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 may affect the normal operation of electronic equipment or damage the corresponding electrical components. Therefore, the electromagnetic compatibility performance of automotive electronic equipment has received more and more attention, and it is currently urgently required to widely apply electromagnetic improvement technology for automotive sub-equipment.

Sources of electromagnetic interference

Automotive electronic equipment works on cars with constantly changing driving environments. The complexity and variability of the electromagnetic energy composition in the environment means that the sources of electromagnetic interference to the system are relatively wide. According to the source classification of electromagnetic interference, it can be divided into external electromagnetic interference, body electrostatic interference and internal electromagnetic interference.

External electromagnetic interference

External electromagnetic interference is the interference that a car experiences when it is driving through various external electromagnetic environments. This type of interference exists in a specific space or a specific time. Such as electromagnetic interference from high-voltage transmission lines, high-voltage substations and high-power radio transmission stations, as well as electromagnetic interference from lightning and sunspot radiation, etc. Other nearby electronic equipment in the environment will also cause interference when working, such as cars that are close to each other while driving.

Electrostatic interference of

the vehicle body Electrostatic interference of the vehicle body is related to both the vehicle and the external environment. As the vehicle body rubs against the air at high speed when the vehicle is driving, unevenly distributed static electricity is formed on the vehicle body. Electrostatic discharge will form interference current on the vehicle body and generate high-frequency radiation, which will cause electromagnetic interference to the vehicle electronic equipment.

Electromagnetic interference in the

vehicle Electromagnetic interference in the vehicle is the internal mutual interference of the vehicle electronic equipment when it is working, including the electronic noise generated by electronic components, the electromagnetic interference generated by the commutation brush during motor operation, and the discharge interference when various switches are working. The most serious is the high-frequency radiation generated by the vehicle ignition system, which has the largest interference energy.

Paths and principles of electromagnetic interference

Electromagnetic interference is classified according to the interference path, mainly into conducted interference, inductive interference and radiated interference, and the corresponding interference principles are as follows.

Conducted interference

Conducted interference is mainly transmitted through the common conductor of the circuit. The typical structure is the common power line and the common ground line. 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 operating voltage of each device is

Therefore, any current change of any device will cause voltage changes of other devices, resulting in interference. To reduce the mutual influence between devices, it is necessary to reduce the R, Z and I values.

Inductive interference

Inductive interference is divided into two types: electric inductive interference and magnetic inductive interference. The basic circuit diagrams are 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 capacitances between wire 1, wire 2 and the ground, M12 is the mutual inductance between the two circuits, and R is the resistance of each circuit.

For inductive circuits, to reduce U2, you can reduce C12, U1 and R, or increase C2g; the first measure is to reduce C12 by increasing the distance between the wires or changing the dielectric parameters between the wires. For magnetic induction circuits, to reduce U2, you can reduce M12 or reduce the rate of change of I1. The basic measure is to reduce M12. For a typical two-loop circuit, L1 and L2 are the lengths of the two loops, m0 is the vacuum magnetic permeability, and r is the distance between the two loop wire segments. Therefore, increasing r and reducing the loop area can reduce M12.

Radiated interference

Radiated interference is emitted by antennas. Since powered 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 equations, the radiated electromagnetic field of a typical monopole antenna is is




spherical coordinates, 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 reduce I, l, or increase r.

In summary, the electromagnetic interference outside the car decreases with the increase of the action distance. Only when its own energy is very large can it affect automotive electronic equipment that is far away. Years of research results have shown that the electromagnetic effect of high energy is harmful to human health. At present, various corresponding electromagnetic standards have been formulated to limit this type of interference, so that automotive electronic equipment is less affected by it.

Body electrostatic interference and in-vehicle electromagnetic interference, because the interference action distance is short and the interference time is long, the interference intensity is relatively large. Since automotive electronic equipment forms an electrical network with batteries and alternators as the core power source and the car body as the common ground, each part of the wiring harness will conduct interference to each other through the power supply and ground wire, and there will be inductive interference between adjacent wires, and non-adjacent wires will also radiate interference due to the antenna effect. This makes the interference in the car a combination of three ways, with more interference components and a wider interference frequency coverage, which is the main electromagnetic interference to automotive electronic equipment. Solving these two electromagnetic interference problems can simultaneously improve the anti-interference ability of automotive electronic equipment to electromagnetic interference outside the car, thereby reducing the possibility of equipment malfunction or damage.

Measures to improve the electromagnetic compatibility of electronic equipment

The electromagnetic compatibility of automotive electronic equipment includes two aspects: one is electromagnetic emission, which measures the emission level of electromagnetic interference generated by the system; the second is electromagnetic sensitivity, which measures the ability of the system to resist electromagnetic interference when working to achieve the expected technical indicators. According to the previous analysis, 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; and the third is to reduce the intensity of interference received by electromagnetic sensitive components of the equipment.

Reduce the electromagnetic interference intensity of the equipment

Optimize the electrical structure of the equipment: The flasher in the automotive electronic equipment is a relay contact structure, and an arc suppressor can be added before the contact; the motor is an inductive load, and the current noise can be reduced by the internal filter circuit; the printed circuit boards of various electronic control units should optimize the wiring to reduce the electromagnetic emission level.

Choose suitable electronic components: The various control units on the car use lower frequency chips to help reduce radiation interference.

Reduce the power of the equipment: Under the condition of meeting the functional requirements, reducing the power of the equipment can reduce the interference voltage and current, thereby reducing the interference intensity.

Suppress the transmission of interference

Shield the interference source equipment and related wiring harnesses: The electronic control units used in the main electronic control systems in the car should be encapsulated in a shielded shell.

Increase wiring harness filtering: For longer wiring harnesses, in order to reduce the conduction and radiation interference, filtering should be added to the wiring harness. It is more convenient to sleeve a suitable ferrite magnetic ring.

Reasonable planning of wiring harnesses: In the wiring harness layout, the low-power sensitive circuit is close to the signal source, the high-power interference circuit is close to the load, and the low-power circuit and the high-power circuit are separated as much as possible to reduce the inductive interference and radiation interference between the wiring harnesses.

Improve the grounding of the equipment: Good grounding layout and improved ground wire bonding can reduce high-frequency impedance. The grounding of automotive electronic equipment is mainly connected to the vehicle body and the wiring harness shielding layer.

Reduce the intensity of interference received by the equipment

Reduce the area of ​​interference received by the equipment: 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: Under the condition that the layout of the interference equipment remains unchanged, modify the installation position of the sensitive components to increase the distance to the interference source.

Experimental study on electromagnetic compatibility improvement measures

At present, electromagnetic compatibility simulation calculations are usually used to make a preliminary estimate of the electromagnetic performance of the vehicle body structure. The electromagnetic performance of automotive electronic equipment is mainly based on tests, so the improvement measures are focused on experimental research. According to the electromagnetic compatibility regulations for complete vehicles and parts GB18655-2002 "Limits and measurement methods for radio disturbance characteristics used to protect automotive receivers", the electromagnetic compatibility test of the electronic equipment of a domestic commercial vehicle model was carried out, and comprehensive improvement measures were adopted. The test results can compare the effects of various measures in actual vehicle use. Structural

adjustment and internal filtering of wiper motors

The wiper motor is a typical inductive load interference source in the equipment, with high power. It is measured by component testing. The commutator structure 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 RF band interference has also been significantly improved.

Circuit filtering of flasher

Flasher is a typical contact-type device in automotive equipment. It is frequently turned on and off during operation, which generates large conducted interference on the wiring harness and thus large radiated interference. By adding a 0.1mF capacitor near the flasher and putting a ferrite ring on the wiring harness, a low-pass filter is formed to suppress its conducted interference and reduce radiated interference. The measurement adopts the whole vehicle test method. Figures 6 and 7 are the test results before and after the improvement. The interference level is reduced above 10MHz.

Conclusion

The electromagnetic compatibility performance of automotive electronic equipment is increasingly valued in China, and it is also very important to improve the competitiveness of domestic automotive products. Through the analysis of the interference source of electronic equipment, it is shown 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, it is necessary to adopt the improvement measures in the article to improve the electromagnetic compatibility performance of automotive electronic equipment. The test shows that the improvement effect is relatively obvious. For most electrical equipment, enhancing circuit filtering is a relatively common improvement measure.