24PCBDesignforRealworldEMIControl

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1.1.2 EMC concepts (Continue)
Electromagnetic Compatibility EMC
Interference (EMI)
Susceptibility (EMS)
Conducted Emission
Radiated Emission
Conducted Immunity
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2.1 The Importance of EMC design
1. EMC Design is important for cost-down. 2. Need certificates to enter into different
countries and regions, common marks: CCC-> China CE -> European Union VCCI-> Japan BSMI-> Taiwan FCC-> United States
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1.5 Introduce the Shielding (Continue)
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2. Basic EMC Design Intention
2.1 The importance for EMC design 2.2 Coupling Mechanisms 2.3 Introduce the Signal Spectra 2.4 Introduce the Resonance Effect 2.5 Potential Interference Source
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2.4 Resonance Effects
Most EMI is narrow band, not broad band. The resonance frequency in cavity is:
1
fmnp 2
(m)2 (n)2 ( p)2 abc
The a, b and c is the dimension of the cavity. The m, n and p is integer, and be both or all zero.
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3.4 Introduce the Self- inductance
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3.2 Electromagnetic Induction
The electromagnetic inductance formula in
Faraday’s law:
uv
Ñ E dl
uv B t
d
ur S
In a small area A, the :
V A B t
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1. Introduce the EMC
1.1 EMI and EMC concepts 1.2 The source of EMI 1.3 Introduce the Inductance 1.4 Introduce the Ground 1.5 Introduce the Shielding
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1.3 Introduce the Inductance
The inductance here is not only for the inductor.
Inductance is critical for high speed signals. A voltage decrease will cause the change of Electromagnetic field. This change of intensity and shape is a source of Emission.
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2.5 Potential Interference Source
Shielded products: Vias and holes, connectors, I/O cables…
Unshielded products PC board, radiators, external cables, wireless antennas…
For a slot with a dimension equal to or less than a half-wave length, the shielding effectiveness is:
S 20log( )
2l
Where l is maximum dimension of slot.
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2.2 Coupling Mechanisms
Electrical Field Coupling
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2.2 Couplingd Coupling
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2.3 Signal Spectra
The ability of systems, equipment, and devices that utilize the electromagnetic spectrum to operate in their intended operational environments without suffering unacceptable degradation or causing unintentional degradation because of electromagnetic radiation or response.
The current when a capacitor discharging:
I C dv dt |
The voltage when a changing current
passing through a inductance:
V L dI
Effect of Power/Ground Inductance dt
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EMC Structure Chart
Radiated Immunity
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1.2 The Source of EMI
➢ High speed clock generator; ➢ High speed bus signal; ➢ Power node; ➢ Digital and Analog GND; ➢ I/O Cable and Connector
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3.2 Electromagnetic Induction (Continue)
The inductive reactance of a conductor is usually more significant than the impedance resistance.
A time-varying current passing through an inductor causes a voltage drop.
PCB Design for Real-World EMI Control
Prepared By Jia Wu 2003-11-20
PCB Design for Real-World EMI Control
The contents
1. Introduce the EMC 2. About the Basic EMC Design Intention 3. Introduce the Electromagnetic Induction 4. The Myth of the Ground 5. Introduce the Return Current Route 6. The Conclusions
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1.4 Introduce the Ground
Ground: The reference point.
Different kinds of Ground should be separated from each others.
Such as Earth reference, Power reference, Chassis reference. This can mitigate the interference between different signals and keep them pure.
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3. The Inductance
3.1 Introduce the Inductance 3.2 Electromagnetic Induction 3.3 Introduce the Mutual inductance 3.4 Introduce the Self-Induction 3.5 Introduce the Partial Induction
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3.1 Introduce the Inductance
The influence of inductance is extremely important in high frequency spectra.
The inductance can be anywhere in all components such as capacitors resistors, lines, and planes.
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3.3 Introduce the Mutual Inductance
M 0 l12l22
2 (l22 r 2 )3
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3.3 Introduce the Mutual Inductance
(Continue)
The Mutual Inductance between two parallel lines
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1.5 Introduce the Shielding
Shielding is a useful way to limit EMI.
The magnitude of reflection loss depends on the ratio of barrier impedance to wave impedance, which in turn depends on its distance from the source and whether the field is electric or magnetic or whether it is a plane wave.
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1.1.1 EMI concepts
EMI: Electromagnetic Interference Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics/electrical equipment. It can be induced intentionally and unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
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1.1.2 EMC concepts
EMC: Electromagnetic Compatibility The condition which prevails when equipment is
performing its individually designed function in a common electromagnetic environment without causing or suffering unacceptable degradation due to unintentional electromagnetic interference to or from other equipment in the same environment.