EMC Components and Filters
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Transcript EMC Components and Filters
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 1F to 1F
Tantalum Electrolytic 0.001F to 10F
Paper and Metallised
Paper. 1F to 1mF
Mylar. 0.01 to 10F
Polystyrene and Polycarbonate. 25pF to 0.25F
Polypropylene. 47pF to 0.15F
Mica and Glass. 1pF to 0.01F
About 1.4nH
0.001
0.01
0.1
1
kHz
10
100
Low Loss Ceramic. 1000pF to 1F
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 jRC
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
jC
Vs R jL 1
jC
1
2
LC jCR 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
0.005F
Max
allowable
N
value shown here
0.005F
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