EMI and Safety: Hazards, Risks, and Designing to Avoid Them

July 15, 2019 Cadence PCB Solutions

Electromagnetic spectrum toward a checkerboard

 

The word “safety” can have dozens—and maybe even hundreds--of informal and formal meanings. A two-year-old child seeks the safety of his or her parents’ arms. Someone caught in a severe storm seeks the safety offered by a shelter. For industrial and manufacturing plants, the unending quest for safety has become a priority. 

In each of those examples, the need for “reliability” and the opportunity for “risk” seem synonymous with “safety.” Reliability, risk, and safety issues also go hand-in-hand when we discuss the impact of electromagnetic interference (EMI) on electronic systems.  Those issues become further amplified when we work with critical systems used for transportation, healthcare, energy production, and other key areas.

EMI, EMS, and EMC

When we work with PCB design and electronic system, we seem to constantly attempt to find methods for eliminating electromagnetic interference (EMI). EMI consists of disruptive electromagnetic energy that transmits from one device to another or from one piece of equipment to another.  As we work with electronic system, we apply the principles of electromagnetic compatibility and look for areas that seem vulnerable to EMI.

Cellphones, welders, motors, and other equipment generate EMI. At the device level, EMI sources include microcontrollers, microprocessors, transmitters, electromechanical relays, and switching power supplies. Using microcontrollers as an example, clock circuitry within the controller generates wide-band noise that contains harmonic disturbances that range up to 300 Megahertz. EMI couples into a circuit through conductors, radiated electric fields, and magnetic fields.

In contrast, Electromagnetic Susceptibility (EMS) represents the amount of performance immunity against electronic discharge (ESD), electrical interference, surges caused by lightning, electromagnetic waves, and electrical fast transients (EFT).

ANSI defines Electromagnetic Compatibility (EMC) as:

“the ability of electrical and electronic systems, equipment, and devices to operate in an intended electromagnetic environment within a defined safety margin, without suffering or causing unacceptable degradation as a result of electromagnetic interference.” (ANSI C64.14-1992).

EMI and EMC Standards

Electromagnetic interference can prevent a system from performing critical functions. An EMI-caused problem in a medical device can interrupt the exchange of biomedical information or give erroneous reports to staff about a patient’s condition. The susceptibility of medical devices to EMI ranges from the impact of RFID on medical devices to the electromagnetic compatibility of hearing aids, powered wheelchairs, and motorized scooters.

The extent of the EMI problem and the impact on consumer, industrial, and military applications becomes evident through the wide range of EMI and EMC standards. Agencies including the Federal Communications Commission (FCC), the International Standards Organization (ISO), the International Electrotechnical Commission (IEC), the American National Standards Institute (ANSI), the Center for Devices and Radiological Health (CDRH), and many other agencies have established standards related to EMI and EMC requirements.

Those standards cover design requirements, emissions testing and immunity testing. As an example, IEC 61508 shows that design requirements must contain information about required EMI levels. The standard goes further by illustrating techniques and measures to control systematic failures. In another example, IEC 60601-1-2 covers the general requirements for safety in medical equipment and electromagnetic compatibility. 

Emissions testing measures devices for the amount and type of generated noise. Immunity measurement standards—such as those listed in IEC 1000-4-4 and IEC 1000-4-3 subject devices to different noise frequencies and measures the ability of the device to tolerate noise emitted by fast transients and radiated electromagnetic fields. The following table describes several emission and immunity tests.

 

Emission or Immunity Testing Type

Description of Emission or Immunity Test

Conducted Emission

Measures frequency range between 150 KHz to 30 MHz to find energy transmitted through a wire or interconnect cable as a propagating wave

Radiated Emission

Measures frequencies from 30 MHz to 1 GHz transmitted through a medium as an electronic field

Conducted Immunity/Susceptibility

Measures the ability of a product to withstand electromagnetic energy in a frequency range from 150 KHz to 100 MHz that penetrates through external cables, power cords, input/output connects, or the chassis

Radiated Immunity/Susceptibility

Measures the ability of a product to withstand electromagnetic energy in the frequency range from 80 MHz to that penetrates through the air

Electrical Fast Transient Burst

Simulates disturbances created at the contacts of AC mains switches or relay contacts due to inductive energy

Power Frequency Magnetic Field Immunity

Simulates effect of the magnetic fields on a product located near power transformers

 

Identify EMI Hazards and Risks

Since the early 1990s, the increasing complexity of components and systems coupled with attempts to save costs have caused an increase of 3 dB in noise margins for electronic devices. Analog circuits have a safety margin that corresponds to the signal-to-noise ratio of devices. While digital circuits have a larger safety margin, the margin shrinks because of low voltage logic and the impact of a failure on digital applications. If EMI interrupts precise switching in a digital circuit, a system can stall or malfunction. With devices operating at higher bandwidths, both noise emissions and circuit susceptibility increase.

The combination of standards and design best practices have the purpose of reducing risks as complexity increases. Because EMI can harm critical applications, risk assessments also include hazard assessments and assessments of hazard probabilities. We define hazards as anything that can produce harm and then consider the level and severity of the harm. When we consider risk, we recognize the not all hazards produce the same level of harm and then determine the probability of the harm occurring.

Hazard and risk assessment encompasses the environment, design, and application of a system. In terms of circuit design and component selection, electromagnetic interference impacts the probability of harm occurring. As you design a circuit, you must recognize how to eliminate or mitigate EMI to achieve lower risk levels. Recognizing potential safety hazards and requirements along with the risks of EMI feeds into the process of designing and producing the circuit and the product.

Electrical hazard next to wiring

Electrical hazards are important to account for in circuit designs

 

Use Design Best Practices to Avoid EMI

Your PCB design should have the objective of achieving excellent signal integrity. That objective also lends itself to building a circuit that rejects EMI and has good electromagnetic compatibility. Obtaining EMC requires study of the entire product ranging from PCBs and power supplies to cables and enclosures. Your design should ensure compatibility between digital and analog circuits, carefully design the layout, and recognize the need for good grounding and shielding practices. Designing for EMC involves reducing radiated emissions and increasing radiated immunity through very low impedance return paths with a continuous ground plan and adding protection circuits for input/output and power signals.

Obtaining signal integrity occurs through keeping noise levels well below signal levels. For digital circuits, the noise margin should remain in the millivolt range. To take this a step further, you must keep EMI emission levels in the microvolt and microamp range. To accomplish these EMC goals, high speed signals must have the proper terminations. You can use differential signals to reduce emissions and decoupling capacitors at power supply pins to decrease switching noise.

In addition, your circuit designs must control impedance. You can maintain impedance control through source terminations for slower signals and by having a continuous return path from plane to plane. Use a decoupling capacitor when your signal crosses a split plane. When designing your PCB layout, identify critical traces that can become susceptible to EMI. Those traces include lines that enter or leave the PCB, lines that carry high-speed clock and data information, analog input lines, and digital lines.

Using Cadence tools for your layout and analytical circuit needs are some of the best choices you can make, particularly when working around EMI and safety concerns. Cadence’s Allegro PCB Editor makes possible all the design rule checks and layout management you need to get your design safely to production. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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