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Practical EMI Filter Design

Workshop IEEE 2008 Detroit

Alexander Gerfer & Michael Eckert

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Capacitor

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Capacitor : Equivalent Circuit

Inductance of connection:SMD-types 1 nH ... 5 nHwired 10 nH ... 50 nH !

ESR:SMD-types 20 mΩ ... 300 mΩ

up to 1 Ω(see Datasheet )

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Impedance vs. Frequency of Capacitors

978-1-4244-1699-8/08/$25.00 ©2008 IEEE

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Simulation Datas for Caps from KEMET Spice

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Simulation Datas for Caps from KEMET Spice

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Capacitance: DC-Bias and Temperature

Capacitance decrease by DC-bias !!!

Effective only 8.6 µF !!

Capacitance decrease temperature increase (@ = 0VDC 18 µF !!)

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Capacitor : KEMET Spice Simulation Model

C0805C104K3RAC @ 25°C, 0VDC, 41,687 kHz LTspicewww.kemet.com www.linear.com

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Pratice-Tip: SRF of SMD-Capacitors

Example 1:Filter with maximum attentuation at f = 500 MHz in a system ZA = ZB = 100 Ω = konst.;T-Filter Step 1:

choose C with SRF @ 500 MHz

=> Nomogramm:

size 1206 ; C = 68 pF

68 pF

Step 2:calculation of Ls

SRF = 500 MHz :

nHCL resSS49,11 2 =⋅= ω

Capacitance [pF]

SRF

[

MHz

]

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What is an Inductor ?

„Headache-Parts“ ?! A mystery.........

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What is an Inductor ?

technical view:a piece of wire wounded on something

a filter

an energy-storage-part (short-time)

examples:

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What is an EMC-Ferrite ?

technical view:Absorber for RF

frequency dependent filter

examples:

EMC Snap-Ferrites EMC-Ferrites forFlatwire

SMD-FerriteWE-CBF

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Current

Magnetic field H wire

The basics of Inductive ComponentsMagnetic Field; example: long, straight wire

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The basics of Inductive ComponentsMagnetic Field; toroidal & rod core

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Magnetic field strength H of some configurations

RIH⋅⋅

=π2

RINH⋅⋅

⋅=π2

lINH ⋅=

long, straight wire

Toroidal Coil

Long solenoid

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Magnetic field H in the air and in a ferrite core

Air Ferrite core

I2xπxRaverage

H1=H2=H= But : B1=B2

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Permeability ?

Permeability (µr):describes the ability to concentrate the magnetic flux in the core material

Typical Permeability µr :

Iron Powder Cores / Superflux : 50 ~ 150

Nickel-Zink : 40 ~ 1500

Mangan-Zink : 300 ~ 20000

HB r ⋅⋅= µµ0

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Permeability µr

HB r ⋅⋅= µµ0HB ⋅= 0µInduction in air:

Linear Function !

Induction in Ferrite:

Non-Linear Function ( µr !)

The relative Permeability is dependent on Field Strength H; Temperature and a frequency-dependent parameter....

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Saturation effect

saturated

not saturated

H~I

B~

I

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Permeabilität vs. Temperatur

0

200

400

600

800

1000

1200

1400

-50 23 50 85 125 150 160 250

Temp. (°C)

µr

Permeability vs. temperature

620- 40%

+ 40%Curie-Temperature

(µr = 1 )

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How to find the best part for my application ?

The key to success is understanding of :Core Material Comparison

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Core Material Properties Measurement

core material

Impedance Analyser core material parameter

XRZ L22 +=

X LR (f)

XL(NiZn)

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Core Material Parameters of Ferrites

ωL

R(f)

Impedance of Coil with core

material

( )jLjZ =−= |||0 µµω

Impedance of same coil but w/o

core material ! core material

ωL0

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Complex Permeability

||0 µω LR =

|0 µω LjX L =

frequency dependentcore losses

frequency dependentinductive portion

L0 inductance without core

( ) jXRjLjZj

+=−=

−=|||

0

|||

µµω

µµµ

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Comparison of core materials: Inductive behaviour

=> It depends on application frequency for EACH Core-Material !FR-PM-3

Comparison of core materials: Losses

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When to use an Inductor ?When to use an EMC-Ferrite ?

- Application: Storage ChokeRequest: lowest possible core losses

at application frequency

- Application: signal-filter in RF-stages:

Request: low losses => high Q in frequency band

- Application: EMI absorber-filter

Request: high core losses

at noise frequency range

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Equivalent circuit : Inductor /Ferrite

Parasitic Capacitance:SMD-EMC-Ferrite 5 fF ... 5 pFwinded Inductors 10 pF ... 500 pF !

Losses:Inductors (at SRF ! ) up to 30 kΩSMD-EMC-Ferrite (broadband !) 10 Ω ... 3 kΩ

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SMD-Ferrite listed in LTspice

Freeware !Download at www.linear.com_FR-PM-3

Normal Type SMD-Ferrites in LTspice

You can sort database by :Part.Nr.; DCR; Current ; Impedance @100MHz ormaximum Impedance @ desired frequency

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Simulation-Modell for EMC-Ferrites

Model as Parallel-Circuit withconstant Parametern for Rp / Lp / Cp

can be used in anySimulationprogram (like P-SPICE or Electronics Workbench, ...) !

Datas: see Book „Trilogy of Inductors“Seite 113ff

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SMD-Ferrites Models in LTspice

Z[I]All High Current Chip Beads (> 1A)

are modeled in their Current Dependend Impedance behaviour

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SMD-Ferrites Models in LTspice

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SMD-Ferrites Models in LTspice: Impedance vs. DC-bias

P/N: 74279252

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Inductors listed in LTspice

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Structured interference suppression

• Recognise the coupling mode:• common mode noise• differential mode noise

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Structured interference suppression

• Procedure to find out– Use split ferrite

• common mode noise⇒ noise reduction / stable noise immunity

• differential mode noise=> no difference to see

1. Basics

2. Coupling mode

3. Filter topologies

4. Measuring

5. Simulation tools

6. Layout recommend.

7. Application

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How can we find out what interference we have on the PCB‘s?

Take a Snap Ferrite and fix it on the cable(both lines e.g. VCC and GND)

if noise is reduced ornoise immuntiy increase

you have Common Mode Interference

if not

you have Differential Mode Interference

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Ferrite as Common-Mode Filter

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Increase impedance with more turns

incr

ease

Impe

danc

e

Fres.-decrease

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Why current compensation?

N1

N2

Power line Noise

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Filtering with two Inductors

Signal before Filter: Signal after Filter:The Filter is effective on both: Differential & Common Mode Currents !

signal rise-time affected !not usefull for high-speed signals

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Filtering with common mode chokes

Signal before Filter Signal after FilterThe Filter is only active for Common Mode Noise !

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sectional winding bifilar winding

< advantage? >

Why bifilar / sectional winding ?

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For example: WE-SL2 744227SCommon Mode suppression

Differential Mode suppressionis high!

Sectional winding:

ATTENTION:SECTIONELL WINDING MUST BEUSED ON MAINS-POWER SUPPLY !

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Bifilar winding:

For example: WE-SL2 744227Common Mode suppression

Differential Mode suppressionis low!

Differential Mode S-Type

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Common Mode Choke

noise (Common Mode )

Data signal (Differential)

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Filtering with Common Mode Choke: USB

example: USB 2.0 Datenline Filter

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USB 2.0 Filtering with WE-CNSW CMC

Measuring Point TP2

EMI-part EMI-part

90 Ohm @ 100 MHz C.M. 600 Ohm @ 100 MHz C.M. WE-CBF 20 Ohm @ 240 MHz D.M. 40 Ohm @ 240 MHz D.M. 120 Ohm @ 100 MHz

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Common Mode Choke Model in LTspice

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Common Mode Choke Model in LTspice

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Common Mode Choke Model in LTspice

P/N: 744212100

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EMI-Filter topologies, Simulation and Layout tips

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Find where the noise come from !

Vcc

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Which impedance do we need ?

0

10

20

30

40

50

60

Level [dBµV/m]

30M 40M 50M 70M 100M 200M 300M 400M 600M 1G

Frequency [Hz]

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Differential noise filter: SMD-Ferrite WE-CBF

(Chip Bead Ferrite)

SMD-Ferrite

WE - CBFLink Aufbau

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Impedance: SMD-Ferrite (Chip Bead)

XL(NiZn)

Advantage of SMD-Ferrites:wideband, frequency-dependent

Absorber for RF-noise in the frequencyrange 10 MHz ... > 2GHz

with very low DC-Resistance (< 0,8 Ohm max. !)

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Insertion Loss Model

Equivalent circuit of source

Equivalent circuit of system impedance

)(log20 dBinZZZZZABA

BFA

+++=Insertion Loss =>

ZA ZF

ZBU1U0 U2

Equivalent circuit of Filter

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Simulation Model Cap

Simulation Model Fer.

The real world........ !

Equivalent Circuit of Filter-ComponentsEquivalent circuit of source

Equivalentcircuit of load

for Inductor and Capacitor onecan find Simulation-Models

? ?

source

Noisesource

device

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Practical figures for ZA / ZB

* Groundplanes : Impedance range 1 ... 2 Ω

* VCC-Distribution: Impedance range 10 ... 20 Ω

* Video-/ Clock-/ Dataline: Impedance range 50 ... 90 Ω

* Long Datalines: Impedance range 90 ... >150 ΩWith this we can make a first decision of a Ferrite-Impedance ZFwhich would suppress noise in our application :

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Which Impedance ZF is needed ?

Example 1: required attentuation = 20dB @ 200 MHz

VCC-Distribution

System impedance ca. 10 Ohm

180Ω

=> goto Nomogramm:

180 Ω => choise: 220 Ω

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case 1: Attentuation reached

The initialsetup that System Impedance isaround 10 Ohm at 200 MHz istrue !

220Ω

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case 2: More Attentuation reached

Measurement of e.g. 40dBwith Ferrit.

The System-impedance ZA and ZB aremuch lower (at around 1 Ohm)!

220Ω

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Analyse of the results

0

10

20

30

40

50

60

Level [dBµV/m]

30M 40M 50M 70M 100M 200M 300M 400M 600M 1G

Frequency [Hz]

--- : Perfect 20 dB attenuation

--- : Only 10 dB attenuation ??

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case 3: less attentuation reached

Measured onlye.g. 10dBwith Ferrite.

The System-impedances ZA and ZB aremuch higherthan expected(around 50 Ohm) !

OR

Additional coupling pathesnot taken intoaccount !

220Ω

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Insertion Loss vs. Impedance of Source/ Load (ZA / ZB)

A = -32 dB max.with ZA = ZB = 10 Ω = constant

A = -18 dB max.with ZA = ZB = 50 Ω = constant

A = - 8 dB max.with ZA = ZB = 200 Ω = constant

A = - 3 dB max.with ZA = ZB = 1000 Ω = constant

with ZA = ZB = 5000 Ω =constantno effect in filtering !

SMD-Ferrite ZF = 600 Ohm @ 100 MHz

Frequency (Hz)

Inse

rtio

n Lo

ss (

dB)

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EMI Problem: Integrated DC/DC Converter

Common Mode Choke and 2 EMI-Ferrites: because of capacitive over coupling (Profibus to 24 V-DC !!)BURST before : 0,9 kV; after up to 3,5kV OK

CMC 2x2000µH 0,6A 744221 WE-SL2 Würth

SMD-Ferrite Z=1200Ohm 0,2A 74279216 WürthFR-PM-3

EMI Problem: Capacitive Coupling Burst-Test

Integrated DC/DC converter with transformer

C coupling ~ 10 pFin outin out

Input : 1 kV Burst / rise time 5 nsOutput ?

!21051000 ApFnsV

dtdvdi Ccoupling ==∗=

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Recommended EMI-Filter circuits

Suggested Circuits

ATTENTION !Self-Resonant-Frequencyof Components !!!

smaller C = higher SRF

If you choose SMD-Ferrite instead of Inductor L = no Resonance with C = broadband filtering

Source Impedance Load Impedance

low

low low

high

high

high

high orunknown

low orunknown

low orunknown

C

L CC

L

L

C

L

high orunknown

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Noise source

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Filter Topologies

50 Ohm Ref. Line

3xC Capacitance Filter

„L“Filter with SMD Ferrite„L-C“ Filter with SMDFerrite und Capacitor

„PI“ Filter

„T“ Filter

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Filter Topologies

50 Ohm Ref. Line

3xC Capacitance Filter

„L“Filter with SMD Ferrite

„L-C“ Filter with SMD Ferrite und Capacitor

„PI“ Filter

„T“ Filter

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SWITCHER CAD III / Simulation

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The „L“ Ferrite –Filter

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Simulation in LTSpice/SwitcherCADIII

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Insertion Loss

-40

-35

-30

-25

-20

-15

-10

-5

0

1 10 100 1000

Frequenz in MHz

in d

B

Ferrite-Filter Simulation modell)

The „L“ Ferrite –Filter

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Ferrite –Filter: Measured vs. Simulation

Insertion Loss

-40

-35

-30

-25

-20

-15

-10

-5

0

1 10 100 1000

Frequenz in MHz

in d

B

Ferrite-Filter (Insertion Loss)Ferrite-Filter Simulation modell)

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Ferrite –Filter: Measuring vs. Simulation

Insertion Loss

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

Ferrite-Filter (Insertion Loss)Ferrits-Filter Simulation modell)Ref.-Meas. (Spectrum Analyzer)

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Ferrite –Filter: Measuring vs. Simulation

Insertion Loss

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

Ferrite-Filter (Insertion Loss)Ferrite-Filter Simulation modell)Ref.-Meas. (Spectrum Analyzer)Ferrite-Filter (Spectrum Analyzer)

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Parallel capacitors

C1= 1nF

C2=10nF

C3=100nF

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Parallel capacitors resonancy

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Paralleled Caps on IC (74HCT)/ NO Ferrite Bead

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Paralleled Caps (w/o Ferrite Bead ) on IC 74HCT

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Paralleled C with Ferrite Bead on IC (74HCT)

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Attenuation

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1 10 100 1000

Frequency in MHz

in d

BC-Filter (Simulationsmodell)

Parallel capacitors

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C-Filter: Measure vs. Simulation

Dämpfung

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1 10 100 1000

Frequenz in MHz

in d

B

C-Filter (Einfügedämpfung)C-Filter (Simulationsmodell)

Attenuation

Frequency in MHz

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Dämpfung

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

C-Filter (Einfügedämpfung)C-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)

C-Filter: Measure vs. Simulation

Attenuation

Frequency in MHz

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Dämpfung

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

C-Filter (Einfügedämpfung)C-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)C-Filter (Spectrum Analyzer)

C-Filter: Measure vs. Simulation

Attenuation

Frequency in MHz

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Basic Design-Rule of EMC-Filters

Low DC-Resistance; high Absorption forRF-noise; no SRF-Point

Low ESR; „zero“-Ohm-Path to GND for RF-noisestabilzie VCC for Pulse-Currents

L1

C1

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Filtercircuit: Lowpass-Filter (ideal)

This is what we learn in the school....

L1

C1

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Step 1: Lowpass-Filter with real Capacitor .....

L1

Cs

Ls

Rs

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Step 2: Lowpass-Filter with real Capacitor and real Inductor /SMD-Ferrite

Lp

Cs

Ls

Rs

Rp

Cp

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Step 3: Lowpass-Filter with real Capacitor ,real Inductor and connection to GND

Capacitor with 1mm wire ~ 1 nH1 via ~ 0.5 nH

Lp

Cs

Ls

Rs

Rp

Cp

1 mm ~ 1nH

1 via ~ 0.5 nH0.5 nH @ 100 MHz = 0.314 Ω

0.5 nH @ 1 GHz = 3.14 Ω !

PCB via

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Layout and Vias

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good

vias to ground

bad

Layout for Ground

IC2aIGND

The GND-Potential is affectedby current driven throughground-pin of semiconductor !

IGND

IC2aSeparated ways for RF-currentof C2a and semiconductor

More vias = smaller DCR/L

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Grounding / Layout

LC-Lowpassfilter Layout Lowpassfilter

Why will this filter not work properly at RF-Frequencies ?

Ground-Level

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Layout

Inductive coupling

Capacitive coupling

Bypass for noise

too long tracks

Contraction for RF

more vias to GND= low ohmic= smaller inductance

Connection to Case

bad solution optimized

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LC-Filter

L=742 792 093

C=100nF

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The LC-Filter

Dämpfung

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1 10 100 1000

Frequenz in MHz

in d

B

LC-Filter (Simulationsmodell)

Attenuation

Frequency in MHz

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Dämpfung

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1 10 100 1000

Frequenz in MHz

in d

B

LC-Filter (Einfügedämpfung)LC-Filter (Simulationsmodell)

LC-Filter: Measured vs. Simulation

Attenuation

Frequency in MHz

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Dämpfung

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

BLC-Filter (Einfügedämpfung)LC-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)

Attenuation

Frequency in MHz

LC-Filter: Measured vs. Simulation

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Dämpfung

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

LC-Filter (Einfügedämpfung)LC-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)LC-Filter (Spectrum Analyzer)

Attenuation

Frequency in MHz

LC-Filter: Measured vs. Simulation

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The Pi-Filter

C1=1nF

L=742 792 093

C2=100nF

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Improvement for VCC-Decoupling with π-Filter

IC

VCC

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Calculation of Minimum Decoupling-Capacitor

nFmVnsmA

dUdtdICS 1200

540 =⋅=⋅=Bsp: allowed Voltage-Drop 200mV at Pulse-Current of @ t = 5ns:

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Dämpfung

-120

-100

-80

-60

-40

-20

0

1 10 100 1000

Frequenz in MHz

in d

B

Pi-Filter (Simulationsmodell)

Attenuation

Frequency in MHz

The Pi-Filter

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Dämpfung

-120

-100

-80

-60

-40

-20

0

1 10 100 1000

Frequenz in MHz

in d

B

Pi-Filter (Einfügedämpfung)Pi-Filter (Simulationsmodell)

Pi-Filter: Measured vs. Simulation

Attenuation

Frequency in MHz

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Dämpfung

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

Pi-Filter (Einfügedämpfung)Pi-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)

Attenuation

Frequency in MHz

Pi-Filter: Measured vs. Simulation

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Dämpfung

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

Pi-Filter (Einfügedämpfung)Pi-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)PI-Filter (Spectrum Analyzer)

Attenuation

Frequency in MHz

Pi-Filter: Measured vs. Simulation

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Improvement for VCC-Decoupling with π-Filter

Description of internal parasitics by ICEM-project (see IEEE publications / Google)

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ICEM Model: parasitics of IC (74HCT)

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ICEM Model: 1nF/ 100nF Caps only (no Ferrite Bead)

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ICEM Model: 1nF/ 100nF and Ferrite Bead 74279213

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Probing & Practical Tip

68 mVpp with 3.3 µF + 1µH + 3.3 µF input Cap

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EMI: Conducted Emission (w/o input filter)

LT3481EMSE Demo Board24V to 3.3V @2A fsw=500kHzCEM 0.15 – 30 MHz

Test without EMC filter:Peak 82dBµV

→ 26dB above limit !

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EMI: Conducted Emission w filter

Test with additional L=10uH,C=3.3uF 50V 1210 input filter

Peak=42dBµV/m ∅=32dBµV/m

Peak & ∅ 14dB below limit

Ferrite bead High ESR Elco to damp cable

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EMI: Radiated Emission

10dB attenuator in software !result should be 10dB lower

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The T-Filter

L1=742 792 040

C=100nF

L2=742 792 092

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Optimisation of T-Filter for 500 MHz (Notch)

SMD-Capacitor

SMD-Ferrite SMD-Ferrite

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Pratice-Tip: SRF of SMD-Capacitors

Example 1:Filter with maximum attentuation at f = 500 MHz in a system ZA = ZB = 100 Ω = konst.;T-Filter Step 1:

choose C with SRF @ 500 MHz

=> Nomogramm:

size 1206 ; C = 68 pF

68 pF

Step 2:calculation of Ls

SRF = 500 MHz :

nHCL resSS49,11 2 =⋅= ω

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Simulation-Model: SMD-Ferrite

Rp = 685 ΩCp = 1,2584 pFLp = 1,4675 nHRDC = 0,30 Ω

Rp = 125 ΩCp = 0,1834 pFLp = 0,2000 nHRDC = 0,30 Ω

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Simulation-Result for T-Filter

Only C = 68 pF

more attentuation with SMD-Ferrite

Broadband attentuation by using SMD-Ferrite

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The T-Filter

L1=742 792 040

C=100nF

L2=742 792 092

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Dämpfung

-120

-100

-80

-60

-40

-20

0

1 10 100 1000Frequenz in MHz

in d

B

T-Filter (Simulationsmodell)

Attenuation

Frequency in MHz

The T-Filter

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Dämpfung

-120

-100

-80

-60

-40

-20

0

1 10 100 1000

Frequenz in MHz

in d

BT-Filter (Einfügedämpfung)T-Filter (Simulationsmodell)

T-Filter: Measured vs. Simulation

Attenuation

Frequency in MHz

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Dämpfung

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

T-Filter (Einfügedämpfung)T-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)

Attenuation

Frequency in MHz

T-Filter: Measured vs. Simulation

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Dämpfung

-120

-100

-80

-60

-40

-20

0

20

1 10 100 1000

Frequenz in MHz

in d

B

T-Filter (Einfügedämpfung)T-Filter (Simulationsmodell)Ref.-Messung (Spectrum Analyzer)T-Filter (Spectrum Analyzer)

Attenuation

Frequency in MHz

T-Filter: Measured vs. Simulation

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You want to know more ??

Look at our Book:

Chap.1: Basicskeep it simple, stupid

Chap.2: ComponentsDescriptions, Applications, Simulation Models and many more

Chap.3: Filter-CircuitsDesign, Grounding, Layout, Tipps

Chap.4: ApplicationsCircuit, suggested parts, Layout

Chap.5: Appendicesfrom A to Z

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You want to know more ??

Look at ABC of Transformers:

Chap.1: Basic Principleskeep it simple and stupid

Chap.2: ApplicationsDescriptions, Applicationsand many more

Chap.3: Step-by-Stepbuild a transformer for the mostimportant switching topologies

Chap.4: Directoriesin depth explanations andfrom A to Z