Study of board immunity measures in PCB design
Electronic interference in automobiles is a major threat to the reliability of electronic circuits and also affects their ability to work. The presence of interference prevents the stable operation of electronic circuits, especially when the operating conditions are very difficult and the sources of interference are strong and varied. Therefore, interference and interference prevention are important considerations in the design process. First, we must ensure that it will not be interfered with by other products, and second, we must not interfere with the work of other products.
However, even the best product industries cannot achieve this goal, because it is almost impossible to completely avoid electrical noise and electromagnetic interference when working. However, noise and interference can affect the stable operation of the equipment, so anti-interference equipment must be installed after the formation of the equipment. This work is not only a waste of labor costs, and additional boards, complex multilayer boards can reach 10 or more layers. It is divided into ordinary circuit boards and flexible circuit boards. Printed circuit board design in automobiles is based on schematics to achieve the functions required by the circuit designer. Printed circuit board design mainly refers to layout design with various factors, such as external connection layout, optimized layout of internal electronic components, optimized layout of metal connections and vias, electromagnetic protection and heat dissipation.
2. Automotive PCB design in the circuit board anti-interference technology
2.1. Interference from the power grid
In the car, AC power is generated, and the DC power used in the circuit is basically obtained through rectification, filtering and stabilization. Therefore, the processing makes it easy to merge interference signals and add interference signals, which makes the system unstable.
2.2. Interference of signal channels
If measurement, control and communication are performed while separating two points, the input and output signal lines of the system will be long and the distance between the two lines will be relatively close to each other. When signals are transmitted within a single line, distortion of the transmitted signal can occur due to interference from the surrounding magnetic field, crosstalk between two similar signal lines, and interference to the ground line.
2.3. Interference from stray electromagnetic fields
When a stray electromagnetic field is generated in the vicinity of an amplifier circuit, the internal input circuit and certain components are placed in a constantly changing electromagnetic field, resulting in interference voltages. In the case of amplifiers, especially in the case of high amplification, if there is interference voltage in the first stage, even a small amount of interference voltage will appear in the final output after gradual amplification.
3. Automotive PCB design of the board anti-interference measures
3.1 Anti-grid interference measures
(1) AC voltage regulator. The main function of this device is to ensure the stability of the power supply, mainly to protect the power supply from overvoltage and undervoltage, and to ensure the stable operation of the entire system. According to the market, AC voltage regulators are not cheap, so the device is not an option for small electronic circuits, but large electronic circuits, or in the case of a clear statement of the need for strict protection against interference.
(2) Power supply filter. It is located in front of the transformer in the circuit, and its function is to emit only the 50Hz AC fundamental, while other high-frequency interference signals are blocked, thus greatly improving the power waveform.
(3) Power transformer with shield. There is usually a distributed capacitance between the primary and secondary coils. This is the most important channel for high frequency interference signals. If a metal shield is added here and the device is then grounded, the value of the distributed capacitance is greatly reduced. This also prevents interference from flowing into the secondary side.
(4) Double-T filter. It is undeniable that the presence of the filter effectively prevents the propagation of interference. In fact, it has been shown that the spectrum of electromagnetic interference from the source of interference is much wider than the received signal. That is, the receiver does not only receive what it wants to receive the purpose added capital costs and can affect product performance. Therefore, interference suppression solutions should be considered at the beginning of the design.
3.2. Anti-interference measures between the signal ground
(1) Single point grounding: This has the advantage of connecting all ground wires to one point. The advantage is that there is no loop ground loop or ground circulating current, and the circuit ground point is only related to the ground current and ground impedance of that circuit. Assuming that some circuits have very low current values and very low voltages between the ground wires, this type of circuit can use single point grounding at relatively short distances. Considering that the ground wire is relatively short and the voltage is relatively small, the mutual interference is much smaller.
(2) Series grounding: Series grounding means that all grounding points are connected to the ground wire in sequence. Due to the nature of the series connection, the common ground wire current is the sum of all connected ground wire currents. Therefore, the potential of each circuit is interfered with by the other circuits and noise is coupled through these common ground wires. At the interference protection level, this routing method is incorrect. However, this connection is very convenient and therefore also suitable for engineering. This is most obvious for printed circuit boards.
(3) Multi-point grounding: Studies have shown that the grounding bus is usually manufactured using wide silver-plated copper wires to reduce impedance. The specific method is to connect the grounding wire of all circuits to a nearby busbar, which has the advantage of effectively reducing impedance. The main component of the circuit is the printed circuit board, whose ground wire is usually connected to the rack bus, and one side of the bus is connected to the DC power supply ground wire. It is worth noting that this mode is only used for high frequency circuits.
(4) Analog and digital ground. Electronic circuit signals are diverse and include not only digital signals, but also analog signals. Digital circuits are often in a start/stop state, and the current often fluctuates significantly. Inevitably, interference will occur and A/D conversion will become diverse.
In recent years, more and more people are aware of the importance of printed circuit board design in automobiles, correct test schematics and test boards, even if the design of the printed circuit board is not correct, it can have a negative impact on the performance of the designed product. Therefore, in the design of automotive printed circuit boards, not only should consider the reasonable layout of the printed circuit board, wiring and pads, but also in the design of anti-interference and reliability measures. Based on the actual situation, we have summarized the reliability and interference countermeasures for printed circuit board design. However, in the actual design, different measures should be taken according to the requirements and specific circuits to minimize noise interference and ensure the reliability of the printed circuit boards in automobiles.