Technical measures for suppressing interference sources of heavy-duty vehicles

Generally speaking, electromagnetic noise is difficult to eliminate, but various measures can be taken to suppress electromagnetic noise to the extent that it does not cause electromagnetic interference. Usually, using only one simple method to solve the electromagnetic noise problem is often difficult to work, so it is best to use several different combined methods.

1. General requirements

The suppression of automobile electromagnetic noise can be carried out on the receiver side, but due to the receiving frequency, the propagation mode of interference waves and other practical situations, it is difficult to take measures on the receiver side. Since the electromagnetic noise of automobile electrical equipment can interfere with other communication equipment and various electronic equipment, it is necessary to consider suppressing the electromagnetic noise generated by the automobile electrical equipment itself. The characteristics and levels of electromagnetic interference waves generated by various electrical appliances in the car are different, so the interference suppression method should also conform to their characteristics and levels. The design of suppressing interference waves can adopt basic measures such as damping, shielding, filtering and connection, and must meet 4 conditions: ① Good suppression effect. ② Does not hinder the performance of the automobile electrical equipment itself. ③ High reliability and easy to use. ④ Reasonable price.

2. Electromagnetic noise suppressor

At present, the suppressors used in automobiles at home and abroad are basically composed of resistors , inductors, capacitors , namely the so-called R, I, C, either individually or in assembly, such as resistors, shielded wires, capacitors, interference suppression inductor coils and interference suppression filters.

III. Methods

shield

Shielding is the most direct way to create an electromagnetic barrier between two areas to protect the circuits in the system from being damaged by the electromagnetic environment.

Shielding can be in various forms, such as partitions, box enclosures, cables or connectors . The effectiveness of shielding is expressed by the shielding effectiveness, which is related not only to the shielding material, but also to the thickness of the material, the frequency of application, the distance from the radiation source to the shielding layer, and the shape and number of discontinuities in the shielding layer.

Shielding has two main purposes: one is to limit the radiated electromagnetic energy to a specific area (called active shielding). Active shielding refers to a structure where the interference source is inside and prevents the interference wave from leaking into the external space. It must be grounded, and the smaller the grounding resistance, the better. The second is to prevent the radiated electromagnetic energy from entering a specific area (called passive shielding). Passive shielding refers to a structure where the interference source is outside and prevents the interference wave from entering the shielded space.

The steps of shielding design are: ① Determine the interference level and energy density to be shielded; ② Estimate the allowable signal level on the other side of the shielding layer; ③ Combine the product structure and shell design to select a detailed shielding design plan. The design principles of shielding are: ① High-frequency electric field shielding uses good conductive materials such as copper, aluminum and magnesium to obtain the maximum reflection efficiency. ② Low-frequency magnetic field shielding uses magnetic materials such as iron and nickel-iron high-permeability magnetic alloy to obtain the maximum absorption efficiency. ③ A shielding layer of sufficient thickness can shield electric fields of any frequency and has high shielding effectiveness. ④ Multi-layer shielding (including housing and cable) can provide high shielding effectiveness over a wide frequency band, but cost and other performance requirements (such as cable flexibility) must be considered. ⑤ Various joint surfaces used to seal gaps must be clean and must not have non-conductive coatings. ⑥ In order to maintain the shielding effectiveness of the shell, conductive linings, spring washers, waveguide attenuators and grids should be added to the necessary threading holes.

[page]Connect

Connection is to establish a low impedance path between two metal surfaces. This path can be established between the ground reference and components, circuits, shields and structural parts. The purpose of connection is to establish a uniform electrical structure, that is, to try to make the path of RF current uniform in the structure to avoid the generation of potential between metal parts, thereby causing interference.

Grounding

Grounding is the creation of a conductive path between two points, one of which is usually an electrical component of the system and the other is a reference point.

The effectiveness of a grounding system depends on the extent to which the potential difference in the grounding system is reduced and the ground current is reduced.

Filtering

Shielding is mainly to solve the radiation interference, while filtering is mainly to solve the interference caused by the conduction path. Both involve connection and grounding technology, and the effectiveness of electromagnetic interference filters is greatly affected by the source impedance and load impedance.

4. Technical measures for anti-electromagnetic interference design

There are many technical measures that can be used to reduce the sensitivity of electronic systems to electromagnetic interference, such as shielding, connection, filtering, grounding, circuit design and component selection. Shielding, connection, filtering and grounding have been described above, and the following focuses on component selection and circuit design.

Component selection and circuit design are one of the key points of anti-electromagnetic interference and electromagnetic compatibility design. By selecting components and screening for anti-interference, components with high anti-interference threshold values ​​can be obtained. This measure can increase the system's anti-interference performance by 10-3dB.

The designed circuit has high signal level and low impedance characteristics, which can greatly reduce the sensitivity to interference. The general method is to shorten the connection between components and circuits and use shielded twisted pair cables for connection.

For circuits, digital circuits are more resistant to interference than linear and analog circuits, and low-speed digital circuits have lower electromagnetic sensitivity than high-speed digital circuits.

When determining components and circuits, in addition to paying attention to their electromagnetic interference sensitivity, you should also pay attention to some components and circuits that will generate electromagnetic interference. They will also have a certain impact on the system, or cause signal distortion, or generate interference voltage , interference current, or cause the system to malfunction. You should pay attention to this during design and testing.

5. Circuit Layout Principles

1. Arrange the position and direction of components correctly. The most sensitive components should be far away from the interference source.

2. The wiring should be reasonable. Wires with different uses and different levels, such as input and output lines, weak current and strong current, should be kept away from each other and cannot be parallel; the length of the grounding wire should be as short as possible and the cross-section should be as large as possible.

3. Key components, circuits and traces must be shielded, and the shielding must be properly grounded.