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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.

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EMC Components and Filters

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

  2. 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

  3. Topics • Components • Capacitors • Inductors • Resistors • Decoupling • Filters

  4. Capacitors – Approx Frequency Ranges. 20 – 25nH About 1.4nH

  5. 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

  6. Capacitors • Electrolytics cont. • Limited reliability and life • Low frequency devices • Ripple current limitations • Parallel inductor improves high frequency (up to 25kHz) response

  7. Capacitors • Paper and Mylar • Lower ESR • Higher ESL • Uses • Filtering • Bypassing • Coupling and noise suppression

  8. Capacitors • Mica and Ceramics • Low ESL and ESR • Keep leads short • Uses • High frequency filtering • Bypassing • decoupling

  9. Capacitors • Polystyrene and Polypropylene • Low ESR • Very stable C – f characteristic • Mylar is a metalised plastic • Polyethelyne terephthlalate • DuPont trade name

  10. Capacitors • Equivalent Circuit R C L

  11. Capacitors • Effect of equivalent Circuit

  12. Inductors • Equivalent Circuit • Now a parallel resonance • R will be low • Winding resistance • C will be low • Inter – winding capacitance

  13. Inductors • Effect of equivalent circuit

  14. 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

  15. 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

  16. 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

  17. Resistors • Equivalent Circuit • Parallel RC Resonance • C will generally be low • L comes from leads and construction • wirewound

  18. Resistors • Effect of Equivalent Circuit

  19. Resistors • As frequency increases resistor begins to look inductive • Wirewound • Highest inductance • Higher power ratings • Use for low frequencies

  20. Resistors • Film Type • Carbon or Metal Oxide films • Lower inductance • Still appreciable because of meander line construction • Lower power ratings

  21. 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

  22. 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

  23. Decoupling • Circuit Diagram

  24. Decoupling • Components of Transmission System form a Transmission Line System • This has a characteristic impedance • Neglect resistance term • Transient current ΔIL gives a voltage

  25. 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

  26. Filtering • Not covering design in this module • Effectiveness quantified by Insertion Loss

  27. 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

  28. Reflective Filters • Generally, filters consist of alternating series and shunt elements

  29. 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

  30. Reflective Filters • Filter Arrangements • Shunt C • Series L • L-C combinations • Classic filter designs • T and Pi Sections

  31. Reflective Filters - Capacitive • Shunt Capacitor Low Pass • Source and Load Resistances Equal

  32. Reflective Filters - Example • Derived Transfer Function • C = 0.1μF and R = 50Ω

  33. Reflective Filters - Example • Effect of strays in Capacitor • Short Leads • Long Leads

  34. Reflective Filters - Inductive • Series Inductor

  35. Reflective Filters - Inductive • Derived Characteristic same as for Capacitive • Strays Effect

  36. 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

  37. 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”

  38. Lossy Filters • Neglect source and load resistors • Transfer Response

  39. Lossy Filters • Natural Resonant Frequency • Damping Factor • Transfer Function becomes

  40. Lossy Filters • Transfer Characteristic • Critically damped for minimum amplification • Best EMI Performance

  41. Ferrite Beads • Very simple component • Equivalent Circuit • Impedance Ferrite Bead Conductor

  42. Ferrite Beads • Frequency Response • Cascade of beads forms lossy noise filter

  43. 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

  44. Mains Filters – Simple Delta Capacitive • Two noise types • Common Mode • Differential Mode • Y Caps filter Common Mode • Max allowable value shown here • X Cap filters Differential Mode Vc Vd Vc

  45. Mains Filters Frequency Response

  46. Feedthrough Capacitors • Takes leads through a case • Shunts noise to ground

  47. Comparison with Standard Capacitor

  48. Typical Mains Filter • C1 and C2 • 0.1 - 1μF • Differential Mode • L provides high Z for Common Mode • None for DM • Neutralising Transformer • L = 5 – 10mH

  49. Typical Mains Filter • C3 and C4 are for CM currents to Ground and the equipment earth • Response

  50. 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

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