The basic requirements and methods of electromagnetic compatibility design in the field of automotive electronic equipment

In order to ensure that electronic products will not affect other equipment in the complex electromagnetic environment, and will not be affected by other equipment, it is necessary to carry out strict electromagnetic compatibility design in terms of structure, technology and other aspects at the early stage of equipment development.

1. Basic requirements for EMC design

Electromagnetic compatibility is one of the key performances of electronic equipment. The electromagnetic compatibility design should be carried out at the same time as the equipment function analysis.

The purpose of EMC design is to make the designed equipment realize EMC in complex electromagnetic environment. Therefore, when designing electromagnetic compatibility, the following requirements should be met: first, establish that the equipment meets the electromagnetic compatibility index, then clarify the sensitive components, interference sources and impact modes of the machine, take targeted measures, and finally master whether the equipment meets the requirements of electromagnetic compatibility index through experiments.

2. What is the method used in EMC design?

For communication vehicles, they usually carry a large number of machines, including power transformation equipment, communication equipment and terminal equipment. Interference signals are easily formed between each device, thus affecting the communication quality. Therefore, three factors (interference source, lotus root mode and sensitive equipment) should be considered when designing the electromagnetic compatibility of the equipment, and various effective measures should be taken to suppress the interference source, eliminate or weaken the influence of lotus root, and improve the anti-interference ability of sensitive equipment.

For example, the automotive electronic equipment is composed of digital ammeter, digital voltmeter, switch switch, circuit breaker, operation and maintenance module, mutual inductor, contactor and other modules and components. The digital ammeter, digital voltmeter, switch switch and circuit breaker are arranged on the front panel, and the operation and maintenance module, mutual inductor, contactor and other modules and components are placed in the chassis. The equipment shall meet the requirements of GJB151A-97 related electromagnetic compatibility indicators. The main measures taken in the overall design are: shield the instrument window; Clearance of shielding case; Reasonably configure and shield each module; Cable laying and power line filtering.

Figure 1. Shielded instrument window

2.2 Shielding of chassis gap

The seam in the shield is an important factor affecting the integrity of the shield. The structure of the vehicle-mounted electronic equipment is formed by bending and welding of aluminum plates. The welding is smooth and continuous and belongs to permanent joint. The RF resistance at this interface is almost the same as that of the metal sheet itself, thus ensuring the electrical continuity at the shield connector. Screw tightening method is usually used for removable joints, such as the joints between the chassis and the cover plate, because the gap between the screws should not be too small, the unevenness of the joint surface and the warping deformation of the cover plate material make the joint surface inevitably form a gap, which reduces the shielding efficiency of the chassis. Therefore, there are two ways to solve this problem: increase the clearance depth. In order to improve the gap depth, the total width of the curved edge of the chassis is 15. Mm, the more overlapping specifications, the better shielding efficiency; Reduce the gap length because it is difficult for the sheet metal chassis to achieve high accuracy of the connecting surface. In order to make up for this shortcoming, an economical and easy method is adopted, and a bronze reed with adhesive is pasted on the connecting surface. Due to the elasticity of the spring plate, the spring plate is deformed after installation, and the contact surface generates certain pressure, which makes the connection surface have certain electrical continuity.

2.4 Adoption and laying of cables

Because the cable is an efficient electromagnetic wave receiving and radiation antenna, it is an excellent channel that affects the transmission. The key to EMC of most machines is cable. One of the important ways to deal with the cable problem is to shield the cable. Therefore, the equipment selects cables with good quality (low impedance) and ensures the 360 degree of cable shielding layer and chassis. Low-impedance lapping makes the shield layer and the case form a complete shield, which can solve the cable radiation problem to a certain extent. In addition, when installing wires, the power distribution line and other lines should be kept at 150. Mm spacing, sensitive circuit and influence circuit shall be laid separately without overlapping, and harness spacing shall be increased to prevent coupling between cables.

2.5 Power line filtering

In order to prevent the influence of high-frequency interference signal at the power input end on the system, EMC power line filter is installed. The filter is different from other electronic components, and its characteristics are closely related to its installation mode. Therefore, a series of countermeasures have been taken for the installation mode of the filter. As shown in Figure 2, first of all, keep away from the input and output lines of the filter to prevent the high frequency filtering effect from decreasing due to the coupling of both sides; Secondly, the low impedance contact between the filter housing and the chassis, and the connection line between the power port and the filter should be shortened. When the current enters the chassis, the filter first flows into the filter for filtering, and then filters along the ports of other filters. The final power signal will also cause the mailbox.