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Nov 15 2018

Important Measures to Prevent Electromagnetic Interference - Filtering Technology

Warm hints: This article contains about 3000 words and reading time is about 15 min.

Summary

Electromagnetic Interference (EMI) is an electronic noise that interferes with cable signals and reduces signal integrity. EMI is typically generated by electromagnetic radiation sources such as motors and machines. Electromagnetic interference is an electromagnetic phenomenon that people have long discovered. It is almost simultaneously discovered with the phenomenon of electromagnetic effects. In 1981, British scientists published an article on "interference", marking the beginning of the study of interference problems. In 1989, the British Post and Telecommunications Department studied the interference problem in communication, and the research on interference problems began to be engineered and industrialized.

Article Core

Filtering Technology

Purpose

Introduction to filtering technology

Article Name

Important Measures to 

Prevent Electromagnetic Interference

 - Filtering Technology

Category

Semiconductor Information 

Application

Anti-electromagnetic interference field

Keywords

Electromagnetic Interference, Filtering Technology

Catalog

Introduction

Type of Online Interference

Filter Classification

Basic Principle of the Filter

The Importance of High Frequency Insertion Loss of Power Supply Filters

Filter Selection

Use of Filters


Introduction

There are three main measures to prevent electromagnetic interference, namely shielding, filtering and grounding. Shielding alone often does not provide complete EMI protection because the cable on the device or system is the most effective interference receiving and transmitting antenna. Many devices have no problem when experimenting with a single station, but when the two devices are connected, the electromagnetic compatibility requirements are not met. This is the function of the cable to receive and radiate the antenna. The only measure is to add a filter to cut off the path of electromagnetic interference along the signal line or power line. Together with the shield, it forms a perfect electromagnetic interference protection. It can be used to suppress the interference source, eliminate the coupling or improve the resistance of the receiving circuit.


Type of Online Interference

The interference currents on the line can be divided into two categories according to their flow paths: one is differential mode interference current and the other is common mode interference current. The differential mode interference current is the interference current flowing between the live line and the neutral line. The common mode interference current is the interference current flowing between the live line, the neutral line and the earth (or other reference object), due to the suppression of these two types of interference. Different, so correctly identifying the type of interference is a prerequisite for implementing a correct filtering method. The two kinds of interference common mode interference are generally induced by the interference electromagnetic waves from the outside or other parts of the circuit in the loop of the cable and the "ground". Sometimes, due to the different "ground" potentials at both ends of the cable, common mode interference is also generated. It is very harmful to electromagnetic compatibility. On the one hand, common mode interference will cause the cable to emit strong electromagnetic radiation, interfere with other parts of the circuit or peripheral electronic equipment; on the other hand, if the circuit is unbalanced, in the cable When the amplitude and phase of the common mode interference current on different wires are different, the common mode interference will be converted into differential mode interference, which will seriously affect the quality of the normal signal, so people are trying to suppress common mode interference.

Differential mode interference is mainly caused by electromagnetic interference generated by other parts of the circuit entering the signal line loop through conduction or coupling, such as higher harmonics, self-oscillation, and grid interference. Since the differential mode interference current flows in the loop simultaneously with the normal signal current in the same direction, its interference with the signal is severe and must be suppressed. In summary, in order to meet the requirements of electromagnetic compatibility, both common mode interference and differential mode interference should be suppressed.


Filter Classification

A filter is a network of resistors, inductors, and capacitors that concentrate parameters, or resistors, inductors, and capacitors that distribute parameters. This network allows some frequencies to pass while suppressing other frequency components. The interference filter has a low-pass filter, a high-pass filter, a band-pass filter, a band-stop filter, and the like according to the relative relationship between the frequency of the interference signal to be filtered and the operating frequency. Low-pass filter type Low-pass filter is the most commonly used one, mainly used when the interference signal frequency is higher than the operating signal frequency. For example, in digital devices, pulse signals have abundant high-order harmonics. These high-order harmonics are not necessary for circuit operation, but they are strong sources of interference. Therefore, in the digital circuit, a common low-pass filter filters out unnecessary higher harmonics in the pulse signal, and only retains the lowest frequency that can maintain the normal operation of the circuit. The power line filter is also a low-pass filter that allows only 50 Hz of current to pass, which greatly attenuates other high-frequency interference signals.

   (1) A commonly used low-pass filter is a combination of an inductor and a capacitor. The capacitor is connected in parallel between the signal line to be filtered and the signal ground (filtering the differential mode interference current) or the signal line to the chassis ground or the earth. The inductance (filtered out common mode interference current) is connected in series to the signal line to be filtered. According to the circuit structure, there are single capacitor type (C type), single inductor type, L type and reverse type, T type, π type.

   (2) The high-pass filter is used when the interference frequency is lower than the signal frequency, such as filtering the interference caused by the power supply harmonics on some sensitive signal lines close to the power line.

   (3) Band-pass filter is used when the signal frequency only occupies a narrow bandwidth. For example, a band-pass filter is installed on the antenna port of the communication receiver, and only the communication signal is allowed to pass.

   (4) The band-stop filter is used for the occasion where the interference frequency bandwidth is narrow and the signal frequency is wide. For example, a band-stop filter with a band-stop frequency equal to the transmission frequency of the station should be installed at a cable port close to the high-power station. There are two main differences between the filter circuits of different structures: 1. The more filter components in the circuit, the greater the attenuation of the filter stopband and the shorter the transition between the filter passband and the stopband. 2. Filter circuits of different structures are suitable for different source impedances and load impedances, and their relationship should follow the principle of impedance mismatch. However, it should be noted that the impedance of the actual circuit is difficult to estimate, especially at high frequencies (electromagnetic interference problems often occur at high frequencies). Due to the influence of circuit parasitic parameters, the impedance of the circuit varies greatly, and the impedance of the circuit is often It is also related to the operating state of the circuit, plus the circuit impedance is different at different frequencies. Therefore, in practice, which filter is effective depends mainly on the results of the test.


Basic Principle of the Filter

The filter is composed of a low-pass filter circuit composed of an inductor and a capacitor. It allows the current of the wanted signal to pass, and the attenuation of the higher-frequency interference signal is greatly attenuated. Since the interference signal has two modes, differential mode and common mode, the filter has an attenuation effect on both types of interference. There are three basic principles:

    A) Using the characteristics of low frequency and low frequency of the capacitor, the high-frequency interference current of the hot line and the neutral line is introduced into the ground line (common mode), or the high-frequency interference current of the live line is introduced into the neutral line (differential mode);

   B) using the impedance characteristics of the inductor to reflect the high frequency interference current back to the interference source;

   C) Using interference suppression ferrite can convert the interference signal absorption of a certain frequency band into heat. Select a suitable interference suppression ferrite magnetic ring for the frequency band of an interference signal, and the magnetic beads are directly placed on the cable to be filtered. can.


The Importance of High Frequency Insertion Loss of Power Supply Filter

Although the limits for conducted emissions in various standards are only 30 MHz (old military to 50 MHz, new military to 10 MHz), the suppression of conducted emissions must not ignore the effects of high frequencies. Because the high-frequency conduction current on the power line causes radiation, the radiation emission of the device exceeds the standard. In addition, the test waveforms in the transient pulse sensitivity test often contain high frequency components. If these high frequency interferences are not filtered out, the sensitivity test of the device will fail. There are two main reasons for the high frequency characteristics of the power line filter. One is the spatial coupling caused by internal parasitic parameters, and the other is the imperfection of the filter components. Therefore, the method of improving the high frequency characteristics is also started from these two aspects. Internal structure: The connection of the filter should be arranged in one direction according to the circuit structure. Under the condition of space, the inductance and the capacitance are kept at a certain distance. If necessary, some isolation plates can be set to reduce the spatial coupling. Inductance: Control the parasitic capacitance of the inductor as described previously. If necessary, use multiple inductors in series. Differential mode filter capacitor: The lead of the capacitor should be as short as possible. To understand the meaning of this requirement: the connection between the capacitor and the wire (hot and neutral) that needs to be filtered is as short as possible. If the filter is mounted on a board, the traces on the board will be equivalent to the leads of the capacitor. At this time, pay attention to ensure that the capacitor lead of the timing is the shortest. Common mode capacitor: The lead of the capacitor should be as short as possible. The understanding and precautions for this requirement are the same as for the differential mode capacitor. However, the common mode high frequency filtering characteristics of the filter are mainly guaranteed by the common mode capacitor, and the frequency of the common mode interference is generally high, so the high frequency characteristic of the common mode filter capacitor is more important. The use of a three-terminal capacitor can significantly improve the high frequency filtering effect. But pay attention to the correct use of the three-terminal capacitor. That is, the grounding wire should be as short as possible, and the length of the other two wires has little effect on the effect. A feedthrough capacitor can be used if necessary, and the performance of the filter itself can be maintained above 1 GHz. Special note: When the radiation emission of the device does not meet the standard requirements at a certain frequency, do not forget to check the common mode conducted emission of the power line at this frequency. The radiation emission is likely caused by this common mode emission current.


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.


Use of Filters

Filters are used to improve the quality of the power supply, the linearity of the circuit, the reduction of various clutter and nonlinear distortion interference, and harmonic interference.

For weapon systems, use the field of the filter:

First, in addition to setting the mains filter on the main distribution system and the distribution system, the power supply to the device must be equipped with a filter, preferably using a line-to-line filter instead of a line-to-ground filter.

Second, for devices that are sensitive to impulse and transient disturbances, filters should be added to the negative terminal when using an isolation transformer.

Third, a filter should be added to the power supply of the weapon system containing the electric blast device. If necessary, the lead of the electric blasting device should also be equipped with a filter.

Fourth, at the interface between each subsystem or device, there should be filters to suppress interference to ensure compatibility. Fifth, the control signals of the equipment and subsystems should be added with filters or bypass capacitors at the input and output terminals.

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