MHzBest over 5MHz
Single bead impedance around 100ΩBest in low impedance circuits
Power supply circuits Class C amplifiers Resonant circuits
Damping of long interconnections between fast switching devices
Mains Filters – Simple Delta Capacitive Two noise types
Common ModeDifferential Mode
Y Caps filter Common ModeMax allowable value
shown here X Cap filters
Differential Mode
L
N
EX
Y
Y
0.1 - 1 F
0.005 F
0.005 F
Vc
Vc
Vd
Mains Filters Frequency Response
0.1 1 10 1000
10
20
30
40
Differential ModeCommon Mode
Frequency (MHz)
Inse
rtio
n L
oss
(dB
)
Feedthrough Capacitors
Takes leads through a case Shunts noise to ground
Lead
Shunt Capacitance
Comparison with Standard Capacitor
Typical Mains Filter C1 and C2
0.1 - 1μFDifferential Mode
L provides high Z for Common Mode
None for DM Neutralising
Transformer L = 5 – 10mH
L
N
E
C1
L
L
C2
C3
C4
Equipment
Typical Mains Filter C3 and C4 are for CM currents to Ground
and the equipment earth Response
0.1 1 10 100Mhz
60
40
20
Summary Various filtering techniques have been
presented Imperfections in components have also been
discussed These strays can be applied to any filter The resultant circuit can become very
complicated Circuit simulator may be a better route