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 h

Topics Components CCapacitors IInductors RResistors Decoupling Filters

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

Capacitors Have Equivalent Series Resistance (ESR) and ESL. Electrolytics rrequire correct DC polarity BBest capacitance to volume ratio HHigh ESR (>0.1Ω) EESR increases with frequency HHigh ESL

Electrolytics cont. LLimited reliability and life LLow frequency devices RRipple current limitations PParallel inductor improves high frequency (up to 25kHz) response

Paper and Mylar LLower ESR HHigher ESL UUses Filtering Bypassing Coupling and noise suppression

Mica and Ceramics LLow ESL and ESR KKeep leads short UUses High frequency filtering Bypassing decoupling

Polystyrene and Polypropylene LLow ESR VVery stable C – f characteristic MMylar is a metalised plastic Polyethelyne terephthlalate DuPont trade name

Equivalent Circuit R C L

Capacitors Effect of equivalent Circuit

Inductors Equivalent Circuit Now a parallel resonance R will be low WWinding resistance C will be low IInter – winding capacitance

Effect of equivalent circuit

Strays give a resonance that is quite sharp. RR and C are low Above resonance inductor looks capacitive Air cored coils are large PProduce unconfined fields SSusceptible to external fields SSolenoid has infinite area return path

Ferromagnetic coils aalso sensitive to external fields oown field largely confined to core SSmaller than air cored devices Permeabiity increase by factors > SSaturate if a DC is present AAir gap reduces this effect Inductance lowered

Ferromagnetic coils CCore material depends on frequency LF – Iron Nickel Alloys HF – Ferrites CCan be noisy caused by magnetostriction in laminations of core RF chokes tend to radiate SShielding becomes necessary

Resistors Equivalent Circuit Parallel RC Resonance C will generally be low L comes from leads and construction wwirewound

Effect of Equivalent Circuit

As frequency increases resistor begins to look inductive Wirewound HHighest inductance HHigher power ratings UUse for low frequencies

Film Type CCarbon or Metal Oxide films LLower inductance Still appreciable because of meander line construction LLower power ratings

Composition UUsually Carbon LLowest Inductance Mainly Leads LLow power capability CC around 0.1 to 0.5pF SSignificant for High values of R Normally neglect L and C except for wirewound

Decoupling Power rails are susceptible to noise PParticularly to low power and digital devices CCaused by common impedance, inductive or capacitive coupling Decouple load to ground UUse HF capacitor CClose to load terminals

Circuit Diagram

Components of Transmission System form a Transmission Line System This has a characteristic impedance NNeglect resistance term Transient current ΔI L gives a voltage

Z 0 should be as low as possible (a few Ω) Difficult with spaced round conductors TTypically Z 0 = Ω SSeparation/diameter ratio > 3 Two flat conductors 66.4mm wide mm apart give 3.4 Ω

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

Impedance Levels IInsertion loss depends on source and load impedance DDesign performance achieved if system is matched LL and C are reflective components RR is Lossy, or absorptive

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

Any power not transmitted is reflected. Series Elements LLow impedance over passband HHigh impedance over stopband Shunt Elements HHigh impedance over passband LLow impedance over stopband Generally use Lowpass filters for EMC

Filter Arrangements SShunt C SSeries L LL-C combinations Classic filter designs TT and Pi Sections

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

Reflective Filters - Example Derived Transfer Function  C = 0.1 μF and R = 50Ω

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

Reflective Filters - Inductive Series Inductor

Derived Characteristic same as for Capacitive Strays Effect

Reflective Filters Cut-off frequency IInsertion loss rises to 3dB IImplies F = 1 or This gives us fc = 63.7kHz BBased on values given earlier

Lossy Filters Mismatches between filters and line impedances can cause EMI problems Noise voltage appears across the inductor RRadiates Interference is not dissipated but “moved around” between L and C. Add a resistor to cause “decay”

Neglect source and load resistors Transfer Response

Natural Resonant Frequency Damping Factor Transfer Function becomes

Transfer Characteristic Critically damped for minimum amplification Best EMI Performance

Ferrite Beads Very simple component Equivalent Circuit Impedance Conductor Ferrite Bead

Ferrite Beads Frequency Response Cascade of beads forms lossy noise filter

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 CCommon Mode DDifferential Mode Y Caps filter Common Mode MMax allowable value shown here X Cap filters Differential Mode VcVc VcVc VdVd

Mains Filters Frequency Response

Feedthrough Capacitors Takes leads through a case Shunts noise to ground

Comparison with Standard Capacitor

Typical Mains Filter C1 and C2 0 μF DDifferential Mode L provides high Z for Common Mode None for DM Neutralising Transformer L = 5 – 10mH

C3 and C4 are for CM currents to Ground and the equipment earth Response

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