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Design For Immunity to Design For Immunity to Electromagnetic Disturbances Electromagnetic Disturbances
"the meter shall be designed in such a way that conducted or radiated electromagnetic disturbances as well as electrostatic discharge do not damage nor substantially influence the meter”
-Section 4.5 IEC1036
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What is EMC?Source-Path-Receptor Model
EMC Stands for Electro-Magnetic Compatibility.
The interference generated by the movement of electrons will travel through a path and result in the energy meter.
Movement of Electrons
RECEPTOR
Energy Meter
SOURCE PATH
Common coupling effects:
• Frequency less than 30MHz (e.g. EFT) – Conduction
• Frequency between 30MHz to 300 MHz (e.g. RFI) – Cable Radiation
• Frequency larger than 300MHz (e.g. ESD, RFI) - Radiation
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EMC Requirement on IEC1036
IEC1036 specifies that the energy meter must be able to withstand:
• Electrostatic Discharge (ESD) up to 8kV
• Radio Frequency Interference (RFI) up to 10V/m near field
• Electrical Fast Transient and Surge (EFT) up to 4kV
The AD7755, core of the energy meter, offers some protection against the above interferences. However, the energy meter circuit and PCB layout should be designed to divert the interference away from the AD7755 to ensure the expected lifespan of the energy meter is not affected.
The AD7755 has passed 60 years life test
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ADE775x Family Architecture
High Resolution Data High Resolution Data ConversionConversion Fixed Function DSPFixed Function DSP
101011...
111001...
D.C. to 5KHz
D.C. to 5KHz
16 bitADC
16 bitADC
16 bitADC
16 bitADC
Multiply
&
LPF
Digital-To-Frequency
Serial Port
Interface
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EMC Certified Electronic Energy Meterbased on AD7755
The fixed function DSP in the The fixed function DSP in the AD7755 is highly effective AD7755 is highly effective against radiated EMC against radiated EMC disturbances, however, disturbances, however, external protection is external protection is essential to minimize the essential to minimize the noise coupling through noise coupling through conduction.conduction.
a
AD7755
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PCB Layout PCB Layout
Layout considerations - Analog Performance Layout considerations - Analog Performance Separate grounds were used to isolate
the AD7755 and low level analog signals from the rest of the design
Ground plane should be made as large as possible to keep the common ground impedance to a minimum
Care was taken with the routing of the ground return current from the capacitor based PSU.
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PCB Layout PCB Layout
Ground Plane helps reduce the impedance of the current path in an ESD/EFT event and reduce interference picks up through radiation.
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PCB Layout PCB Layout
Layout considerations - EMC Layout considerations - EMC
Separate grounds were used to isolate the AD7755 from electromagnetic disturbances.
Power and ground connections are made through ferrite bead.
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PCB Layout: Some key areas
Large digital/power supply ground plane (noisy)
Ground shielding helps separate strong voltage signal from affecting weak current signal
Weak differential signal lines are shielded by ground fill and are parallel and closed to each other
Large analog ground plane (quiet)
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Inductance of the ShuntInductance of the Shunt
Shunt inductance can create a high impedance path at high frequency: ESD/EFT and RFI picked up through the cable will travel to the AD7755’s signal inputs V1P and V1N. Protection is needed!
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Design For Immunity to Design For Immunity to Electromagnetic Disturbances Electromagnetic Disturbances
ESD (IEC1000-4-2) ESD (IEC1000-4-2)
Ferrite
RC Filter
R > 1k
Attenuation network
R > 1M
EMI Filter on
PSU
Passed 25kV Contact discharge Specification requires for 8kV
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Design For Immunity to Design For Immunity to Electromagnetic Disturbances Electromagnetic Disturbances
EFT (IEC1000-4-4) EFT (IEC1000-4-4)
Ferrite & RC slows down edge. EFT appears as CM signal
Ferrite & C slows down edge. MOV suppresses large differential EFT
PCB layout isolatesAD7755 from EFT in the signal ground
Passed 5kV @ 15kHzSpecification requires for 4kV @ 5kHz
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Design For Immunity to Design For Immunity to Electromagnetic Disturbances Electromagnetic Disturbances
HF Field (IEC1000-4-3) HF Field (IEC1000-4-3)
EMI Filter on PSUshunting cap acts
as bypass for RF
Ferrite & RC make up effective HF
filter PCB layout/grounding
Passed 20V/m near fieldSpecification calls for 10V/m
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Conclusion Conclusion
• Good EMC design is essential to help ensure the lifespan and accuracy of a solid-state energy meter.
• Large ground plane, short analog signal lines help greatly reduce EMC problems.
• Components’ parasitic characteristics will usually dominant in a EMC situation.
• EMC Protection does not have to come with additional cost to a meter if the right component is used. For example, RC used in anti-aliasing filter can be effective EMC filter.
• Mechanical meter is also susceptible to EMC issues.