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Aug 6 2019

EMI Filter Basics - Operational Principles and Faulty Installation


Article Core: Electromagnetic Interference Filter
Ⅰ IntroductionⅡ Adaptation principle of EMI Filters


Ⅲ Operating Principle

Common Mode Noise

Ⅳ Selection

Differential Mode Noise

Ⅴ Installation


The EMI filter is a filter circuit composed of a capacitor, an inductor and a resistor, also called RFI Filters or radio-frequency interference filters. Its filter circuit composed of 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 and the power supply and load side, the more effective the attenuation of electromagnetic interference is. The filter can effectively filter out the frequency of a specific frequency in the power line or the outside frequency to obtain a power signal of a specific frequency or eliminate the power signal after a specific frequency point.

The EMI filter is a common electrical elements in powers supplies. In this video we open one of them and study the circuit of them.

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).


 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 direct 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 to power the radio, thus turning the pulsating DC into a waveform-smooth DC is necessary, which is filtering. In other words, the task of filtering is to reduce the fluctuation component of the rectifier output voltage as much as possible, and transform it into a nearly constant DC power.

According to the electromagnetic interference characteristics of the power port, an EMI filter can transmit AC power to the power supply without attenuation, and greatly attenuates the EMI noise transmitted with the AC, and at the same time effectively suppresses the EMI noise generated by the power supply devices, preventing them from entering the AC grid to interfere other electronic devices.

This is a passive network structure that is suitable for both AC and DC power supplies and has two-way suppression. Insert it between the AC grid and the power supply, which is equivalent to adding a blocking barrier between the EMI noise of them, that is, a two-way noise suppression, thus it is widely used in various electronics.

The EMI filter is designed for the characteristics of electromagnetic interference of the power terminals. It is generally a selective two-terminal network composed of an inductor, a capacitor, a resistor or a ferrite device. It is actually called a reflective filter according to the working principle. It provides high series impedance and low parallel impedance within the filter stopband, causing it to be heavily mismatched with the noise source's impedance and load impedance, thereby transmitting unwanted frequency components back to the noise source.


 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, too large to 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 choke 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 

Fig. 1 Typical Circuit Diagram of An EMI Filter

To judge a EMI filter good or not, it is necessary to understand the performance indicators of it. The main parameters: rated voltage, rated current, leakage current, insulation resistance, withstand voltage, operating temperature, insertion loss, etc. The most important one is the insertion loss. The insertion loss is often expressed by “IL”, sometimes it is also called the insertion attenuation. This indicator is the main indicator of the performance of 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

 Fig. 2 The Insertion Loss of An EMI Filter


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 requirements of the product, and the leakage current cannot be chosen too large. The EMI filter with the relevant certification system can be selected, the volume and shape are determined according to the actual application, stronger suppression ability when having big insertion loss, and so on.

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.



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 a 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 the 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 a 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 not means 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.

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    • 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.

    • 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.

    • 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.

    • Berkeley on 2019-8-10 11:35:23

    Excellent work and good explanation, pl keep it up!

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