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EMI Filter Basics - Operational Principles and Faulty Installation

Author: Apogeeweb
Date: 6 Aug 2019
faulty emi filter


1.1 EMI Filter Definition

The EMI filter (electromagnetic interference filter), also called RFI Filters or radio-frequency interference filters, is a filter circuit composed of a capacitor, an inductor and a resistor. Its filter circuit composed of a capacitor, inductor and resistor. A passive two-way network: One end is the power supply and the other end is the load. The principle of the EMI filter is an impedance matching network: the greater the impedance adaptation between the input and output sides of the EMI filter, the power supply and load side, the more effective the attenuation of electromagnetic interference is. The filter can effectively filter out a specific frequency or external frequency in the power line, thereby obtaining a specific frequency power signal, or eliminating the power signal after a specific frequency point. In fact, an EMI filter is an electrical device/circuit that mitigates the high-frequency electromagnetic noise present on the power and signal lines. 

The EMI filter is a common electrical element in power supplies. In this video, we introduce it and study its circuit.


Ⅰ Introduction

1.1 EMI Filter Definition

1.2 EMI Sources

1.3 Common Mode Noise and Differential Mode Noise

1.4 Why Do We Need EMI Filters?

Ⅱ Adaptation Principle of EMI Filters

Ⅲ Operating Principles

Ⅳ EMI Filters Classification

Ⅴ Selection

Ⅵ Installation

1.2 EMI Sources

EMI is electronic noise that interferes with electrical signals and reduces signal integrity. Every electrical or electronic device connection may become a potential source of EMI. It is generated externally by cosmic energy, such as solar flares, lightning strikes, atmospheric noise, electronic equipment, power lines and so on. A large part of it is generated along the power line and transmitted to the equipment through the power line. EMI filters are devices or internal modules designed to reduce or eliminate noise interference.


1.3 Common Mode Noise and Differential Mode Noise

Common Mode and Differential Mode Circuit

Figure 1. Common Mode and Differential Mode Circuit

With this characteristic of the EMI filter, a square wave group or composite noise passing through the power supply filter can be converted into a sine wave of a specific frequency.

The noise to be suppressed by the line filter can be divided into the following two types:

1) common-mode: The same noise on two (or more) power lines can be seen as the noise of power lines to the ground.

2) differential mode: Noise between power lines.

An EMI filter will have different suppressing capabilities for common-mode noise and differential mode noise, and will generally be described by the spectrum of the frequency corresponding to the suppression (in decibels).


1.4 Why Do We Need EMI Filters?

Electromagnetic compatibility (EMC) is an important indicator to measure the quality of electronic products, and it has increasingly become the key in the design of electronic products. In the design process of the power system, the introduction of electromagnetic compatibility design can improve the overall anti-interference ability of the power system, extend the service life of the system, and ensure the safety of use. Therefore, the electromagnetic interference filter is a device that provides good electromagnetic compatibility.


 Adaptation Principle of EMI Filters

The filter circuits commonly used in power supply filters are passive filtering and active filtering. The main forms of passive filtering are capacitor filter, inductance filter and complex filter (including inverted L-type, LC filter, LCπ-type filter and RCπ-type filter, etc.). The main form of active filter is active RC filters, also known as electronic filters. The magnitude of the pulsating component in the DC current is represented by the pulsation coefficient S. The larger the value, the worse the filtering effect.

Pulsating coefficient (S) = fundamental maximum of output voltage AC component / DC component of output voltage

The specific working principle is as follows: After the alternating current is rectified by the diode, the direction is single, but the current is still changing constantly. This pulsating DC is generally not directly used for radio power supply. Therefore, it is necessary to convert the pulsating DC into a smooth wave DC, which is filtering. In other words, the task of filtering is to reduce the fluctuation component of the rectified output voltage as much as possible and convert it into an almost constant DC power supply.

According to the electromagnetic interference characteristics of the power port, the EMI filter can transmit AC power to the power source without attenuation. This not only greatly reduces the EMI noise of AC transmission, but also effectively suppresses the EMI noise generated by the power supply, preventing them from entering the AC grid to interfere with other electronic devices.

This is a passive network structure suitable for both AC and DC power supplies and has a two-way suppression function. Inserting it between the AC power grid and the power supply is equivalent to adding a blocking barrier between the EMI noise of the AC power grid and the power supply, that is, two-way noise suppression, so it is widely used in various electronic products.

Aiming at the characteristics of electromagnetic interference from power terminals, an electromagnetic interference filter is designed. It is usually a selective two-terminal network composed of an inductor, a capacitor, a resistor or a ferrite device. According to the working principle, it is called a reflection filter. It provides high series impedance and low parallel impedance in the filter stopband, which causes it to be seriously mismatched with the impedance of the noise source and the load impedance, thereby transferring unwanted frequency components back to the noise source.

 emi filter

 Operating Principles

The following figure is a typical circuit diagram of the EMI filter: C1and C2 are differential-mode capacitors, generally called X capacitors, the capacitance is often between 0.01μF and 0.47μF; Y1 and Y2 are common-mode capacitors, generally called Y capacitor, the capacitance should not be too large, generally in the tens of nanofarads, if it is too large, it will easily cause leakage; L1 is a common-mode choke, which is a pair of coils wound in the same ferrite ring in the same direction. The inductance is about a few millihenries. For the common-mode interference current, the magnetic fields generated by the two coils are in the same direction, and the common-mode chokes coil exhibits a large impedance to attenuate the interference signal. For the mode signal, the magnetic field generated by the two coils offsets, so it does not affect the performance of the circuit. It should be noted that this is a primary filter circuit, if you want better results, you can use secondary filtering.

Typical Circuit Diagram of An EMI Filter 

Figure 2. Typical Circuit Diagram of An EMI Filter

To judge an EMI filter good or not, it is necessary to understand its performance indicators. The main parameters: rated voltage, rated current, leakage current, insulation resistance, withstand voltage, operating temperature, insertion loss, etc. The most important one is insertion loss. The insertion loss is often expressed by "IL", sometimes it is also called insertion attenuation. This indicator is the main indicator of the performance of the EMI filter. It is usually expressed by the decibel number or the frequency characteristic curve. It refers to the power ratio or terminal voltage ratio of the test signal from the power supply to the load before and after the filter is connected to the circuit. The larger the number of decibels, the stronger the ability to suppress interference. For example, some insertion loss can be tested with a 50-ohm test system. The following figure shows the insertion loss of an EMI filter.

The Insertion Loss of An EMI Filter

 Figure 3. The Insertion Loss of An EMI Filter

 Ⅳ EMI Filters Classification

There are two main types of EMI filters: conducted EMI and radiated EMI. Conducted EMI passes through conductors such as wire, while radiated EMI propagates through air. In high-speed PCB design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may all become radiated interference sources, which can emit electromagnetic waves and affect the normal work of systems or subsystems.



Therefore, when purchasing the EMI filter, the phase number, rated voltage, rated current, leakage current, certification, volume and shape, insertion loss, etc. should be fully considered. The rated voltage/current should meet the product requirements, and the leakage current cannot be too large. EMI filter with relevant certification system can be selected. Determine its volume and shape according to the actual application. When the insertion loss is large, the suppression ability is strong, etc.

In addition to these, there are some details needed to consider. For example, some EMI filters are military-grade and some are industrial-grade. Some are dedicated to household equipment, some are dedicated to inverters, and some are dedicated to medical equipment. Only when the object is determined can you choose a suitable one. As long as the basic conditions are met, the price is the key factor to consider.


power emi filter


1. The EMI filter cannot have an electromagnetic coupling path.

1) Power lines are too long.

2) Power lines are too close.

Both of these are incorrect installations. The point of the problem is that there is an obvious electromagnetic coupling path between the input wire of the filter and its output wire. In this way, the EMI signal present at one end of the filter escapes the suppression of the filter and is directly coupled to the other end of the filter without attenuation. Therefore, the filter input and output lines must be effectively separated first.

In addition, if the above two types of power supply filters are installed inside the shield of the device, the EMI signal on the internal circuits and components of the device will be directly coupled to the outside of the device due to the EMI signal generated by the radiation on the (power) terminal of the filter. Therefore, the device shielding loses the suppression of EMI radiation generated by internal components and circuits. Of course, if there is an EMI signal on the filter (power supply), it will also be coupled to the components and circuits inside the device due to radiation, thereby damaging the suppression of the EMI signal.

2. Don't bundle cables together.

In general, when installing an EMI filter in an electronic device or system, be careful not to bundle the wires between the power end and the load end together, because this undoubtedly aggravates the electromagnetic coupling between them to cause poor EMI signals suppression.

3. Try to avoid using long grounding wires.

It is advisable to connect the inverter or the motor to the output of the EMI filter at a length of not more than 30 cm. Because an excessively long ground line means a large grounding inductance and resistance, it can severely damage the filter's common-mode suppression. A better method is to secure the shield of the filter to the housing at the power inlet of the unit with metal screws and star spring washers.

4. The input line and output lines must be pulled apart.

Having distance does not mean parallel connection, because this will reduce the filter performance.

5. The EMI filter housing must be in good contact with the case shell.

The inverter-specific filter metal case and the case shell must be connected well, as well as the ground wires.

6. The connection lines should be twisted pair.

The input and output connection lines are preferably choosing shielded twisted pairs, which can effectively eliminate some high-frequency interference signals.


Frequently Asked Questions about EMI Filter Basics

1. What is an EMI filter?

EMI Filters, or electromagnetic interference filters, also called RFI Filters or radio-frequency interference filters, are an effective way to protect against the harmful impacts of electromagnetic interference.


2. What causes EMI?

Conducted interference
Conducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused by induction (without physical contact of the conductors). For lower frequencies, EMI is caused by conduction and, for higher frequencies, by radiation.


3. What is an EMI filter used for?

Most electronics contains an EMI filter, either as a separate device, or embedded in circuit boards. Its function is to reduce high frequency electronic noise that may cause interference with other devices. Regulatory standards exist in most countries that limit the amount of noise that can emitted.


4. What is DC EMI filter?

Filter provides noise suppression in both directions protecting your DC lines from noise generating by a particular piece of equipment, or protecting your sensitive equipment from noise coming from DC power supply or other loads.


5. Where should I place my EMI filter?

A power line or mains EMI filter is placed at the power entry point of the equipment that it is being installed into to prevent noise from exiting or entering the equipment. Essentially, an EMI filter is made up of two basic types of components–capacitors and inductors.


6. What is the difference between RFI and EMI?

The terms EMI and RFI are often used interchangeably. EMI is actually any frequency of electrical noise, whereas RFI is a specific subset of electrical noise on the EMI spectrum. ... Radiated EMI is similar to an unwanted radio broadcast being emitted from the power lines.


7. How can I reduce EMI?

Use twisted pair shielded cable to carry instrumentation signals. Twisting the wires equalizes the effect of EMI on both wires, greatly reducing error due to EMI. Surrounding the instrument wires with a shield protects them from EMI, and provides a path for EMI-generated current to flow into ground.


8. How does EMI filter works?

EMI, or Electro-Magnetic Interference, is defined as unwanted electrical signals and can be in the form of conducted or radiated emissions. ... The capacitors provide a low impedance path to divert the high frequency noise away from the input of the filter, either back into the power supply, or into the ground connection.


9. How do you test an EMI filter?

Checking your EMI filter use an ohm meter. Measure from one prong of where you connect the power cord (line)to one of the output lines past the EMI filter (load). You should get around 0.4 ohm.


10. What is EMI filtering?

When attached to devices or circuits, EMI filters can suppress electromagnetic noise transmitted through conduction. These filters extract any unwanted current conducted through wiring or cables, while allowing desirable currents to flow freely.


You May Also Like

Principle and Function of the Filter

Common Applications of Filter

Classification of Electronic Filters

What Is A Low Pass Filter Circuit?

Important Measures to Prevent Electromagnetic Interference - Filtering Technology

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Related Articles

pinglun 5 comments

    • pingluntus
    • Billy Chan on 2019/8/10 10:42:47

    The function of a band stop filter is too pass all those frequencies from zero (DC) up to its first (lower) cut-off frequency point, and pass all those frequencies above its second (upper) cut-off frequency, but block or reject all those frequencies in-between.

    • pingluntus
    • Adley TIO on 2019/8/10 11:19:42

    How the emi filter remove a band of unwanted frequencies ? pl give me more details about it, many thanks.

    • pingluntus
    • Jimmy Wan on 2019/8/10 11:29:08

    I want to learn more about emi filter from your site, so can you write more info about it? It is better to give more examples to explain your idea.

    • pingluntus
    • Berkeley on 2019/8/10 11:35:23

    Excellent work and good explanation, pl keep it up!

    • pingluntus
    • Tom Adams on 2020/7/14 16:16:26

    thank you very much for excellent information ....

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