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EMI Shielding Techniques Analysis

Author: Apogeeweb Date: 15 Nov 2018  1553

electromagnetic interference definition

Introduction

With the rapid development of electronic technology, modern electronic devices have been widely used in various fields of human life. At present, electronic equipment has been in a period of rapid development, and this process is still continuing at an increasing rate. The wide application and development of electronic equipment will inevitably lead to the continuous increase of the electromagnetic field level they generate in the surrounding space. In other words, electronic devices inevitably work in an electromagnetic environment, thus electromagnetic interference cannot be avoided.

Catalog

Introduction

Ⅰ Electromagnetic Interference EMI

Ⅱ Electromagnetic Interference Source

Ⅲ How To Reduce Electromagnetic Interference?

Ⅳ Filtering Interference Technique

4.1 Filtering Summary

4.2 Filtering Circuit Analysis

4.3 Example Analysis: EMI Filter

Ⅴ Filter Selection Requirements

5.1 Selecting Rules

5.2 Filter Selection


Ⅰ Electromagnetic Interference EMI

Electromagnetic interference (EMI) is electronic noise that interferes with cable signals and reduces signal integrity, also called radio-frequency interference (RFI) when in the radio frequency spectrum. It is usually generated by electromagnetic radiation sources such as motors and machines. EMI is an electromagnetic phenomenon that people have discovered long time ago, and was discovered almost at the same time as the phenomenon of electromagnetic effect.

Electromagnetic Interference EMI Explained

There are two types of EMI: conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to the interference source coupling its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources which is similar to antenna characteristics. They can emit electromagnetic waves and affect other systems or subsystems in the system.

 

Ⅱ Electromagnetic Interference Source

EMI Source

The occurrence of any electromagnetic interference must meet three basic conditions: first, there should be an interference source; second, there must be a way and channel to spread the interference energy; third, there must be a response from the interfered object. Conducted transmission must have a complete circuit connection between the interference source and the sensitive elements. And the interference signal is transmitted to the sensing parts along this connection circuit, then interference occurs. This transmission circuit can include wires, conductive components, power supply, common impedance, ground plane, resistance, inductance, capacitance, and mutual inductance components. Radiation transmission is the propagation of electromagnetic waves through the medium. Interference energy is emitted to the surrounding space according to the law of electromagnetic fields. In actual engineering, interference between two devices usually involves coupling in many ways. Since the simultaneous existence of multiple ways of coupling, repeated cross-coupling, and mutual interference, electromagnetic interference becomes difficult to control.

 

Ⅲ How To Reduce Electromagnetic Interference?

All electronic circuits may be subject to EMI, which affects the normal operation of electronic equipment widely. Therefore, electromagnetic compatibility is a problem that must be considered in the design of electronic equipment. There are three commonly used methods to suppress EMI, including grounding, shielding and filtering.
1. Grounding and shielding are relatively easy. Make a standard grounding for the interference source. Sometimes, even a single-point grounding is required, depending on the nature of the interference source.
2. Shielding means to make a metal case for the interference source to isolate the interference source.
3. As for filtering, you can choose reactors, filters, filtering magnetic loops, zero-phase reactors, neutral line reactors, common mode chokes and other components. Installing it on the power supply of the interference source or output end, different interference frequency and the applicable filter components are different.

 

Ⅳ Filtering Interference Technique

4.1 Filtering Summary

The filter can suppress the conducted interference from the electronic equipment and the power grid. Filtering is an important measure to suppress and prevent interference. The filter can significantly reduce the level of conducted interference, because the filter has good suppression capabilities for these components that are different from the necessary signal frequency. Therefore, the use of a filter network is a powerful measure whether it is to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving equipment. The resistance-capacitance and inductance-capacitance decoupling network can isolate the circuit from the power supply, eliminating the coupling between the circuits, and preventing interference signals from entering the circuit. For high frequency circuits, a filter consisting of two capacitors and an inductor (high frequency choke) can be used. There are many types of filters, and choosing an appropriate filter can eliminate unwanted coupling.

 

4.2 Filtering Circuit Analysis

Filtering Circuit

Figure 1. Filtering Circuit

The basic of the filtering circuit is shown in Figure 1. The five-terminal device has two input terminals, two output terminals and a ground terminal. The shell should be connected to the ground when in use. The circuit includes common mode choke (also known as common mode inductance) L, filter capacitors C1 ~ C4. L has no effect on series mode interference. When common mode interference occurs, due to the magnetic flux direction of the two coils is the same, the total inductance increases rapidly after coupling. Resulting in a large inductance, so it is not easy to pass. Two coils of L are wound on a low-loss and high-permeability ferrite magnetic ring, respectively. When a current passes through, the magnetic fields on the two coils will strengthen each other. The inductance of L is related to the rated current I of the EMI filter. It should be pointed out that when the rated current is large, the wire diameter of the common mode choke should also be increased correspondingly in order to be able to withstand a large current. In addition, an appropriate increase in inductance can improve low-frequency attenuation characteristics. C1 and C2 are film capacitors (the capacity range is roughly 0.01μF ~ 0.47μF), and mainly used to filter series mode interference. C3 and C4 are connected across the output terminal, and the midpoint of the capacitor is grounded, which can effectively suppress common mode interference. C3 and C4 can also be connected in parallel at the input end, belonging to  ceramic capacitors, and the capacitance range is 2200pF~0.1μF. In order to reduce the leakage current, the capacitance should not exceed 0.1μF, and the midpoint of the capacitor should be connected to the earth. The withstand voltage values of C1C4 are 630VDC or 250VAC.

 

4.3 Example Analysis: EMI Filter

4.3.1 EMI Filter Circuit

EMI filters are mainly used to suppress interference. The EMI filter consists of a linear component circuit, which is installed between the power line and the electronic equipment. It allows the power frequency to pass, and prevents high-frequency noise from passing, which plays an important role in improving the reliability of the equipment.

Figure 2. Basic Circuit Diagram

Cx: Anti-differential mode capacitor, used to attenuate differential mode interference.
Cy: Anti-common-mode capacitor, used to attenuate common-mode interference.
R: Resistor, used to eliminate static electricity accumulation that may appear on the filter.
L1, L2: Common mode inductance coils. They have the same number of turns and opposite directions. When the filter is connected to the circuit, the magnetic fluxes generated by the currents of the two coils cancel each other out in the magnetic ring, so that the magnetic ring will not reach the magnetic saturated state. So keeping the two inductance values unchanged, a better filtering effect can be obtained.
When choosing X and Y capacitors in filtering design, we must pay attention to the requirements of their capacitance, withstand voltage level and safety level, because they are directly related to the safety performance of the EMI filter.

 

4.3.2 EMI Filter Installation
When using EMI filters, pay attention to the operating frequency and installation location. The installation requirements are as follows:
The filter housing and the equipment metal housing should be firmly and reliably fixed together, otherwise it will increase the contact resistance and reduce the filtering performance. Avoid coupling between the input wires and the output wires of the filter, so as not to reduce the ability of the filter to suppress EMI signals. The most effective solution is to install the filter at the inlet of the equipment chassis.

Recommended Reading: EMI Filter Basics - Operational Principles and Faulty Installation


Ⅴ Filter Selection Requirements

5.1 Selecting Rules

The selected filter is required to meet the attenuation characteristics required by the load within the corresponding working frequency range. If not, multi-stage parallel can be used to obtain higher attenuation performance than a single stage. Because different filters can be cascaded to obtain good attenuation characteristics in a wide frequency band. There are other rules needed to remember.
1) Do insertion loss test on the filter.
2) Comparing the output impedance of the filter circuit and the power supply, whether it will affect the stability of the filter.
3) Choose a multi-stage filter that can suppress self-resonance as much as possible.
4) High input impedance filter is matched with low power supply impedance.
5) The filter must be able to withstand occasional high-voltage transients, such as lightning impact.
6) The filter must have a certain withstand voltage capability, according to the rated voltage of the interference source.
7) The current allowed by the filter must be consistent with the rated current of the circuit.
8) The filter should have sufficient mechanical strength, install easily, and be safe and reliable.


5.2 Filter Selection

According to the characteristics of the interference source, frequency range, voltage and impedance, and load characteristics, the filter should be properly selected. Generally consider:
First, the filter is required to meet the attenuation characteristics of the load in the corresponding operating frequency range. If a filter attenuation cannot meet the requirements, multi-cascade can be used, and higher attenuation than single-stage can be obtained. Different filter cascades can obtain good attenuation characteristics in a wide frequency band.

Second, to meet the load circuit operating frequency and the need to suppress the frequency, if the frequency to be suppressed and the useful signal frequency are very close, then a filter with very steep frequency characteristics is needed to filter out the interference frequency of the suppression, only Allows the passage of useful frequency signals.

Third, at the required frequency, the impedance of the filter must be mismatched with its connected interference source impedance and load impedance. If the load is high impedance, the output impedance of the filter should be low impedance; if the power supply or interference source The impedance is low impedance, then the input impedance of the filter should be high impedance; if the source impedance or the interference source impedance is unknown or varies over a wide range, it is difficult to obtain stable filtering characteristics, in order to obtain the filter Good relatively stable filtering characteristics, can be connected to a fixed resistor at the input and output of the filter.

Fourth, the filter must have a certain withstand voltage capability, and the filter should be selected according to the rated voltage of the power source and the interference source so that it has a sufficiently high rated voltage to ensure reliable operation under all expected working conditions. Can withstand the impact of input transient high pressure. 

Fifth, the filter allows the pass to be consistent with the rated current for continuous operation in the circuit. If the rated current is high, the volume and weight of the filter will be increased; the rated current will be low, and the reliability of the filter will be reduced. 

Sixth, the filter should have sufficient mechanical strength, simple structure, light weight and small size. Easy to install, safe and reliable.

Ordering & Quality

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2520+: $0.17737
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