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Electromagnetic Interference (EMI)Williams, Tim. EMC for product designers. Newnes, 2016.

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The Heusweiler Motorway Faraday Cage

Electromagnetic Interference (EMI)

Electronic equipment must be switched off during take-off and landing…Clothing, shoes, and optical stores…

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EMC Directive

• The ability of the system to operate without interfering with other systems

• The ability of the system to operate despite interference from other systems

• Under typical conditions (domestic, commercial, industrial)

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Sources of Interference

• Supply voltage interruptions: dips, surges, and fluctuations• Transient over-voltages on supply, signal, and control lines.• Radio-frequency fields, both pulsed (radar) and continuous,

coupled directly onto equipment or onto its connected cables. • Electrostatic discharge (ESD) from a charged object or person.• Low frequency magnetic or electric fields.

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Radio Spectrum

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Disturbances on the Mains Supply

• An perfect power supply is not cost-effective!• Voltage variations • UK: ±10%• US: National Electric Code (NEC) recommends ±3% in households• Voltage fluctuations• Voltage interruptions• Waveform distortion• Reactive impedances and harmonic currents• Transients and surges

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Disturbances on the Mains Supply

• Mains Signaling• Superimposed signals (3kHz-150kHz)• No extra wiring/aerial emission required• Installation can be as simple as plugging in the system components• No frequency variation from country to country or licensing issues

• ….Same frequency band as motors, power supplies, fluorescent

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Electromagnetic Interference

Source equipment

victim equipment

peripheral

input

Case to mains cable

Common earth impedance

Common mains impedance

External mains interference

Case to caseCable to cable

ESD to interface

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Common Impedance Coupling

System A System BInput

Inductance, L, of the wireVN

System B: input = VIN+VN

PROBLEM: VINLoad

ILOAD

System AInput

ILOAD

SOLUTION: VINLoad

System B

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Mains Coupling

Source Victim

50Ω 50μH

Mains equivalent circuit model

System A System B

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Magnetic Inductance

VN = -M*dIL/dtM = mutual inductance [H]M depends on… • Loop area• Loop orientation• Distance between loops• Screening material

RS

VN

INPUTLOAD

IL

MUTUAL INDUCTANCE, M

VIN

VIN VN

RS

ZIN

EQUIVALENT CIRCUIT (magnetic coupling)

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Electric Inductance

VN = CC*dVL/dt * Zin//RSCC is the coupling capacitanceZIN//RS is victim impedance to groundCC depends on…• Distance between conductors• Their effective areas• Screening material

System A System BRS

V

LOADIL

VIN

VL RS

CC

May be stray capacitances to ground…

VIN

RS

ZIN

EQUIVALENT CIRCUIT (electric coupling)

RS

IN

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Mutual Capacitance and Inductance

MUTUAL CAPACITANCE

MUTUAL INDUCTANCE

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• Electric field coupling increases with increasing ZIN

• Electric coupling is more of a problem for high impedance circuits

• Magnetic field coupling decreases with increasing ZIN

• Magnetic coupling is more of a problem for low-impedance

circuits

Mutual Capacitance and Inductance

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Radiated Couplingd [m]

E-field

V

V

E

NEAR FIELD

RADIATING SOURCE

HComplex field geometry, Ratio E/H varies with position

I

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Radiated Couplingd [m]

E-field

V I

r [m]

H-field

V

E

NEAR FIELD

RADIATING SOURCE

HComplex field geometry, Ratio E/H varies with position

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FAR FIELD

Plane of E-field

Plane of H-field

Ratio of E/H is constant

direction of propagation

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Distance from source, normalized to λ/2π

Wav

e im

peda

nce,

Ω

Magnetic field predominates

Electric field predominatesNear-field impedance may be anywhere in this region

Radiated Coupling Near field; wave impedance determined by Maxwell’s equations

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Distance from source, normalized to λ/2π

Wav

e im

peda

nce,

Ω

Magnetic field predominates

Electric field predominates

transition region

Radiated Coupling

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Radiated Coupling Far field; plane wave. E and H decay with distance at the same rate, therefore the impedance is constant.

Distance from source, normalized to λ/2π

Wav

e im

peda

nce,

Ω

Magnetic field predominates

Electric field predominates

Plane wave, Z0 = 377 Ωtransition

region

Far-field

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Radiated Coupling modes

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Radiated Coupling modes

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Radiated Coupling modes

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Radiated Coupling modes

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Emissions

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Emissions

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Electrostatic Discharge

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Electrostatic Discharge

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Electrostatic Discharge

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PCB Layout and Grounding

“Ground is a low-impedance path by which current can return to its source”

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PCB Layout and Grounding

“Ground is a low-impedance path by which current can return to its source”

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PCB Layout and Grounding

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PCB Layout and Grounding

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Good Practices for EMI Immunity

• Control the flow of interference into and out of the equipment

• Keep interference paths away from critical logic circuitry

• Add I/O filters / isolation

• Use high-noise threshold logic (e.g. 74HC)

• Avoid edge triggered inputs if possible

• Use a watchdog

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System Partitioning

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Twisted Wires!

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Shielded wires

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