Electromagnetic compatibility is becoming increasing important as more and more systems are becoming electronic and digitalised. It guarantees that electronic devices and systems do not interfere with each other and operate as expected in a given environment.

What is EMI?

Electromagnetic interference (EMI) in the context of power converters is generally caused by electrical flow and switching which produces a magnetic field which can affect nearby or connected equipment. Generally higher power electrical equipment will produce a larger electromagnetic (EM) field which is more likely to interfere with other systems.

Radiated EMI

All electrical conductors such as wires act as an antenna for EM waves, these waves are generated by the power supply or an interaction between the power supply and load. Radiated emissions problems are typically solved by shielding and filtering components to attenuate the energy and reduce their effects.

Conducted EMI

This form of EMI occurs when a device sends energy along the power conductors (such as wires or bus bars), a common source of this is a large motor or power supply. Generally power converters have internal filter components to suppress conducted emissions and improve immunity.

EMC testing

EMC testing is a series of tests carried out to determine if a product or complete system meets the criteria for EMC standards. Different industries have different requirements for EMC, some of the most stringent come from the defence industry.
The testing procedures cover radiated immunity, radiated emissions, conducted immunity, and conducted emissions. Each category has its own test and standards to meet depending on the industry or application.

How to minimise EMI issues

When designing an SMPS there are several methods to improve EMC, through control/switching schemes to PCB design. However, when designing for an application the power supply is generally selected as a ready product and not designed specifically for the system. Below are a few after market solutions.
Filtering is one of the most employed ways of minimising EMI, it prevents unwanted signals from travelling between the supply and power converter, or the converter and load. The trade off here is that the system size is increased, and the filters are relatively expensive. Additionally, the system efficiency is slightly reduced as there are more components in the conduction path. Passive filters such as inductors and capacitors are used to attenuate the energy caused by EM waves. Active filters use amplifiers and smaller passive components than their passive filter counterparts, they work by cancelling out the noise by injecting an opposing current.
Shielding is most often used to protect equipment from radiated or coupled EMI, it is often done by wrapping a wire in an isolated metal or enclosing a system in a metallic box. This protects the conductor or system and prevents them from radiating their emissions to other devices.

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