Anti-jamming design of a vehicle driving recorder

• Electromagnetic compatibility design of automotive electrical appliances
• Anti-interference design of car drive recorder
• Anti-interference problem needing attention in circuit board design

• The hardware structure of the car tachograph
• Anti-interference design of the power supply part

Source of interference on the car

There are various loads on the automotive electrical system, including small impedance, high current resistance to the sexy load, small current, high voltage pulse generating device, and high frequency oscillating signal source, which are not only externally interfered with the emission. Source is also a source of interference for automotive electronics. In addition, due to the high maneuverability, the car may also be in various imaginable complex electromagnetic fields from low frequency to high frequency. The resulting electromagnetic interference coupling also affects the normal operation of the automotive electrical and electronic system. The voltages in automotive electrical systems can be grouped into the following categories: normal operating voltage, abnormal steady-state voltage, radio interference voltage, transient overvoltage, and electrostatic discharge.

Electromagnetic compatibility design of automotive electrical appliances

The electromagnetic compatibility environment of automotive electrical equipment should be an environment where equipment coexists and does not interfere with each other, which requires the system to have good EMI and EMS characteristics. Electromagnetic interference that causes degradation or failure of electrical functions must have three elements at the same time: interference sources, interference coupling paths, and sensitive equipment. Suppressing sources of interference, blocking coupling, and increasing the immunity threshold of sensitive devices are fundamental measures to address electromagnetic compatibility issues.

1 transmission and transmission of electromagnetic interference

The occurrence of electromagnetic interference must have the propagation and propagation channels of interference energy. There are two basic ways to transmit interference: conduction and radiation. Radiative coupling is subdivided into: antenna-to-antenna coupling, field-to-line coupling, and line-to-line coupling. For the propagation and coupling of interference, the following system methods should be adopted in the practice of automotive electrical engineering to improve EMC characteristics: filtering, shielding, grounding and wiring.

2 electromagnetic compatibility design of interference sources and sensitive equipment

In the functional circuit that has been determined by the scheme, check whether the electromagnetic compatibility index meets the requirements; if the requirements are not met, the parameters can be modified to achieve the index, such as adjusting the working frequency of the digitizing controller, the rising rate of the rounding pulse, or re- Select components and so on. Secondly, the protection design, including filtering, shielding, grounding and lap design, and even the improvement measures such as space-time isolation and frequency avoidance. Finally, the layout adjustment design, including the inspection of the overall layout, the inspection of the gap of the shield, the inspection of the components and the printed board layout. The electromagnetic compatibility design of the circuit and subsystem includes the following steps: component selection, circuit selection, filtering technique application, grounding design, shielding design, circuit layout, and system layout planning.

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Figure 1 Schematic diagram of the hardware structure of the car tachograph

3ESD protection design

In order to eliminate the danger of electrostatic discharge, the measures that can be taken include: establishing a perfect shielding structure, releasing the static charge to the ground through the metal shell of the ground iron; the connection between the internal circuit and the metal shell should be a little grounding; increasing such as silicon transient A fast protection element such as a voltage snubber diode (STVS) bleeds high-voltage charge to ground; a protective circuit strip is added to the printed circuit board design to discharge the charge from the hand-pull board to the ground through the shortest path.

Figure 2 switching power supply circuit

Anti-interference design of car drive recorder

1 car tachograph hardware structure

Figure 1 shows the hardware structure of the car tachograph. The anti-interference processing of the signal is achieved by photoelectric isolation. The power system of the vehicle equipment has a great influence on the reliable operation of the equipment. A good power circuit can filter out many interference signals that are transmitted through the power line.

2 anti-interference design of the power supply part

The power supply circuit of this control module is shown in Figure 2. The vehicle battery-generator power supply enters from 24V_1, and the diode D16 mainly prevents the positive and negative poles of the power supply from being mistakenly connected. The inductors L1 and L2 filter the power supply, and cooperate with the common mode inductor L3 and the capacitor C8 to filter out spikes in the power supply. Diode D24P6KE51A is a pulse interference suppressor (TVS), which can quickly absorb spikes exceeding the rated voltage. The instantaneous current is very large, and the maximum power can reach 500~1000W. Capacitors E5 and C9 further filter the power supply. The filtered current is converted to the 5V power supply used by the system through the switching power supply chip LM2576. Capacitors E7 and C11 filter the output 5V supply, and resistor R18 and LED D28 indicate whether there is current power.

Figure 3 DC / DC power circuit

In some power systems with higher reliability requirements, DC/DC modules with a wide input voltage range can be considered for power conversion, as shown in Figure 3. The input and output of this DC/DC module are completely isolated and have a wide input range. The allowable input range for DC/DC power supplies with a nominal input voltage of 12V is 9 to 18V; the allowable input range for DC/DC power modules with a nominal input voltage of 24V is 18 to 36V.

Both of these circuits have been tested in practical applications, and it has been proven to be stable and reliable on both gasoline and diesel vehicles.

Anti-interference problem needing attention in circuit board design

The wiring of the circuit board has a great influence on the anti-interference performance of the system. The wiring of this system is mainly considered from the following points.

● Try to thicken the power cable and ground wire. Use large storage capacitors at the power inlet and lay copper on the circuit board. This will greatly enhance the anti-interference performance of the circuit board.

● Add decoupling capacitors at the power supply and ground terminals of the chip. The capacitance value is 0.01~0.1μF. It is best to use ceramic capacitors, which must be added at each chip. This is very important.

● Add a decoupling capacitor to the IO pin to filter out a large amount of external interference.

● Minimize the wiring length of high-frequency signal lines, especially the clock signal lines. Due to the high frequency, the longer the wiring, the stronger the electromagnetic interference. At the same time, try to reduce the length of other signal lines.

● Use surface mount chips as much as possible, so that not only the circuit length can be shortened, but also looseness due to vibration or shock.