System-level EMC design for electric vehicles

Abstract: Based on the analysis of the current status of vehicle EMC design, this paper uses the design and testing experience of a large number of components and vehicles as support, draws on the advanced design ideas of some foreign models, and proposes an electric vehicle system-level EMC development method from the perspective of EMC engineering design. This method has been successfully applied to various R&D models, changing the situation in which prototypes were difficult to pass EMC regulations, while ensuring EMC within the system.

　　introduction

　　In order for the onboard electrical components of electric vehicles to meet the corresponding EMC technical requirements, the reasonable arrangement of their internal components and wires should be considered, and corresponding testing and optimization work should be carried out. Since the electrical system of the whole vehicle is an integration of various electrical components and connecting cables, the mutual influence between the equipment increases the complexity of the electromagnetic environment, and it is difficult to correlate and analyze the component-level EMC test and the vehicle EMC test. At the same time, there may be large differences in functions, market positioning, system architecture and layout, electromagnetic characteristics of components, and integration of various models. It is difficult to give one or a set of unified quantitative indicators suitable for all electric vehicles.

　　In terms of EMC design and management, domestic electric vehicle factories generally have the following problems:

　　①EMC work is mainly carried out by EMC engineers, lacking collaboration within the system;

　　②EMC work mainly focuses on EMC testing of electrical components and complete vehicles, and EMC design is insufficient;

　　③ The EMC design of electrical components is out of touch with the EMC design of the entire vehicle, and the responsibility for EMC problems is almost entirely borne by the on-board electrical components;

　　④ The company has a short history, lacks professional EMC design experience, and lacks standardized EMC R&D and management processes.

　　This paper refers to the system-level electromagnetic compatibility design concept and draws on the excellent EMC design methods of foreign electric vehicles to propose a system-level EMC development method for electric vehicles. The system development process established by this method is implemented throughout the various vehicle development processes. The entire vehicle passes the EMC regulatory test at one time and achieves good compatibility within the system.

　　1. System-level EMC design ideas for electric vehicles

　　System electromagnetic compatibility issues are system engineering issues in terms of analysis methods, design methods, and test methods.

　　System-level EMC design ideas for electric vehicles: Comprehensively consider various factors such as electrical component performance and functional integrity, reliability, technical cost, lightweight body, product launch cycle, etc., determine the layout and technical control status, select materials, structures and processes, and implement grounding, shielding and filtering design ideas and specific measures into products or systems in the lowest cost and most effective way at each stage of vehicle development. Make detailed EMC test evaluation, optimization and management in the testing phase, and finally form a set of highly feasible forward development design methods or processes.

　　In the advanced product quality planning (APQP) process, the new product development process generally consists of five stages: plan definition and project, product design and development verification, process design and development verification, product and process confirmation, as well as feedback, evaluation and corrective measures. The APQP progress chart is shown in Figure 1.

　　Figure 1 APQP progress chart

　　Drawing on the APQP process, the electric vehicle system-level EMC development process may include: EMC planning stage, EMC system architecture layout stage, EMC design stage, EMC system testing and status freezing stage, and EMC evaluation, review and optimization stage.

　　The above stages require the collaborative participation of the chief vehicle designer, project manager, EMC experts, EMC engineers, electrical engineers, wiring harness engineers, general layout engineers, structural engineers, test engineers and electrical component suppliers to complete them together.

　　2. Electric vehicle system-level EMC design and development process

　　2.1 EMC planning stage

　　The work content of this stage is to study the contents listed in Table 1 based on the analysis of the first draft of the vehicle technical specification ( VTS ), focus on mastering the EMC characteristics of existing electrical components, and write reports such as the vehicle EMC design guide to provide important basis for the EMC system architecture layout.

　　Table 1 Main tasks in the EMC planning phase

 2.2 EMC system architecture layout stage

　　This stage is the most critical step in the vehicle system-level EMC development process. Its core work content can be summarized as "first build points from surfaces, and then connect points with lines."

　　The "surface" is the reference ground established by the vehicle body, body brackets, 12 V battery negative terminal, etc.

　　"Points" are on-board electrical components. Guided by reports such as "Guidance Design Report on Layout of High-Voltage Components" and "Guidance Design Specification for Layout of CAN Network Harnesses" compiled in the planning stage, the layout of key on-board electrical components is carried out by comprehensively considering the digital model of the vehicle body and the initial digital model of electrical components. Priority is given to the layout of power batteries; the general layout of the motor body is determined according to the drive mode, cooling system, installable position, center of mass coordinates, etc.; the layout of other electrical components is determined in combination with functional requirements, collision safety regulations, IP protection, installation convenience, aesthetics, etc. "Points" also include abstract grounding points, which are determined as the layout position of electrical components is confirmed. The selection of grounding points should be based on the principles of nearby grounding, reasonable system grounding network, and maintainability.

　　The "wire" is the interconnection cable between the "points" established above, and is an important part of the vehicle electrical system. The basic principles of cable layout are: as short as possible, avoid crossing, beautiful direction, and easy installation and fixation. Taking the cable layout under the chassis of the i-MiEV (see Figure 2) as an example, its characteristics of short cables and no crossing of cables are obvious.

　　Figure 2 Cable layout under the i-MiEV chassis (Internet data)

　　The strategy of giving priority to system layout is the most cost-effective EMC design method. Dividing the system layout makes it possible to control the interference current.

　　The EMC architecture layout of the whole vehicle needs to comprehensively consider various technical requirements and integrate EMC technology into the product architecture design. Figure 3 is a comparison diagram of the layout differences of a certain model of electric vehicles. Compared with Figure 3 (a), the layout scheme shown in Figure 3 (b) is more reasonable, the cable direction is more standardized, and the collision safety of the whole vehicle is also higher. There are large differences in the design of the housing of electrical components and the selection of connectors under the two layout schemes. This shows that if the "point" planning in the layout stage is unreasonable, it will lead to changes in the layout of the electrical system architecture of the whole vehicle, which will bring about significant changes in the design cost and listing cycle of the whole vehicle. In the early stage of vehicle design, it is not recommended to make a mold plan for all electrical components. At the same time, from the perspective of vehicle design, the "point" should also conform to the "surface" planning. Even if some electrical components have been molded in the early stage and are suitable for some models, they should be re-formulated after review according to the layout requirements of this model.

　　(a) Messy layout

　　(b) Neat layout

　　Figure 3 Comparison of layout differences of a certain model of electric vehicles (online data)

　　Figure 4 shows an unreasonable motor system (motor and motor controller) layout for a certain vehicle model. This layout causes the U, V, and W cables to be too long. Based on design experience, this solution has the risk of exceeding the radiation emission standard, and the EMC review failed, so the layout solution was not approved.

Reasonable layout is the most basic and has the highest economic efficiency. The increasing number of electronic communication devices in the car has greatly increased the layout density of the interconnection system. Coupled with the small internal space of the vehicle system, higher requirements are placed on the early system architecture layout.

　　Table 2 lists the main output reports of this phase.

 2.3 EMC Design Phase

　　Although EMC design is not a new technology, it requires a lot of support from professional design, manufacturing process and management knowledge, and must refer to all information, standards, specifications, rules and experience that can guide the team and employees to make decisions or take actions, and finally form a design knowledge system to guide production. The knowledge flow and conversion block diagram during the R&D process is shown in Figure 5.

　　Figure 5: Flow and transformation of knowledge in new product development

　　The EMC design phase mainly revolves around three EMC measures (i.e. grounding, shielding and filtering). The main design output report of this phase is shown in Table 3.

　　Table 3 Main output reports of EMC design phase

　　Figure 4: Unreasonable motor system layout of a certain model in the early stage

　　Reasonable layout is the most basic and has the highest economic efficiency. The increasing number of electronic communication devices in the car has greatly increased the layout density of the interconnection system. Coupled with the small internal space of the vehicle system, higher requirements are placed on the early system architecture layout. Table 2 lists the main output reports of this stage.

　　The grounding design mainly includes the process of grounding wire, the selection of grounding bolts and nuts, the anti-corrosion treatment process design of grounding points, etc. Figure 6 shows the grounding design details of a certain model of electric vehicle, which can be used as a reference.

　　(a) Grounding wire and grounding bolt

　　(b) Ground wire and ground nut

　　Figure 6 Grounding design of a certain model of electric vehicle (Internet data)

　　One of the keys to shielding design is the design of the housings of high and low voltage electrical components. How to skillfully combine industrial design and other technologies with housing shielding design technology to embody the perfect combination of EMC design technology and art is the difficulty of this part. Due to the high cost of housing mold making, it is recommended that a new mold be made after passing the review.

　　It should be pointed out that when selecting shielded cables, not only the shielding performance should be considered, but also the cost, mechanical strength and other characteristics. When the entire cable is subjected to excessive mechanical, weather and moisture effects, the most seriously affected shielding part is the connection, and the performance will usually drop by an order of magnitude (20dB) after 5 years of use.

　　For chassis with multiple cable entries, to ensure the continuity of the shield connection, the cable shield connection method can refer to Figure 7.

　　(a) Cable shield and housing termination

　　(b) Cable end connection fixture

　　Figure 7 Electrical connection between multiple cable shields and housing (Internet data)

　　(a) Motor body (b) Motor and integrated controller

　　Figure 8 Design of a certain model of electric vehicle motor system (Internet data)

　　Considering the cost, it is difficult to achieve perfect component shielding design, so system-level solutions can be considered. Figure 8 shows the design of a certain model of electric vehicle motor system. In order to reduce the radiation emission problem that may be caused by U, V, and W cables, a metal shielding box is added to the motor end, which improves the EMC design and the IP protection level.

　　2.4 EMC system testing and status freezing stage

　　System electromagnetic compatibility test technology includes: test specification formulation, standard formulation, project selection, implementation methods, site construction, error handling and other technologies and processes. In order to ensure the consistency of EMC testing, system testing must be carried out in a standard test environment. It is a good choice to establish a corresponding test environment or choose a test organization according to your own conditions. The practice of sacrificing the EMC performance of components or vehicles to save testing costs will inevitably pay a heavy price.

　　If it is separated from the vehicle test verification link, the EMC design of components is likely to be under-designed or over-designed. EMC system testing is an important link in the system-level EMC design process. It is used to verify the rationality of the vehicle EMC design and provide a basis for design optimization, review and freezing. On the premise of verifying the EMC design compliance of each electrical component, verify the correlation between the EMC test data of the component and the vehicle test data. According to the problems exposed in the vehicle test, first try to optimize and rectify the grounding measures in the vehicle system. Under the premise that the rectification effect is difficult to meet the vehicle test requirements, make targeted changes to the EMC indicators of the components. Confirm the proportion of component changes based on factors such as rectification convenience, cost, reliability, and development cycle, and ensure sufficient margin, so as to reduce errors caused by factors such as uncertainty and ensure the consistency of vehicle testing.

　　In the state freezing stage, it is necessary to randomly sample multiple electrical components from the same batch for testing. After the test data consistency review is passed, the EMC design of the components is frozen. Similarly, only when the vehicle test has sufficient consistency and margin can the vehicle EMC design data be frozen.

　　The main output reports of this stage are: "EMC test and analysis report of electrical components", "Vehicle test and analysis report", "System design optimization analysis report", "XX component EMC optimization design analysis report", "Grounding wire (including grounding bolts, nuts) salt spray and other test analysis report)", "Grounding wire impedance test report", "Grounding point anti-corrosion treatment process design review report", "Grounding point maintainability review report", "Electrical component housing digital model freezing report", "Electrical component EMC design plan freezing report", "XX vehicle model EMC design plan freezing report", etc.

　　2.5 EMC Assessment, Review and Optimization Phase

　　This stage runs through the entire process of system-level EMC design. The evaluation, review and optimization of each stage must ensure that the design of components and the design of the whole vehicle are synchronized to a certain extent. When evaluating and reviewing, it is necessary to consider design factors such as functional integrity, technological advancement, reliability, and safety, and technical guidance from EMC experts is also required. At the same time, other factors such as design aesthetics, maintainability, engineering feasibility, and cost must be comprehensively considered.

　　A simple and reasonable design is the best design. This will undoubtedly save costs, improve product yield, speed up time to market, and minimize the risk of EMC design for electric vehicles. Therefore, the principle of simplicity should be adhered to during the evaluation and review stages.

　　The power components of electric vehicles are increasingly showing a technical trend of miniaturization and integration, and the EMC design of power components will still be one of the important contents of the EMC design of the whole vehicle. In order to increase the driving range, the battery structure is enlarged, making the layout of the whole vehicle electrical system more compact, and the EMI problem between components is more prominent. The installation of intelligent and high-frequency electronic appliances has increased the difficulty of the whole vehicle passing the GB 14023 test, so the evaluation and review stage should also adhere to the principle of keeping pace with the times.

　　3. Conclusion

　　This article describes the vehicle system-level EMC design process in detail from the perspective of engineering application design, but does not specifically describe the design details and the quantification of EMC indicators, but the entire design process is still very clear. A systematic approach is adopted to organize various fuzzy and intertwined R&D activities in the R&D process according to a specific logic, minimize the repetition and coupling of R&D activities, and streamline the complex, fuzzy, and chaotic EMC R&D activities, thereby improving the efficiency and quality of EMC design work, shortening the development cycle, and reducing R&D costs and the total cost of the product life cycle.

　　In the process of designing and selecting power supply filters, system engineers found that adding filters had little effect and even caused the noise in certain frequency bands to increase.

　　Filter units are designed inside OBCs, but due to unprofessional design of the filter units (including matching of the filter output impedance and the OBC input impedance, unreasonable filter topology design, etc.) or unreasonable installation position due to limited layout space, the actual interference suppression capability of the filter is poor, and the phenomenon of conducted emission exceeding the standard is obvious.

　　4. Summary

　　The examples described in this article illustrate the importance of correct and reasonable design of grounding, shielding and filtering measures. At present, there are more and more electronic and electrical components in electric vehicles, and the electromagnetic environment established by the vehicle electrical system (including various electrical components, interconnecting cables and body structure, etc.) is becoming more and more complex. How to reasonably reflect EMC measures in the vehicle design based on the EMC characteristics of each electrical component and the electromagnetic environment in which it is located should be the focus and key of research.