EMC Components and Filters

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.

Al Electrolytic 1  20 – 25nH Tantalum Electrolytic 0.001

 F Paper and Metallised Paper. 1  Mylar. 0.01 to 10  Polystyrene and Polycarbonate. 25pF to 0.25

 Polypropylene. 47pF to 0.15

 About 1.4nH

Mica and Glass. 1pF to 0.01

 F Low Loss Ceramic. 1000pF to 1  0.001 0.01 0.1 1 10 100 1 10 100 1000

kHz Mhz

Capacitors

 Have Equivalent Series Resistance (ESR) and ESL.

 Electrolytics  require correct DC polarity  Best capacitance to volume ratio  High ESR (>0.1

Ω)  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 100 C    6 R  0.02

L    9 10 1 0.1

0.01

1  10 3 100 1  10 3 1  10 4 Frequency (M Hz) Cap acitive Reactance Equivalent Circuit Imp edance 1  10 5 1  10 6

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 8 C    L   R  0.02

 3 12 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

 1  10 3 Effect of Equivalent Circuit C    6 R  L  1000   6 100 10 1 0.1

1 10 100 1  10 3 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

 Rs C T Distribution System Source R T Noise Voltage Decoupling Capacitor Load Load

Decoupling

 Components of Transmission System form a Transmission Line System  This has a characteristic impedance  Neglect resistance term 

0 

L T C T

Transient current ΔI L 

V L

 

I L Z

0 gives a voltage

Decoupling

 Z 0 should be as low as possible (a few Ω)  Difficult with spaced round conductors  Typically Z 0 = 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



output output Voltage Voltage without with filter filter E

 20 log  

1  

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 C C RL High RL Low Rs Low L/2 C L/2 RL Low L Rs High C/2 C/2 RL High

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  Classic filter designs  T and Pi Sections

Reflective Filters - Capacitive

 Shunt Capacitor Low Pass  Source and Load Resistances Equal R Vs C R Vo

V o V s

 2  1



RC V o

 2 1 1 

V s where F

 

fRC

Reflective Filters - Example

 Derived Transfer Function

 20 log  1 

2   1 2 80  10 log  1 

2   C = 0.1

μF and R = 50 Ω 40 60 20 0 0.1

1 Frequency (M Hz) Derived Characteristic 10 100

Reflective Filters - Example

 Effect of strays in Capacitor 60 80  Short Leads C  1  10   7   9 R  50 Rc 0.01

 Long Leads C L   1  1  10  10  7 8 R  50 Rc 0.01

40 20 0 0.1

Long Leads Short Leads 1 Frequency (M Hz) 10 100

Reflective Filters - Inductive

 Series Inductor R L Vs

V o

 2 1 1 

V s

R Vo

V o V s

 1  1



L R where F

 

f L R

Reflective Filters - Inductive

 Derived Characteristic  same as for Capacitive 80 Strays Effect 60 L  2.5

 10  4 40 C  5  10  11 R  50 20 0 0.1

With Strays 1 Frequency (M Hz) 10 100

Reflective Filters

 Cut-off frequency 3   Insertion loss rises to 3dB 10 log  1 

2  1  

fRC

 Implies F = 1 or  This gives us fc = 63.7kHz

 Based on values given earlier C  1  10  7 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 Vs

V o V s



 1



C j



 1



  2

 1



 1 C Vo

Lossy Filters

 Natural Resonant Frequency  0  1

 Damping Factor  

L C

 Transfer Function becomes

V o V s

 1      0   2  2

     0    1

Lossy Filters

20  Transfer Characteristic  Critically damped for minimum amplification  Best EMI Performance       0.1

0.5

10     0 20 40 60 0.01

0.1

1 Normalised Frequency Overdamped Critically Damped Underdamped 10

Ferrite Beads

 Very simple component  Equivalent Circuit R L Conductor Ferrite Bead  Impedance



2   2

Ferrite Beads

150  Frequency Response  Cascade of beads 100 forms lossy noise filter 50 0 1 High L High R 10 Frequency (M Hz) 100 1  10 3

Ferrite Beads

 Noise suppression effective above 1MHz   Best over 5MHz Single bead impedance around 100 Ω  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  Common Mode  Differential Mode  Y Caps filter Common Mode  Max allowable value shown here N  X Cap filters Differential Mode L V c V c V d 0.1 - 1  F X Y Y 0.005

 0.005

 E

Mains Filters Frequency Response

40 30 20 10 0 0.1

1 Differential M ode Common M ode Frequency (M Hz) 10 100

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 - 1 μF   Differential Mode L provides high Z L for Common Mode L C1  None for DM L   Neutralising Transformer L = 5 – 10mH N E C2 C3 Equipment C4

Typical Mains Filter

 C3 and C4 are for CM currents to Ground and the equipment earth  Response 60 40 20 100 0.1

1 10 Mhz

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