EMC technology improvement for automotive electronics

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, leading to increasingly prominent electromagnetic interference problems, which may affect the normal operation of electronic equipment or even 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 in a car with a constantly changing driving environment. The complexity and variability of the electromagnetic energy in the environment means that the system is subject to a wide range of electromagnetic interference sources. According to the source of electromagnetic interference, it can be divided into external electromagnetic interference, body electrostatic interference and internal electromagnetic interference.

External electromagnetic interference

External electromagnetic interference refers to the interference that a car experiences when it is driving in various external electromagnetic environments. This type of interference exists in a specific space or at a specific time. For example, 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 devices in the environment will also generate interference when they are working, such as cars that are close to each other while driving.

Static electricity interference

The 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. The electrostatic discharge will form an interference current on the vehicle body and generate high-frequency radiation, which will cause electromagnetic interference to the vehicle's electronic equipment.

Electromagnetic interference in the car

Electromagnetic interference inside the car is the internal mutual interference when the car's electronic equipment is working, including electronic noise generated by electronic components, electromagnetic interference generated by commutation brushes during motor operation, and discharge interference when various switches are working. The most serious is the high-frequency radiation generated by the car's ignition system, which has the greatest interference energy.

The paths and principles of electromagnetic interference

Electromagnetic interference is classified according to the interference path, mainly into conducted interference, induced interference and radiated interference. 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.


Conducted interference circuit

Since the operating voltage of each device is

Formula

Therefore, any current change in one device will cause voltage changes in other devices, causing 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 induction interference and magnetic induction 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 electric induction 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-circuit circuit,, L1, L2 are the lengths of the two circuits, m0 is the vacuum magnetic permeability, and r is the distance between the two circuit wire segments. Therefore, increasing r and reducing the loop area can reduce M12. Radiated interference Radiated interference is emitted by the antenna. Since the energized 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

a 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 reduce I, l, or increase r.

In summary, the electromagnetic interference outside the car decreases as the distance increases. Only when its energy is very large can it affect the automotive electronic equipment far away. Years of research results show that the electromagnetic effect of large energy is harmful to human health. At present, various corresponding electromagnetic standards have been formulated to limit this type of interference, so that the impact on automotive electronic equipment is reduced.

The electrostatic interference of the vehicle body and the electromagnetic interference inside the vehicle are relatively strong because of the short interference distance, long interference time. Since the automotive electronic equipment forms an electrical network with the battery and the alternator as the core power source and the vehicle body as the common ground, the wiring harnesses of each part will conduct interference to each other through the power supply and ground wires, and there is inductive interference between adjacent wires, and there is also radiation interference between non-adjacent wires due to the antenna effect. This makes the interference inside the vehicle a combination of three channels, with more interference components and a wider interference frequency coverage. It 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 external electromagnetic interference, thereby reducing the possibility of equipment malfunction or damage.

Measures to Improve the Electromagnetic Compatibility of Electronic Equipment

The electromagnetic compatibility performance of automotive electronic equipment includes two aspects: electromagnetic emission, which measures the emission level of electromagnetic interference generated by the system; and electromagnetic sensitivity, which measures the ability of the system to resist electromagnetic interference in order to achieve the expected technical indicators when working. According to the previous analysis, to comprehensively improve the electromagnetic performance of automotive electronic equipment, three aspects can be considered: first, reducing the intensity of electromagnetic interference emitted by the equipment; second, suppressing the transmission of electromagnetic interference; and third, reducing 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 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 through the internal filtering circuit; the printed circuit boards of various electronic control units should optimize the wiring and reduce the level of electromagnetic emission.

Choose appropriate electronic components: For various control units in the car, using lower-frequency chips can help reduce radiation interference.

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

Suppressing interference transmission

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

Increase wiring harness filtering: For longer wiring harnesses, in order to reduce conducted and radiated interference, filtering should be added to the wiring harness. A more convenient method is to sleeve appropriate ferrite rings.

Reasonable planning of wiring harnesses: Arrange the wiring harnesses so that low-power sensitive circuits are close to signal sources, and high-power interference circuits are close to loads. Separate low-power circuits and high-power circuits as much as possible to reduce inductive interference and radiation interference between wiring harnesses.

Improve equipment grounding: 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 device

Reduce the area where the device receives 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 between the device and the interference source: Under the condition that the layout of the interference device 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 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 electromagnetic compatibility regulations for complete vehicles and parts GB18655-2002 "Limits and measurement methods for radio disturbance characteristics used to protect automotive receivers", an electromagnetic compatibility test was conducted on the electronic equipment of a domestic commercial vehicle model, and comprehensive improvement measures were adopted. The test results can compare the effects of various measures in actual vehicle application.

Structural adjustment and internal filtering of wiper motor

The wiper motor is a typical inductive load interference source in the equipment. It has a large power. We use the component test method to measure it. We first adjust the commutator structure of the motor and filter the circuit inside the motor. Figures 4 and 5 show the results before and after the improvement. The RF interference has also been significantly improved.

Structural adjustment and internal filtering of the wiper motor

Flasher circuit filtering

The 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 generates 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 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 sources of electronic equipment, it is shown that the electromagnetic interference in the car is the main interference to the equipment. In order to reduce the electromagnetic interference of the system, it is necessary to adopt the improvement measures in this article to improve the electromagnetic compatibility of automotive electronic equipment. Tests show that the improvement effect is quite obvious. For most electrical equipment, enhancing circuit filtering is a more common improvement measure.