How to design for emc
Introduction
The ultimate goal of EMC design is to ensure your product does not interfere or suffer from any noise (either radiated or conducted) with other electrical product or system in its vicinity.
Background
Electromagnetic Compatibility of an electronic equipment is often overlooked in early stage of the product design cycle until the product fails regulatory compliance tests such as FCC and CE in later stage. Mostly due to insufficient understand of EMC principles and underestimate the EMC compliance test time and cost.
Most commonly seen EMC failures seen in consumer electronics design are immunity and emission.
Failing a regulatory compliance certification at this stage often results in teams scrambling to find a fast band-aid solution that compromises product performance; Hence, we strongly recommend engineers to to start EMC design early. A good EMC design is one of the main differentiating factor that separates between a premium and consumer friendly branded products.
Definition
EMC the ability of the electronic equipment to operate under its intended usage and environment without unacceptable user experience due to electromagnetic interference.
Note: the source of interference in the above context can be from an external source or self-generated.
EMC Overview
EMC can be breakdown in two parts:
Immunity
A good immunity design allows the electronic equipment to operate with minimal user impact (e.g. poor wireless connectivity) under strong external electromagnetic interference.
Emission
A good emission design minimizes its own electromagnetic emission to the environment without affecting the operational states of nearby electrical equipment such as TV, radio, wireless speakers, etc.
EMC Impacts
Immunity
Degradation in functional performance
Poor wireless connectivity causing low throughput
Noisy audio output
Permanent damage in electronic circuits
Unexpected freeze and shutdown of the device
Emission
Creates interference with other household electronics such as radio, TV, cellphone, bluetooth speakers, etc.
Understand emission and immunity
Emission is categorized as
Radiated Emission (RE)
Unwanted noise radiated out from you product in the form of electromagnetic radiation.
Conducted Emission (CE)
Unwanted noise conducted out from your product to the power cord in the form of voltage waveform.
Immunity is categorized as
Electrostatic Discharge (ESD)
High spike of voltage wave form generated by external source such as a human finger conducted through either spark in the air or physical contact
Radiated Immunity (RI)
Electromagnetic interference generated by external source such as a hobbyist ham radio picked up by the radio antenna on the electrical equipment
Conducted Immunity (CI)
Noisy voltage waveform generated by external source such as an external power supply picked up by the power cable to the electrical equipment.
EMC Mitigation
Emission Mitigation
Common sources of emission is listed below
Immunity Mitigation
Common sources of external interference is listed below
EMC Compliance Test Overview
Most common regulatory compliance bodies are FCC [US] and CE [EU]. However, CE has more mandated test cases such as ESD, power surge, harmonic current, etc., but both regulatory bodies cover the same RE/CE and RI/CI test criteria. Often meeting CE compliance will suffice for FCC.
Radiated Emission Test
A test receiving antenna and equipment under test (EUT) are placed inside an approved EMC chamber during testing. EUT is then put in normal operation state during testing
The purpose of emission test is to determine whether or not the emission level is under required regulatory limits.
Conducted Emission Test
A line impedance stabilization network (LISN) is inserted between EUT and wall outlet to decouple noise current existing out of EUT power cord from main AC signal and provides a decoupled noise port for measurement.
The purpose of conducted test is determine whether or not the noise current level is under required regulatory limits.
Recommended EMC criteria
ESD
Contact - 8KV Criteria C
Air - 15KV Criteria C
Criteria
Criteria A: No deterioration
Criteria B: Recoverable with no user intervention
Criteria C: Recoverable with user intervention
Criteria D: Cannot recover
Immunity
Radiated Immunity (RI) - 6V per meter Criteria A
Conducted Immunity (CI) - 6Vrms Criteria A
Emissions
recommend 3dB to 6dB margin from the regulatory CE limit.
Note: Emission margin is needed due to antenna tolerances over a large quantity. 3dB can be used if component tolerance and volume is not that large. For higher volume product, 6dB is Strongly Recommended.
Example Standards:
IEC CISPR 32 : Emission product limit for multi-media equipement.
IEC 61000-6-3: Generic Emission standards for residential and commercial.
IEC 61000-6-1: Generic Immunity standards for residential and commercial.
FCC Title 47, Part 15, Subpart B ; USB FCC EMC Standards
Conclusion
EMC needs to be design early on to meet both functional and regulatory requirements.
Understand EMC principles and possible causes is important.
PCB Layout and stack up is critical to reduce emission and improves immunity.
EMC component selection, placement, and package size impacts is critical to EMC evaluation.
Adding shield enclosure and use proper cables reduces emission and improves immunity.
Pretest EMC Compliance as early as possible and iterate through out each development stage.
Further Reading
"Basic Knowledge of EMC Standards", https://product.tdk.com/en/products/emc/guidebook/eemc_basic_04.pdf