PCB Tech - Suppress the source of electromagnetic interference in electronic equipment

Electromagnetic interference is widely present in all kinds of electronic and electrical equipment, and various electronic and electrical equipment will more or less emit electromagnetic waves when they are working, which will cause interference to the normal operation of the entire equipment. In the design of electronic products, due to insufficient consideration of electromagnetic compatibility, some electrical and electronic products are not qualified. Therefore, the author summarizes some points that should be paid attention to.

Ground connection

The analog and digital circuits have independent power and ground paths. Try to widen the power and ground lines of the two parts of the circuit, or use separate power and ground layers to reduce the impedance of the power and ground loops and reduce any Possible interference voltage in the power and ground circuit.

The analog ground and digital ground of a separately working PCB can be connected at a single point near the system ground point. If the power supply voltage is the same, the power supply of the analog and digital circuit is connected at the power inlet at a single point. If the power supply voltage is inconsistent, the two power sources are closer. Place a -1～2μf capacitor to provide a path for the signal return current between the two power sources.

The ideal ground wire is a zero-impedance, zero-potential physical entity. It is not only a reference point for the signal, but also does not produce a voltage drop when current flows. In actual electrical and electronic equipment, this kind of ideal ground wire does not exist, and a voltage drop will inevitably occur when current flows through the ground wire. According to this, the formation mechanism of interference in the ground wire can be attributed to the following two points. First, reduce the low impedance and power feeder impedance. Second, correctly choose the grounding method and block the ground loop. According to the grounding method, there are floating ground, single-point grounding, multi-point grounding, and mixed grounding. If the interference of the sensitive line mainly comes from the external space or the system shell, the floating ground can be used to solve this problem. However, the floating ground equipment is prone to static electricity accumulation. When the charge reaches a certain level, electrostatic discharge will occur, so the floating ground is not suitable for use For general electronic equipment.

PCB component layout requirements

The layout of circuit components and signal paths must minimize the mutual coupling of unwanted signals:

(1) The low-level electronic signal channel should not be close to the high-level signal channel and unfiltered power lines, including circuits that can generate transient processes.

(2) High, medium, and low-speed logic circuits use different areas on the PCB.

(3) When arranging the circuit, the length of the signal line should be minimized.

(4) Ensure that there are no excessively long parallel signal lines between adjacent boards, between adjacent levels of the same board, and between adjacent wiring on the same level.

(5) The electromagnetic interference (EMI) filter should be as close as possible to the EMI source and placed on the same circuit board.

(6) DC/DC converters, switching elements and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires.

(7) Place the voltage regulating component and filter capacitor as close as possible to the rectifier diode.

(8) The printed board is partitioned according to frequency and current switching characteristics, and the noise components and non-noise components should be farther apart.

(9) The wiring sensitive to noise should not be parallel to the high-current, high-speed switching line.

Multilayer board design

In the multi-layer board design, the power plane should be close to the ground plane and arranged below the ground plane. In this way, the capacitance between the two metal plates can be used as a smoothing capacitor for the power supply, and the ground plane also shields the radiation current distributed on the power plane; in order to generate flux cancellation, the wiring layer should be arranged adjacent to the entire metal plane The printed lines in the middle layer form a planar waveguide, and a microstrip line is formed on the surface. The transmission characteristics of the two are different; clock circuits and high-frequency circuits are the main sources of interference and radiation. They must be arranged separately and far away from sensitive circuits; all Printed boards with a certain voltage will radiate electromagnetic energy into space. In order to reduce this effect, the physical size of the printed board should be 20H smaller than the physical size of the nearest grounding board, where H is the surface of the two printed boards. spacing. According to the general typical printed board size, 20H is generally about 3mm,

In order to avoid electromagnetic crosstalk caused by the relatively small distance between two printed lines, any line spacing should be kept at least 2 times the printed line width, that is, not less than 2W, where w is the width of the printed line.

Set decoupling capacitor

A good high-frequency decoupling capacitor can remove high-frequency components as high as 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor must be added between the power and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, which provides and absorbs the charging and discharging energy at the moment of opening and closing of the integrated circuit; on the other hand, it bypasses the high frequency noise of the device.

Suppress electromagnetic coupling between lines

Reduce the loop area of interference sources and sensitive circuits. The best way is to use twisted-pair wires and shielded wires, so that the signal wire and the ground wire (or current-carrying circuit) are twisted together so that the distance between the signal and the ground wire (or current-carrying circuit) is the shortest; increase The distance between the lines makes the mutual inductance between the interference source and the induced line as small as possible; if possible, the interference source line and the induced line are wired at right angles (or close to right angles), which can greatly reduce the two Coupling between lines;

Other ways to reduce noise and electromagnetic interference

(1) Enclose the clock area with a ground wire and keep the clock wire as short as possible.

(2) Try to provide some form of damping for relays, etc.

(3) Use the lowest frequency clock that meets the system requirements.

(4) The clock generator is as close as possible to the device that uses the clock. The shell of the quartz crystal oscillator should be grounded.

(5) The I/O drive circuit should be as close to the edge of the printed board as possible, and let it leave the printed board as soon as possible. The signal entering the printed board should be filtered, and the signal from the high-noise area should also be filtered. At the same time, a series of terminal resistors should be used to reduce signal reflection.

(6) The input terminal of the gate circuit that is not in use should not be left floating. The positive input terminal of the unused operational amplifier should be grounded, and the negative input terminal should be connected to the output terminal.

(7) The printed board should try to use 45-degree fold lines instead of 90-degree fold lines to reduce the external emission and coupling of high-frequency signals.

(8) The clock, bus, and chip selection signals should be far away from I/O lines and connectors.

(9) The analog voltage input line and reference voltage terminal should be as far away as possible from the digital circuit signal line, especially the clock.

For A/D devices, the digital part and the analog part would rather be unified than crossed.

(10) Do not route wires under the quartz crystal and under noise-sensitive devices.

in conclusion

In the PCB design, it is necessary to take into account the impact of various interferences. A complete design can effectively simulate electromagnetic interference, shorten the product design cycle, and improve the stability and reliability of the system.