Technical solutions for improving electromagnetic compatibility of automotive electronic equipment
With the continuous development of automotive electronic control technology, the number of automotive electronic equipment has greatly increased, the operating frequency has gradually increased, and the power has gradually increased, making the automotive working environment full of electromagnetic waves, resulting in increasingly prominent electromagnetic interference problems, which can affect the normality of electronic equipment. Work, damage the corresponding electrical components. Therefore, the electromagnetic compatibility performance of automotive electronic equipment is getting more and more attention. At present, it is urgently required to widely apply the electromagnetic improvement technology for automotive sub-devices.
Sources of electromagnetic interference
Automotive electronic equipment works on cars with constantly changing driving environments. The complexity and variability of electromagnetic energy in the environment means that the sources of electromagnetic interference experienced by the system are relatively wide. According to the source classification of electromagnetic interference, it can be divided into electromagnetic interference outside the vehicle, electrostatic interference from the vehicle body and electromagnetic interference inside the vehicle.
Electromagnetic interference outside the car
Electromagnetic interference outside the car is the interference that the car experiences when it experiences various external electromagnetic environments. Such interference exists in a specific space or a specific time. Such as the electromagnetic interference of high-voltage transmission lines, high-voltage substations and high-power radio transmission stations, as well as the electromagnetic interference of lightning, sunspot radiation, and so on. Other nearby electronic devices in the environment can also cause interference when working, such as cars that are close to each other while driving.
Car static interference
The static interference of the car body is related to the car and the external environment. Due to the high-speed friction between the car body and the air when the car is traveling, an unevenly distributed static electricity is formed on the car body. Electrostatic discharge will cause interference currents on the car body, and at the same time produce high-frequency radiation, which will cause electromagnetic interference to automotive electronic equipment.
Electromagnetic interference in the car
The electromagnetic interference in the car is the internal mutual interference during the operation of automotive electronic equipment, including the electronic noise generated by electronic components, the electromagnetic interference generated by the commutation brush during the operation of the motor, and the discharge interference when the various switches work. The high-frequency radiation generated by the ignition system has the largest interference energy.
The way and principle of electromagnetic interference
Electromagnetic interference is classified according to interference channels, which are mainly divided into conducted interference, inductive interference and radiated interference. The corresponding interference principles are as follows.
Conducted interference
Conducted interference is mainly propagated through the common conductor of the circuit. The typical structure is a common power line and a common ground. Figure 1 is a schematic diagram of a typical conducted interference circuit. R is the resistance on the power line, Z is the resistance on the ground line, U is the branch voltage, and I is the branch current.
Since the working voltage of each device is
Therefore, changes in the current of any one device will cause changes in the voltage of other devices, causing interference. To reduce the mutual influence between the devices, you need to reduce the R, Z, and I values.
Inductive interference
Inductive interference is divided into electrical induction interference and magnetic induction interference. The basic circuit diagram is shown in Figure 2 and Figure 3. U1 is the voltage of wire 1, I1 is the current on wire 1, U2 is the interference voltage on wire 2, C12 is the capacitance between the two wires, C1g and C2g are the capacitance of wire 1, wire 2 and ground, M12 is between the two circuits Mutual inductance, R is the resistance of each circuit.
For electric induction circuits, 
, To reduce U2 can reduce C12, U1 and R, or increase C2g; the first measure is to reduce C12, the method is to increase the wire distance or change the dielectric parameters between wires. For magnetic induction circuits, 
To reduce U2, you can reduce M12 or I1 rate of change. The basic measure is to reduce M12. For typical two circuits, 
, L1, L2 are the length of the two loops, m0 is the vacuum permeability, r is the distance between the two loops. Therefore, increasing r and decreasing the loop area can reduce M12.
Radiated interference
Radiated interference is emitted by the antenna. Since the conducting wires and cables can be regarded as equivalent antennas, the wiring harness radiation interference of automotive electronic equipment is very serious. According to Maxwell's equation, the radiated electromagnetic field of a typical monopole antenna is 
Is the spherical coordinate, I is the antenna current, l is the antenna length, r is the distance from the antenna to the field point, w is the angular frequency, e0 is the air dielectric constant, and l is the electromagnetic wavelength. To reduce H and E, you can decrease I, l, or increase r.
In summary, the electromagnetic interference outside the vehicle decreases with the increase of the working distance. Only when the energy of the vehicle itself is very large can it affect the electronic equipment of the vehicle that is far away. The results of many years of research indicate that the electromagnetic effect of large energy is harmful to human health, and various corresponding electromagnetic standards have been formulated to limit such interference, which reduces the impact of automotive electronic equipment.
The electrostatic interference of the vehicle body and the electromagnetic interference in the vehicle, because the interference action distance is short, the interference time is long, and the interference intensity is relatively large. Since the automotive electronic equipment forms an electrical network with the battery and alternator as the core power supply and the car body as the common ground, each part of the harness will conduct interference with each other through the power supply and the ground wire, and there is inductive interference between adjacent wires, but not adjacent The interference between the wires is also radiated due to the antenna effect, which makes the interference in the car integrate three ways. The interference is more composed and covers a wider range of interference frequencies. It is the main electromagnetic interference received by automotive electronic equipment. Solving these two kinds of electromagnetic interference problems can simultaneously improve the anti-interference ability of automotive electronic equipment against electromagnetic interference outside the vehicle, thereby reducing the possibility of equipment malfunction or damage.
Measures to improve the electromagnetic compatibility performance of electronic equipment
The electromagnetic compatibility performance of automotive electronic equipment includes two aspects: one is electromagnetic emission, which measures the emission level of electromagnetic interference generated by the system; the other is electromagnetic sensitivity, which measures the ability of the system to resist electromagnetic interference in order to achieve the expected technical indicators during operation. . According to the previous analysis, in order to comprehensively improve the electromagnetic performance of automotive electronic equipment, three aspects can be considered: one is to reduce the intensity of electromagnetic interference emitted by the equipment; the second is to suppress the transmission of electromagnetic interference; the third is to reduce the interference received by the electromagnetic sensitive components of the equipment strength.
Reduce the electromagnetic interference intensity of the equipment
Optimize the electrical structure of the device: the flasher in the automotive electronic device is a relay contact structure, and an arc suppressor can be added in front of the contact; the motor is an inductive load, which can reduce the current noise through the internal filter circuit; the printed circuit of various electronic control units Board, we must optimize the wiring and reduce the level of electromagnetic emissions.
Choose appropriate electronic components: various control units on the car, and use lower frequency chips to help reduce radiated interference.
Reduce the power of the device: When the functional requirements are met, reducing the power of the device can reduce the interference voltage and current, thereby reducing the interference intensity.
Suppress interference transmission
Shield interference source equipment and related wiring harness: The electronic control unit used in the main electronic control system in the car should be encapsulated in a shielded shell.
Increase wiring harness filtering: For longer wiring harnesses, in order to reduce conduction and radiation interference, filtering should be added to the wiring harness. It is more convenient to sleeve suitable ferrite magnetic rings.
Reasonably plan the wiring harness: the wiring harness is arranged so that the low-power sensitive circuit is close to the signal source, and the high-power interference circuit is close to the load. Separate the low-power circuit and the high-power circuit as much as possible to reduce inductive interference and radiated interference between the wiring harness.
Improve equipment grounding: A good grounding arrangement and improved ground connection can reduce high-frequency impedance. The grounding of automotive electronic equipment is mainly connected to the vehicle body and the grounding of the wiring harness shielding layer.
Reduce the intensity of equipment receiving interference
Reduce the area of ​​equipment receiving interference: the wiring harness should be designed with the minimum length, minimum impedance and minimum loop area. It is best to use a power supply method with a small loop area such as twisted pair. Increase the distance from the equipment to the interference source: In the case of the same arrangement of the interference equipment, modify the installation location of the sensitive components and increase the distance to the interference source.
Experimental Research on Improvement Measures of Electromagnetic Compatibility
At present, electromagnetic compatibility simulation calculations are usually used to estimate the electromagnetic performance of the car body structure. The electromagnetic performance of automotive electronic equipment is mainly based on testing, so the improvement measures are focused on experimental research. According to the GB18655-2002 "Limits and Measurement Methods for Protecting the Radio Disturbance Characteristics of Automobile Receivers", the electromagnetic compatibility test was carried out on electronic equipment of a domestic commercial vehicle model, Comprehensive improvement measures are adopted, and the test results can compare the effects of various measures in the actual vehicle application.
Wiper motor structure adjustment and internal filtering
The wiper motor is a typical source of inductive load interference in the equipment, and the power is relatively large. It is measured using the component test method. The structure of the commutator of the motor is adjusted first, and the circuit is filtered inside the motor. Figures 4 and 5 are the results before and after the improvement, and the radio frequency interference has also been significantly improved.
Circuit filtering of flasher
The flasher is a typical contact-type device in automotive equipment, and it is frequently switched on and off during operation, which generates large conduction interference on the wiring harness and thus generates large radiation interference. By adding a 0.1mF capacitor near the flasher and putting a ferrite magnetic ring on the wire harness, a low-pass filter is formed to suppress its conducted interference while reducing radiated interference. The measurement adopts the vehicle test method. Figures 6 and 7 are the test results before and after the improvement, which reduces the interference level above 10MHz.
Conclusion
The electromagnetic compatibility of automotive electronic equipment is increasingly valued in China, and it is also very important for improving the competitiveness of domestic automotive products. The analysis of the interference sources of electronic equipment shows that the electromagnetic interference in the car is the main interference suffered by the equipment. In order to reduce the electromagnetic interference of the system, the improvement measures in the article need to be adopted to improve the electromagnetic compatibility performance of the automotive electronic equipment. Tests have shown that the improvement effects are all More obvious. For most electrical equipment, enhanced circuit filtering is a more general improvement measure.
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