EMC Components and Filters When Capacitors aren’t ……..

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