PCB is the support body of electronic components, is the electrical connection provider of electronic components, and is the most basic component of various electronic equipment. With the rapid development of microelectronics technology, various electronic products often work together, and the interference between them is more and more serious. The design of printed circuit board has a great impact on the interference and anti-interference ability of the circuit, and its performance is directly related to the quality of electronic equipment. This paper introduces the anti-interference measures that can be taken in the design and configuration of printing plates, and discusses in detail the methods of improving the reliability of the equipment and realizing electromagnetic compatibility.
1. printed circuit board (also known as printed board) is the MCU system of devices, signal lines, power lines of the high-density assembly, printed circuit board design is good or bad against interference ability to greatly affect, so printed circuit board design is not only the simple layout of the device, line, but also must meet the anti-interference design principles. The anti-interference design of printed circuit board not only directly affects the reliability of electronic products, but also relates to the stability of products. Based on the principle of electromagnetic compatibility, the noise suppression of the printed board should start from the design stage, and run through a series of links such as circuit schematic design, printed board schematic design, component selection, and printed board installation leads. The following design principles can achieve the purpose of reducing or eliminating the common interference in the design of printed plates, and improving the reliability and electromagnetic compatibility of the equipment.
2. the wiring method of the printed board has a direct impact on the anti-interference performance.
2.1. If the working speed of the logic circuit on the printed board is lower than the TTL speed, the shape of the wire strip has no special requirements; If the high-speed logic device is used, and the working speed is high, the copper foil used as a wire will make the impedance of the wire discontinuous at the 90 degree bend, which may cause reflection interference. The 90 degree wire is bent to 45 degrees, which helps to reduce the reflection interference.
2.2. do not leave a blank copper foil layer on the printed board. The blank copper foil layer can act as a transmitting or receiving antenna, so they can be grounded.
2.3. double-sided wiring of the printed board, should make the two sides of the lines vertical cross, to reduce the magnetic field coupling, conducive to the suppression of interference.
2.4. the distance between the wires should be as large as possible. For the signal loop, the distance between the printed copper chaff strips should have a certain size, and this distance should increase with the increase of the signal frequency. Pay more attention when the frequency is very high or the pulse front is very steep. Because only in this way can the effect of distributed capacitance between the wires be reduced.
2.5. high voltage or high current lines are easy to interfere with other lines, while low level or small current signal lines are susceptible to inductive interference. Therefore, when wiring, the two should be kept away from each other as far as possible, avoid parallel laying, and adopt shielding measures.
2.6. Use isolated cabling. In the wiring of many circuits that have to walk the line, the method can be considered first, that is, two signal lines plus a grounded isolation line.
2.7. for the signal line that is easy to accept interference on the printed board, do not lay parallel to the line that can cause interference or transmit interference within a long range. If necessary, a ground wire can be set between them to achieve shielding.
2.8. all lines as far as possible along the DC laying, as far as possible to avoid laying along the AC laying.
2.9. Short wiring. In the case that the line cannot be arranged or can only be routed around a large circle, it is simply connected with an insulated "fly wire" without a printed line, or with a double-sided printed "fly wire" or a direct crossover of the resistance-capacitance element leads.
2.10. in order to prevent "interference", AC and DC should be separated; Input leads with high input impedance are separated from adjacent leads; Separate input cables and outgoing cables.
2.11. Anti-interference protection ring is adopted at the terminal of the sensitive element. The protection ring cannot be used as a signal loop and can only be grounded at a single point. The part surrounded by the protective ring effectively inhibits the interference caused by leakage, and also reduces the radiation of the surrounded part.
2.12. in addition to the wiring of the power line to make the width of the conductor as thick as possible, the direction of the power line, the ground line and the direction of data transmission are suppressed, which will help to enhance the anti-noise ability.
2.13. the line does not have branches, which can avoid reflection interference or harmonic interference in the transmission of high-frequency signals.
2.14. Reasonable and proper arrangement of signal transmission lines inside and outside the printed board can also inhibit high-frequency radiation noise. High-speed signal line to use SMS; The loop area formed between the signal lines should be minimum; The main signal lines are best gathered in the center; The clock generating circuit is arranged near the center; In order to avoid interference between signals, the two signal lines should not be parallel, and should take a vertical cross, or open the distance between the two lines, you can also add a ground line between the two parallel signal lines. Pay particular attention to the signal lines that are connected to the outside world and send signals to the outside world, and sometimes can receive interference signals from the outside world and play a similar role as an antenna.
3. Layout of decoupling capacitors
Printed boards are equipped with multiple integrated circuits, and when some of the components consume a lot of power, there will be a large potential difference on the ground. The method of suppressing potential difference is to connect decoupling capacitors between the power line and ground line of each integrated device to shorten the flow path of switching current and reduce the resistance voltage drop. This is a common practice in printed board design.
3.1. Power decoupling
Power decoupling is to connect the decoupling capacitor between the power cord and the ground wire at the entrance of each printed board. The combined capacitor should be a large-capacity electrolytic capacitor (10 to 100µF) and a non-electrolytic capacitor (0.01 to 0.1µF). We can decompose the interference into high-frequency interference and low-frequency interference two parts, and access the large capacitor to remove the low-frequency interference component, access the small capacitor to remove the high-frequency interference part. Low frequency decoupling capacitors use aluminum or tantalum electrolytic capacitors, high frequency decoupling capacitors use mica or ceramic capacitors with small inductance.
3.2. Integrated chip decoupling
In principle, each integrated chip should be installed with a 0.1µF ceramic capacitor, if the printed circuit board gap is too small to fit, you can install a 1 to 10µF noise limiting tantalum capacitor for every 4 to 10 chips. The high frequency impedance of this capacitor is particularly small, less than 1 ohms in the range of 500kHz to 200MHz, and the leakage current is very small (less than 0.5µA). For devices with weak noise capacity and large turn-off current and ROM and RAM memory, the decoupling capacitor should be directly connected between the power line (V) and the ground (GND) of the chip. The location of the chip decoupling capacitor is shown in Figure 6.
When installing the capacitor room, be sure to shorten the lead of the capacitor as much as possible, especially the high-frequency bypass capacitor. The hole distance of the printed board should be just moderate with the lead distance of the capacitor, and the lead of the capacitor is the shortest.
To install the decoupling capacitor type of each chip, the decoupling capacitor must be installed on the V and GND lines of the chip, if it is incorrectly installed to other GND positions, it will lose the anti-interference effect.
4. power cord and ground design
4.1. ground width
The thickened ground wire reduces the wire resistance so that it can pass through three times the allowable current on the printed board. If possible, the ground wire width should be more than 2 ~ 3mm.
4.2. The ground wire constitutes a closed-loop road
The closed-loop circuit formed by ground wire can obviously improve the anti-noise ability compared with comb. The closed-loop shape can significantly shorten the loop of the line, reduce the line impedance, and thus reduce interference. However, it should be noted that the smaller the area surrounded by the loop, the better.
4.3. the printed circuit board partition centralized parallel point grounding
When there are multiple circuits with different functions on the same printed board, the components of the same functional unit can be concentrated in one point grounding, and the independent circuit can be formed. This allows the ground current not to flow into the circuit of other functional units, avoiding interference with other units. At the same time, the ground block of each functional unit should also be connected to the power supply ground of the host. This connection is called "zoned centralized parallel point grounding". In order to reduce the impedance, the ground and power lines should use large area bus.
Digital and analog separately designed, the two ground wires are connected at the power end, and the ground wire should be as thick as possible.
4.4. Grounding consideration when printed board works at high frequency
When the components and wires on the printed board work at high frequencies, they will emit radiation interference to the space. The source of radiation interference comes from those high frequency digital signals, such as high frequency oscillators. In order to suppress high-frequency radiation noise, the following measures should be taken in high-frequency circuits:
a. try to make the ground wire thicker to reduce the noise to the ground impedance.
b. Fully grounded. In addition to the printed wire used for transmitting signals on the printed board, the area not occupied by the device on the circuit board is all used as the ground wire. It's called "fully grounded."
c. Install the floor. An aluminum or iron plate can be attached to the back of the printed circuit board to make a floor, or the printed circuit board can be placed between two aluminum or iron plates to make it a double-sided floor. When installing, the single or double floor should be as close as possible to the printed board in order to achieve a good effect of suppressing radiation noise. In addition, the installed ground floor must be connected with the signal ground end of the system, and find the best ground point, otherwise the effect of suppressing radiation noise will be reduced.
4.5. Layout of power cord
In addition to making the width of the conductor as thick as possible according to the size of the current, the power line and the ground line should be taken in the same direction as the data transmission, which will help to enhance the anti-noise ability.
5. memory wiring
The EPROM chip memory and RAM chip data memory configured on the host board have weak anti-noise ability, large current change and high frequency when shut off, so as to prevent external electromagnetic interference. Therefore, the anti-interference measures taken are:
5.1. data line, address line, control line as short as possible to reduce the capacitance to the ground. In particular, the address line, the length of each line, the wiring mode should be as consistent as possible, in order to cause the impedance difference is too large, the formation of non-synchronous interference control information.
5.2. Due to serious switching noise, decoupling capacitors should be connected at the power supply entrance and between V and GND of each memory chip.
5.3. due to the large load current, the power cord and ground wire should be thick, and the cable should be as short as possible. The three buses on both sides of the printed board are perpendicular to each other to prevent electromagnetic interference between the buses.
5.4. the beginning and terminal of the bus should be configured with appropriate pull-up resistance to improve the high level noise tolerance, increase the anti-interference ability of the memory port in the high resistance state and weaken the reflection wave interference. Therefore, it is a common practice to configure pull-up resistors.
5.5. if the three buses of the host board need to be drawn out and connected with other expansion boards, it should be connected with other expansion boards after the three-state buffer gate (74LS244 74LS245). In this way, it can effectively prevent external electromagnetic interference, improve waveform and weaken reflection interference.
6. Device layout of printed board
For the device layout on the printed board, the related logic circuit devices should be placed as close as possible to obtain a better anti-noise effect.
When the logic circuit is arranged on the printed board, in principle, the high-speed device should be placed near the outgoing terminal, and the low-speed circuit and memory should be placed slightly farther away, which can reduce the common impedance coupling, radiation and interference noise. If the fastest logic element in the printed board is slower than the TTL speed, the layout of the device has little effect on interference.
Devices prone to noise, high-current circuits, etc. are as far away from logic circuits as possible, and if conditions permit, you can also do another printed plate.
The layout of the device should take into account the heat dissipation, when the printed board is installed vertically, ROM, RAM, clock generator and other hot devices are arranged above the printed board, or easy to pass the wind heat dissipation place.
In order to reduce the interference introduced by the external line, the photoelectric isolator, the isolation transformer and the filter, etc., should usually be placed closer to the outlet terminal.
7. printed board wiring, connection and installation
The principle that the installation of the interboard distribution box of multiple printed boards should follow is to suppress the interference introduced on the line and reduce the temperature rise. For wiring between multiple printed boards, the following points should be noted:
7.1. when the logic circuit is a TTL integrated circuit, if the operating frequency is less than 1MHz and the wiring length does not exceed 40cm, a single strand of wire can be used; When the wiring length exceeds 40~90cm, twisted pair cables with characteristic impedance of 100~200 ohms should be used, one of which is connected to the signal line in the board, and should be connected to the load resistance matching the terminal; When the wiring length exceeds 150cm, an integrated dedicated line driver-receiver and coaxial cable with a wave impedance of 50~60 ohms should be used.
7.2. the insulation of the wire should be good.
7.3. the shorter the signal line between the board and the board, the better.
From the point of view of heat dissipation and temperature rise reduction, when installing and using multiple printed boards, the heat dissipation performance of the vertical installation is better than that of the horizontal installation. In addition, a circuit board should be considered in the direction of placement in the chassis, and the device with large heat output should be placed on the top.
In the MCU application system composed of several printed boards, connectors are often used to connect each board and each board to the reference power supply. It is also easy to cause interference between the pins of the connector, which is related to the distance between the pins of the connector and the distance between the pins and the ground wire. Design selection should pay attention to:
a. Reasonably set the connector, the power connector and the signal connector should be as far away as possible, and the connector of the main signal should be shielded.
b. The number of grounding pins should be increased on the plug seat. When setting the pin signal number, a part of the pin should be used as the grounding pin and distributed evenly between the signal pins to play an isolation role to reduce the mutual interference between the pins. It is best to set the ground needle on both sides of each signal needle, so that the ideal ratio of signal and ground needle is 1:1.
c. the signal needle as far as possible scattered configuration, increase the distance between each other.
d. the design takes into account the reversal time difference of the signal, and puts the pins that are not flipped at the same time together. The needle that turns at the same time should leave as much as possible, because the signal turns at the same time will increase the interference.
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