The loads in automotive electrical systems are diverse, ranging from low-impedance, high-current resistive loads to low-current, high-voltage pulse generators and high-frequency oscillation signal sources. These are not only potential sources of external interference but also sources of interference to in-vehicle electronic products. Furthermore, due to high mobility, automobiles may be exposed to various imaginable complex electromagnetic fields, from low to high frequencies, and the resulting electromagnetic interference coupling can also affect the normal operation of automotive electronic and electrical systems. Voltages within automotive electrical systems can be categorized as follows: normal operating voltage, abnormal steady-state voltage, radio interference voltage, transient overvoltage, and electrostatic discharge. Electromagnetic Compatibility Design of Automotive Electrical Systems: The electromagnetic compatibility environment for automotive electrical systems should be one where devices coexist without interference, requiring the system to possess good EMI and EMS characteristics. Electromagnetic interference causing functional degradation or failure of electrical equipment requires the simultaneous presence of three elements: an interference source, an interference coupling path, and sensitive equipment. Suppressing the interference source, blocking coupling, and increasing the immunity threshold of sensitive equipment are fundamental measures for solving electromagnetic compatibility problems. 1. Transmission and Transmission Path of Electromagnetic Interference: The occurrence of electromagnetic interference necessarily involves the propagation of interference energy and a propagation path. Interference transmission occurs in two basic ways: conduction and radiation. Radiative coupling is further subdivided into antenna-to-antenna coupling, field-to-line coupling, and line-to-line coupling. In automotive electrical engineering practice, the following systematic methods should be adopted to improve EMC characteristics in response to interference propagation and coupling paths: filtering, shielding, grounding, and wiring. 2. Electromagnetic Compatibility Design of Interference Sources and Sensitive Equipment In the functional circuit of the established scheme, verify whether the electromagnetic compatibility indicators meet the requirements. If not, the requirements can be met by modifying parameters, such as adjusting the operating frequency of the digital controller, the rise rate of the rounded pulse, or reselecting components. Next, conduct protection design, including filtering, shielding, grounding and connection design, and even adopting improvement measures such as spatiotemporal isolation and frequency avoidance. Finally, perform layout adjustment design, including verifying the overall layout, the gaps in the shielding, and the layout of components and printed circuit boards. The electromagnetic compatibility design of circuits and subsystems includes the following steps: component selection, circuit selection, application of filtering technology, grounding design, shielding design, circuit layout, and system layout planning. [align=center]Figure 1 Hardware Structure Schematic Diagram of Car Driving Recorder[/align] 3 ESD Protection Design To eliminate the hazards of electrostatic discharge, the following measures can be taken: establish a complete shielding structure to release static charge to ground through a grounded metal shell; the connection between the internal circuit and the metal shell should use a single-point ground; add fast protection components such as silicon transient voltage absorption diodes (STVS) to discharge high-voltage charge to ground; add a protective ring in the printed circuit board design to discharge the charge from manual plugging and unplugging of the circuit board to ground through the shortest path. [align=center]Figure 2 Switching Power Supply Circuit[/align] Anti-interference Design of Car Driving Recorder 1 Hardware Structure of Car Driving Recorder Figure 1 shows the hardware structure principle of the car driving recorder. Signal anti-interference processing is achieved through opto-isolation. The power supply system of the vehicle equipment has a great impact on the reliable operation of the equipment. A good power supply circuit can filter out many interference signals transmitted through the power line. 2 Anti-interference Design of Power Supply Section The power supply circuit of this control module is shown in Figure 2. The vehicle battery-generator power supply enters from 24V_1. Diode D16 mainly prevents the positive and negative terminals of the power supply from being mistakenly reversed. Inductors L1 and L2 filter the power supply, and together with common-mode inductor L3 and capacitor C8, they can filter out power spikes. Diode D24 P6KE51A is a TVS (Transient Voltage Suppressor), which can quickly absorb voltage spikes exceeding the rated voltage. It can handle a large instantaneous current, with a maximum power of 500-1000W. Capacitors E5 and C9 further filter the power supply. The filtered current is converted into the 5V power supply used by the system by the switching power supply chip LM2576. Capacitors E7 and C11 filter the output 5V power supply, and resistor R18 and LED D28 indicate whether there is power. [align=center] Figure 3 DC/DC power supply circuit[/align] In some power supply systems with higher reliability requirements, a DC/DC module with a wide input voltage range can be considered for power conversion, as shown in Figure 3. This type of DC/DC module features complete input and output isolation and a wide input range. The allowable input range for a DC/DC power supply with a nominal input voltage of 12V is 9–18V; the allowable input range for a DC/DC power supply module with a nominal input voltage of 24V is 18–36V. Both circuits have undergone practical application testing and have proven to be stable and reliable in both gasoline and diesel vehicles. Regarding anti-interference issues in circuit board design, the wiring of the circuit board significantly impacts the system's anti-interference performance. The wiring of this system is mainly considered from the following points: ● Use thicker power and ground lines as much as possible, use large energy storage capacitors at the power input, and lay copper foil in the empty spaces of the circuit board. This will greatly enhance the anti-interference performance of the circuit board. ● Add decoupling capacitors at the power and ground terminals of the chips. The capacitance value should be 0.01–0.1μF, preferably ceramic capacitors. Adding these at each chip is crucial. ● Add a decoupling capacitor at the IO pins to filter out a large amount of external interference. ● Minimize the length of high-frequency signal lines, especially clock signal lines. Due to their high frequency, longer lines generate stronger electromagnetic interference. Also, minimize the length of other signal lines. ● Use surface-mount chips whenever possible. This not only shortens circuit length but also prevents loosening due to vibration or impact.