Basic principles of EMC design for automotive wiring harnesses

【Abstract】 Through the analysis of the principle of electromagnetic interference of automobile wiring harnesses, based on the principles of three wiring harness interference coupling modes, the EMC design principles that need to be followed in the design of automobile wiring harnesses are given, and some design principles are simulated by CST simulation software to verify the consistency between the simulation results and the design principles, providing a basis for the electromagnetic compatibility design of the entire vehicle. Finally, through the analysis of actual cases, the EMC design principles of the entire vehicle wiring harness are verified again.

Driven by the global trend of electrification of automotive power, intelligent control, and information networking, intelligent networked vehicles have become the forefront of the international automotive engineering field and the core of future market competition. The application of new electronic technologies has led to increasingly complex electromagnetic environments inside and outside the vehicle, which not only affects the EMC regulatory performance of the vehicle, but also affects the reliable implementation of intelligent driving and networked communication functions, and also poses a severe test to the electromagnetic compatibility of the entire vehicle.

As the carrier for connecting all electronic and electrical components of the vehicle and the electrical components to the vehicle body nodes, the automotive wiring harness is usually the transmission path of interference in electromagnetic interference. Therefore, whether the layout and routing of the wiring harness conform to the EMC design principles will directly affect the EMC performance of the vehicle, and in serious cases, it will lead to vehicle regulations not passing, functional failure, etc. Therefore, when designing the vehicle wiring harness, it is necessary to clearly define which EMC design principles need to be followed.

1.1 Common impedance coupling

When two or more wiring harness loops pass through the same conductor, the impedance in the conductor is called common impedance. As shown in Figure 1, the voltage generated on the common impedance is affected by all loop currents flowing through the conductor, and the voltage will also affect the voltages in other loops, thus forming common impedance coupling. Simple common impedance coupling is shown in Figure 1. The voltage Us formed on the common impedance ZG is affected by the currents on circuits 1 and 2 at the same time. At the same time, the voltage Us will also affect the loop voltages of circuits 1 and 2. When circuit 1 is the wiring harness of the sensitive source and circuit 2 is the wiring harness of the interference source, the noise current generated by circuit 2 will be coupled to the wiring harness of circuit 1 through the common impedance ZG, causing interference to the sensitive source of circuit 1. In the case of DC, the common impedance is very small and can be ignored. However, due to the existence of parasitic parameters, as the frequency increases, the common impedance will become larger and larger, and will change with frequency.

1.2 Capacitive coupling

An electric field will be generated between two circuit systems or wires with a potential difference, thereby generating parasitic capacitance. This parasitic capacitance becomes the path for the transmission of high-frequency interference signals between conductors, that is, capacitive coupling is formed. Figure 2 is a typical capacitive coupling equivalent model composed of parallel wires. Assuming that the circuit where wire 1 in Figure 2a is located is the interference source circuit, wire 2 is the interfered circuit, there are loads Z21 and Z22 on wire 2, C12 is the distributed capacitance between wire 1 and wire 2, C1 and C2 are the grounding parasitic capacitances of wire 1 and wire 2, and U2 is the interference voltage induced on wire 2. The equivalent circuit of the capacitive coupling model is shown in Figure 2b.

The relationship between the voltage U2 generated on wire 2 and the interference source voltage U1 is:

Based on the mechanism of electromagnetic interference, when designing automotive wiring harnesses, in order to better improve the electromagnetic compatibility of the entire vehicle, the following principles generally need to be followed.

1) In the wiring harness design, the distance between two metal conductors affects the parasitic capacitance and mutual inductance between the conductors. The farther the distance, the smaller the capacitance and mutual inductance. Therefore, when arranging the interference source harness, it should be kept away from sensitive components and their connecting harnesses, and arranged separately as much as possible. The distance between the connecting harness of the interference source and the connecting harness of the sensitive component should be no less than 100mm. When the connecting harness of the interference source cannot be far away from the connecting harness of the sensitive component, the two (geometric shape) should be arranged vertically and crosswise. If this cannot be achieved, a certain angle should be maintained as much as possible.

2) The wiring harness should not be placed far away from the metal body. The wiring harness should be no more than 100mm away from the metal structure of the body. In principle, the closer the better. The metal body is connected to the negative pole of the battery and can be used as the "0V" reference ground plane of the vehicle electrical system. If it is placed close to the body, the shielding and grounding of the metal body can be used to protect the wiring harness and reduce interference. At the same time, it can also reduce the external radiation generated by the wiring harness itself. Therefore, the wiring harness should be placed in the angle or groove of the metal body, or close to the metal body.

3) In order to reduce the impact of electromagnetic radiation coupling, the area of ​​the current loop of the interference source and sensitive components (as shown in the shaded part of Figure 5) and the length of the wiring harness should be minimized as much as possible. This is also the most basic principle for reducing electromagnetic radiation coupling when designing the wiring harness of the entire vehicle. An example is shown in Figure 5. In the same current loop, the power line and the grounding line need to be arranged in parallel to reduce the loop area of ​​the current, reduce the electromagnetic radiation generated by the wiring harness, and reduce the coupling between external interference and the wiring harness, thereby enhancing the anti-interference ability and reducing the intensity of external radiation.

2.2 High current wiring harness design principles

High-current wiring harnesses are interference sources. For the connection harnesses of high-current components, the following principles are generally followed.

1) A high current conductor will form a strong magnetic field around it, as shown in Figure 6. Generally, the vehicle body cannot be used as a current loop for high current components. If the vehicle body is used as a current loop, the positive power line should be placed close to the vehicle body to reduce the area of ​​the current loop.

2) The layout distance between the load and the power supply of the high-current loop should be as short as possible. At the same time, the positive and negative power lines should be routed in parallel to avoid the positive and negative power lines running in different wiring harness branches, thereby reducing the loop area and reducing the external radiation of the high-current wiring harness.

3) High-current harnesses should be arranged separately to avoid being in the same harness as harnesses of sensitive components. If this cannot be avoided, the length of the common lines must be kept as short as possible. At the same time, shielded wires or twisted pair wires should be used for harnesses of sensitive components, or high-current harnesses should be bundled separately and then bundled together with other harnesses, or shielded wires should be used for high-current harnesses to reduce coupling risks.

2.3 RF signal harness design principles

The following principles should be followed for wiring harnesses that pass RF signals.

1) The wiring harnesses of RF components and those of strong interference components are bundled in different wiring harnesses.

2) Do not place strong electrical interference components and their wiring harnesses near wiring harnesses that pass radio frequency signals.

3) Do not place high-current components (such as wipers, blowers, etc.) near the RF antenna.

2.4 Layout principles of twisted pair cables

Twisted pair is a general wiring made of two mutually insulated wires twisted together (usually clockwise) according to certain specifications. It is recommended to use twisted pair for connecting sensitive components, such as engine oxygen sensor, knock sensor, CAN bus, etc.

The twisted pair model is shown in Figure 7. In a twisted pair, the induced electromotive force caused by the change in magnetic flux generated by the electromagnetic disturbance current in two adjacent loops tends to cancel each other out, thereby weakening the influence of the electromagnetic field and reducing inductive coupling, which is effective in reducing electromagnetic emission and improving anti-interference capabilities. The more times the twisted pair is twisted per unit length, the better the effect of canceling the induced electromotive force generated by electromagnetic disturbance. Therefore, the more times the twist is twisted per unit length, the better the effect of reducing inductive coupling.