How to Understand EMI Filters

What is EMI (Electromagnetic Interference)

Electromagnetic interference is broadly defined as an electrical or magnetic emission that negatively affects the functionality of electronic equipment. Everything that uses electricity can generate emissions that can turn into EMI. 

Causes and Effects of EMI

EMI is a class of electrical and electronic phenomena caused by the presence and flow of non-inductive currents in conductors, usually due to electromagnetic induction. Everything that uses electricity can generate emissions that can turn into EMI. 

1. Narrowband Emissions

Narrowband emissions are easy to filter out because they only occupy a small area of the spectrum. These emissions are caused by digital devices. These types of power supplies are called narrowband because their frequency range is very narrow and does not interfere with surrounding electronics.

2. Broadband Emissions

Broadband emissions (or wideband emissions) occupy a large section of the frequency spectrum and represent the most difficult type to filter out. Common sources of broadband EMI include power lines, microwaves, AM radio signals, FM radio signals, light bulbs, fluorescent lighting fixtures, and electrical wiring that is close to an appliance during usage. Devices with wideband power supplies are often used in high-end audio systems since the higher quality sound requires higher electric currents that can generate a lot of EMFs.

Effects of EMI

In industrial practice, EMI can affect the functionality of machines by causing failure, disruption, or damage. The interactions between an electronic device and EMI from a power line can lead to a failure in the device, failure of the power line, or a failure that damages both. In more critical situations such as medical devices or aircraft equipment failure or failure that damages both are not acceptable failure modes.

To reduce the number of emissions, many manufacturers have designed new power supplies that minimize the amount of EMI that is generated. Nearly all modern devices have a step-down voltage converter with a ferrite core to lower the EMFs.

Different Types of EMI Filters: Passive, Active & Balanced

EMI filtering is the process of isolating electronic circuits from unwanted high-frequency signals. Active EMI filtering typically involves the use of a low-pass, series RC, or shunt RC filter to stop high-frequency interference currents on power lines. Passive EMI filtering typically uses wires and cables with ground conductors for protection against electric fields. The actual choice of method for a given application is not always obvious.

Conventional EMI Filtering Schematic Diagram

Three main types of EMI filtering are used in electronic circuits. 

1. Passive EMI Filters

By utilizing inductors and capacitors to create an impedance mismatch in the EMI current path, passive filtering reduces a power electronic circuit’s conducted emissions. This type of filtering is used for filtering noise generated in high-frequency circuits such as pulse generators and oscillators. The noise currents are induced in the printed circuit board (PCB) ground plane and then using a long wire structure which is kept at a right angle to the direction of noise flow, or by using a cable which is kept at a right angle to the direction of noise flow. 

2. Active EMI Filters

This type of filtering is used for low-frequency circuits such as digital circuits, or analog circuits which use power supplies with large voltage changes (switching power supplies, DC/DC converters). The low-frequency currents are induced in the printed circuit board (PCB) ground plane. Then using a low-pass RC filter, the unwanted currents are stopped in the ground plane of the printed circuit board (PCB). 

3. Balanced EMI Filters

This type of filtering is mainly used for digital circuits to reduce common-mode noise which is induced on both ground and power lines. For a pure common mode noise, a balanced filter is used which inverts and sums the signal in a way that cancels out the unwanted interference. This type of filtering is also called “symmetrical EMI filtering”.

Active EMI Filters VS Passive EMI Filters

A passive EMI filter blocks differential-mode noise currents from flowing through power supply wiring to sensitive circuits. It does not do anything to block common-mode noise that appears on both signal ground and power ground at the same time. 

Active EMI filters are more complicated because they must have different properties for differential-mode currents, which flow only on the power supply wires, and common mode currents, which flow on both the power supply wires and also on all of the circuits signal wires carrying both power and signal. 

Thus, active EMI filters are designed for each wire carrying both power and signals, with no common connections to other wires.

How to Implement EMI Filters

A passive or active filter can be implemented by placing a low-pass RC circuit at the connection of the sensitive device’s ground pin to the PCB’s ground plane as shown below, or by using a wire to connect the ground pin to the ground plane, which is typically done when filtering individual wires.

To reduce EMI, power supply wiring should be kept as far away from signal wiring as possible. For example, supplies for analog-to-digital converters (ADCs) should not share PCB layers with other digital signals that could induce noise into the supply. 

Avoiding twisted power supply wiring helps reduce the common-mode voltage, which is a form of common-mode noise. Sometimes a critical signal wire must cross a noisy supply wire to reach another part of the board. In this case, a single ground plane cut can be used to separate the two wires as shown below.

For example, for a single-ended power supply, if the differential voltage exceeds 4 V, this indicates that noise coupling is significant. A good practice is to try to avoid using differential circuits in noisy environments. Oftentimes, this simply means routing the supply wires so they do not cross. 

Power-supply bypassing can also help to reduce noise by reducing the impedance between noisy power supply pins and ground, which reduces the voltage drop across the impedance. However, this is not always practical because it can increase parts count and board space.

Ending Words

We hope you enjoyed this blog post by Easybom on EMI (Electromagnetic Interference) and its causes, effects, different types of filters, and how to implement them. If you have any suggestions or questions about this post, please let us know!

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