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

Components
 Capacitors
 Inductors
 Resistors
Decoupling
 Filters

Capacitors – Approx Frequency
Ranges.
20 – 25nH
Al Electrolytic 1F to 1F
Tantalum Electrolytic 0.001F to 10F
Paper and Metallised
Paper. 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
About 1.4nH
0.001
0.01
0.1
1
kHz
10
100
Low Loss Ceramic. 1000pF to 1F
1
10
100
Mhz
1000
Capacitors
Have Equivalent Series Resistance (ESR)
and ESL.
 Electrolytics

 require
correct DC polarity
 Best capacitance to volume ratio
 High ESR (>0.1Ω)
 ESR increases with frequency
 High ESL
Capacitors

Electrolytics cont.
 Limited
reliability and life
 Low frequency devices
 Ripple current limitations
 Parallel inductor improves high frequency (up
to 25kHz) response
Capacitors

Paper and Mylar
 Lower
ESR
 Higher ESL
 Uses
Filtering
 Bypassing
 Coupling and noise suppression

Capacitors

Mica and Ceramics
 Low
ESL and ESR
 Keep leads short
 Uses
High frequency filtering
 Bypassing
 decoupling

Capacitors

Polystyrene and Polypropylene
 Low
ESR
 Very stable C – f characteristic
 Mylar is a metalised plastic
Polyethelyne terephthlalate
 DuPont trade name

Capacitors

Equivalent Circuit
R
C
L
Capacitors

Effect of equivalent Circuit
6
C   0 .1 1 0
R   0 .02
9
L   1 .5 1 0
Magnitude of Reactance & Impe dance
100
10
1
0.1
0.01
1 10
3
100
1 10
3
1 10
Fr equency (MHz)
Capacitive Reactance
Equivalent Circuit I mp edance
4
1 10
5
1 10
6
Inductors
Equivalent Circuit
 Now a parallel resonance
 R will be low

 Winding

resistance
C will be low
– winding
capacitance
 Inter
Inductors
Effect of equivalent circuit
 12
C   1 001
 0
3
L   5 0 1 0
R   0 .02
Magnitude of Reactance & Impedance

1 10
8
1 10
7
1 10
6
1 10
5
1 10
4
1 10
3
1
10
100
Frequency (kHz)
Inductive Reactance
Equivalent Circuit Impedance
1 10
3
1 10
4
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 fields
 Susceptible to external fields
 Solenoid has infinite area return path
Inductors

Ferromagnetic coils
 also
sensitive to external fields
 own field largely confined to core
 Smaller than air cored devices

Permeabiity increase by factors > 10000
 Saturate
if a DC is present
 Air gap reduces this effect

Inductance lowered
Inductors

Ferromagnetic coils
 Core
material depends on frequency
LF – Iron Nickel Alloys
 HF – Ferrites

 Can
be noisy caused by magnetostriction in
laminations of core

RF chokes tend to radiate
 Shielding
becomes necessary
Resistors
Equivalent Circuit
 Parallel RC
Resonance
 C will generally be low
 L comes from leads
and construction

 wirewound
Resistors
3
Effect of Equivalent Circuit
6
C   0 .00 11
 0
R   1 00 0
6
L   1 1 0
Magnitude of Reactance & Impedance

1 10
100
10
1
0.1
1
10
3
100
1 10
Frequency (kHz)
Equivalent Circuit Impedance
1 10
4
1 10
5
Resistors
As frequency increases resistor begins to
look inductive
 Wirewound

 Highest
inductance
 Higher power ratings
 Use for low frequencies
Resistors

Film Type
 Carbon
or Metal Oxide films
 Lower inductance

Still appreciable because of meander line
construction
 Lower
power ratings
Resistors

Composition
 Usually
Carbon
 Lowest Inductance

Mainly Leads
 Low
power capability
 C around 0.1 to 0.5pF
 Significant for High values of R

Normally neglect L and C except for
wirewound
Decoupling

Power rails are susceptible to noise
 Particularly
to low power and digital devices
 Caused by common impedance, inductive or
capacitive coupling

Decouple load to ground
 Use
HF capacitor
 Close to load terminals
Decoupling

Circuit Diagram
L
RT
T
Noise Voltage
Rs
CT
Source
Distribution System
Decoupling
Capacitor
Load
Load
Decoupling
Components of Transmission System form
a Transmission Line System
 This has a characteristic impedance

 Neglect
resistance term
LT
Z0 
CT

Transient current ΔIL gives a voltage
VL  I L Z0
Decoupling
Z0 should be as low as possible (a few Ω)
 Difficult with spaced round conductors

Z0 = 60 - 120 Ω
 Separation/diameter ratio > 3
 Typically

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

output Voltage without filter E2
IL 
output Voltage with filter E1
 E2 
IL  20 log 
 E1 
dB
Filtering

Impedance Levels
 Insertion
loss depends on source and load
impedance
 Design performance achieved if system is
matched
 L and C are reflective components
 R is Lossy, or absorptive
Reflective Filters

Generally, filters consist of alternating
series and shunt elements
L
L
Rs High
Rs Low
RL Low
C
RL High
C
L/2
L
L/2
Rs Low
Rs High
RL High
C
RL Low
C/2
C/2
Reflective Filters
Any power not transmitted is reflected.
 Series Elements

 Low
impedance over passband
 High impedance over stopband

Shunt Elements
 High
impedance over passband
 Low impedance over stopband

Generally use Lowpass filters for EMC
Reflective Filters

Filter Arrangements
 Shunt
C
 Series L
 L-C combinations

T
Classic filter designs
and Pi Sections
Reflective Filters - Capacitive
Shunt Capacitor Low Pass
 Source and Load Resistances Equal
Vo
1

C
Vo
Vs
Vs 2  jRC

R
R
Vo 
1
2 1 F
2
Vs
where F  fRC
Reflective Filters - Example
Derived Transfer
Function
1

IL  20 log 1  F


2  2
 10 log 1  F 2

80
60
C
= 0.1μF and R =
50Ω
Insertion Loss (dB)

40
20
0
0.1
1
10
Frequency (MHz)
Derived Characteristic
100
Reflective Filters - Example
80
Effect of strays in
Capacitor
 Short Leads
7
C  1  10
9
L1  1 .25 1 0
R  50
Rc  0 .01

Long Leads
7
C  1  10
8
L  1  10
R  50
Rc  0 .01
Insertion Loss (dB)

60
40
20
0
0.1
1
10
Frequency (MHz)
Long Leads
Short Leads
100
Reflective Filters - Inductive

Series Inductor
R
L
R
Vo
Vs
Vo 
1
2 1 F
2
Vs
Vo
1

Vs 1  j L
R
L
where F  f
R
Reflective Filters - Inductive
Derived Characteristic
same as for Capacitive
 Strays Effect

80
Rc  0 .2
 11
C  5  10
R  50
Insertion Loss (dB)
4
L  2 .5 1 0
60
40
20
0
0.1
1
10
Frequency (MHz)
With Strays
100
Reflective Filters

Cut-off frequency
 Insertion

loss rises to 3dB
3  10log 1  F
2

1  fRC
 Implies

F = 1 or
This gives us fc = 63.7kHz
 Based
on values given earlier
7
C  1  10
R  50
Lossy Filters
Mismatches between filters and line
impedances can cause EMI problems
 Noise voltage appears across the inductor

 Radiates
Interference is not dissipated but “moved
around” between L and C.
 Add a resistor to cause “decay”

Lossy Filters
Neglect source and load resistors
R
 Transfer Response

L
C
Vs
1
Vo
jC

Vs R  jL  1
jC
1
 2
 LC  jCR  1
Vo
Lossy Filters

Natural Resonant Frequency

Damping Factor

Transfer Function becomes
0 

Vo
1

2
Vs
 
 
    2 j    1
 0 
 0 
1
LC
R
2
L
C
Lossy Filters
20
Transfer
Characteristic
 Critically damped
for minimum
amplification
 Best EMI
 0 .1 
Performance   0 .5
Insertion Loss (dB)

 
 10 
0
20
40
60
0.01
0.1
1
Normalised Frequency
Overdamped
Critically Dam ped
Underdampe d
10
Ferrite Beads

Very simple component

Equivalent Circuit
Ferrite Bead
Conductor
R
L

Impedance
Z  R2   2 L2
Ferrite Beads
Frequency
Response
 Cascade of beads
forms lossy noise
filter
Bead Impedance (Ohms)

150
100
50
0
1
10
100
Frequency (MH z)
High L
High R
1 10
3
Ferrite Beads

Noise suppression effective above 1MHz
 Best

over 5MHz
Single bead impedance around 100Ω
 Best
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
L
 Common
Mode
 Differential Mode

Vc
Vd
Y Caps filter
Common Mode
0.1 - 1 F

X Cap filters
Differential Mode
Y
E
X
Y
0.005F
 Max
allowable
N
value shown here
0.005F
Vc
Mains Filters Frequency Response
40
Insertion Loss (dB)
30
20
10
0
0.1
1
10
Frequency (MHz)
Differential Mode
Comm on Mode
100
Feedthrough Capacitors
Takes leads through a case
 Shunts noise to ground

Shunt Capacitance
Lead
Comparison with Standard
Capacitor
Typical Mains Filter

C1 and C2
 0.1
- 1μF
 Differential Mode
L
L provides high Z
for Common Mode
 None for DM
 Neutralising
Transformer
 L = 5 – 10mH

L
Equipment
C1
C2
C3
L
N
E
C4
Typical Mains Filter
C3 and C4 are for CM currents to Ground
and the equipment earth
 Response

60
40
20
0.1
1
10
Mhz
100
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