PCB Technical - EMC considerations in PCB design

EMC considerations in PCB design

With the development of the electrical age, there are more and more electromagnetic wave sources in the human living environment, such as radio broadcasting, television, microwave communication; household appliances; power frequency electromagnetic fields of power transmission lines; high frequency electromagnetic fields, etc. When the field strength of these electromagnetic fields exceeds a certain limit and the action time is long enough, it may endanger human health; at the same time, it will also interfere with other electronic equipment and communications. In this regard, protection is required. Concepts such as electromagnetic interference and shielding are often put forward during the development, production and use of electronic products. The core of electronic products during normal operation is a coordinated working process between the circuit board and the components and parts installed on it. It is very important to improve the performance index of electronic products and reduce the influence of electromagnetic interference.

1 PCB board design

Printed circuit board (PCB) is the support of circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. It is the most basic component of various electronic equipment. The performance of PCB is directly related to electronics. The quality and performance of the equipment are good or bad. With the development of integrated circuits, SMT technology, and micro-assembly technology, there are more and more high-density, multi-functional electronic products, resulting in complicated wire layout on PCB, many parts and components, and dense installation, which will inevitably cause more interference between them. It is getting more serious, so the problem of suppressing electromagnetic interference has become the key to whether an electronic system can work normally. Similarly, with the development of electrical technology, the density of PCB is getting higher and higher, and the quality of PCB design has a great influence on the interference and anti-interference ability of the circuit. In order to obtain the best performance of electronic circuits, in addition to the selection of components and circuit design, a good PCB design is also a very important factor in electromagnetic compatibility.

1.1 Reasonable PCB layer design

According to the complexity of the circuit, a reasonable choice of the number of layers of the PCB can effectively reduce electromagnetic interference, greatly reduce the size of the PCB and the length of the current loop and branch wiring, and greatly reduce the cross interference between signals. Experiments show that when the same material is used, the noise of the four-layer board is 20dB lower than that of the double-layer board. However, the higher the number of layers, the more complicated the manufacturing process and the higher the manufacturing cost. In multi-layer board wiring, it is better to use a "well"-shaped mesh wiring structure between adjacent layers, that is, the directions of the respective wiring of the adjacent layers are perpendicular to each other. For example, the upper side of the printed board is wired horizontally, and the lower side is wired vertically, and then connected by vias.

1.2 Reasonable PCB size design

When the PCB board size is too large, the printed wires will increase, the impedance will increase, the anti-noise ability will decrease, and the equipment volume will increase and the cost will increase accordingly. If the size is too small, heat dissipation is not good, and adjacent lines are easily disturbed. In general, the mechanical layer (Mechanical Layer) determines the physical frame, that is, the outline size of the PCB, and the Keepout Layer (Keepout Layer) is forbidden to determine the effective area of layout and wiring. Generally, according to the number of functional units of the circuit, all the components of the circuit are integrated, and the best shape and size of the PCB board are finally determined. Usually a rectangle is selected, and the aspect ratio is 3:2. When the size of the circuit board is larger than 150 mmx200 mm, the mechanical strength of the circuit board should be considered.

2 PCB layout

In PCB design, product designers often only focus on increasing the density, reducing the occupied space, making simple, or pursuing aesthetics and uniform layout, ignoring the influence of circuit layout on electromagnetic compatibility, causing a large number of signals to radiate into the space to form mutual interference . A poor PCB layout can cause more electromagnetic compatibility problems, rather than eliminate them.

The characteristics of the component layout and wiring of digital circuits, analog circuits, and power circuits in electronic equipment are different, and the interference they produce and the methods of suppressing interference are different. Due to the different frequencies of high-frequency and low-frequency circuits, their interference and methods of suppressing interference are also different. Therefore, in the component layout, the digital circuit, analog circuit and power circuit should be placed separately, and the high-frequency circuit and the low-frequency circuit should be separated. If possible, they should be isolated or made into a circuit board separately. In the layout, special attention should be paid to the device distribution of strong and weak signals and the signal transmission direction.

2.1 PCB component layout

The layout of PCB components is the same as that of other logic circuits, and the components related to each other should be placed as close as possible, so that a better anti-noise effect can be obtained. The position of the components on the printed circuit board should fully consider the problem of anti-electromagnetic interference. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high-current switches, etc.) should be reasonably separated to minimize the signal coupling between each other.

Clock generators, crystal oscillators, and CPU clock input terminals are all prone to noise, so they should be closer to each other. Noise-prone devices, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, another circuit board should be made, which is very important.

General layout requirements for PCB components: The layout of circuit components and signal paths must minimize the coupling of unnecessary signals.

1) Low-level signal channels cannot be close to high-level signal channels and unfiltered power lines, including circuits that can generate transient processes.

2) Separate low-level analog circuits and digital circuits to avoid common impedance coupling between analog circuits, digital circuits, and power common loops.

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

4) When arranging the circuit, the signal line length should be minimized.

5) 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.

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

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

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

9) The printed board is partitioned according to frequency and current switching characteristics, and the distance between noisy components and non-noise components should be farther.

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

11) The component layout should pay special attention to the heat dissipation problem. For high-power circuits, those heating components such as power tubes and transformers should be placed as far as possible to facilitate heat dissipation. Do not concentrate in one place, and do not have high capacitance too close to avoid Make the electrolyte age prematurely.

2.2 PCB wiring

The composition of a PCB is a series of multilayer structures of lamination, wiring and prepreg on the vertical stack. In a multi-layer PCB, in order to facilitate debugging, the signal line will be laid out on the outermost layer.

In the case of high frequency, the wiring, vias, resistors, capacitors, and the distributed inductance and distributed capacitance of the connectors on the printed circuit board cannot be ignored. Resistance will cause reflection and absorption of high-frequency signals. The distributed capacitance of the trace will also play a role. When the length of the trace is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect occurs, and noise is emitted through the trace.

Most of the wire connections of the printed circuit board are completed through vias. One via can bring about 0.5 pF distributed capacitance, and reducing the number of vias can significantly increase the speed.

The packaging material of an integrated circuit itself introduces a capacitance of 2 to 6 pF. A connector on a circuit board has a distributed inductance of 520 nH. A dual-in-line 24-pin integrated circuit socket introduces 4-18 nH distributed inductance.

The general requirements that should be followed to avoid the influence of PCB wiring distribution parameters:

1) Increase the spacing of the traces to reduce the crosstalk of capacitive coupling.

2) In double-panel wiring, the wires on both sides should be perpendicular, oblique, or curved to avoid parallel to each other to reduce parasitic coupling; printed wires used as the input and output of the circuit should be avoided as far as possible. In order to avoid feedback, it is best to add a grounding wire between these wires.

3) Lay sensitive high-frequency lines away from high-noise power lines to reduce mutual coupling; the high-frequency digital circuit traces should be thinner and shorter.

4) Widen the power line and the ground line to reduce the impedance of the power line and the ground line.

5) Try to use 45° fold lines instead of 90° fold lines to reduce the external emission and coupling of high-frequency signals.

6) The length of the address line or the data line should not be too different, otherwise the short line part will have to be bent artificially for compensation.

7) Pay attention to isolation between large current signals, high voltage signals and small signals (the isolation distance is related to the withstand voltage to be withstood. Normally, the distance between the boards is 2 mm at 2 kV, and the ratio is calculated above this. For example, to withstand the 3 kV withstand voltage test, the distance between the high and low voltage lines should be above 3.5 mm. In many cases, to avoid creepage, the high and low voltage lines on the printed circuit board should be opened. groove).

3 Circuit design in PCB

When designing electronic circuits, more consideration is given to the actual performance of the product, rather than too much consideration of the product's electromagnetic compatibility characteristics and electromagnetic interference suppression and electromagnetic anti-interference characteristics. In order to achieve the purpose of electromagnetic compatibility when using the circuit schematic diagram for PCB layout, necessary measures must be taken, that is, to add necessary additional circuits on the basis of the circuit schematic diagram to improve the electromagnetic compatibility performance of its products. The following circuit measures can be used in actual PCB design:

1) A resistor can be connected in series on the PCB trace to reduce the transition rate of the lower edge of the control signal line.

2) Try to provide some form of damping (high-frequency capacitors, reverse diodes, etc.) for relays, etc.

3) Filter the signal entering the printed board, and filter the signal from the high-noise area to the low-noise area. At the same time, use a series of terminal resistors to reduce signal reflection.

4) The useless end of the MCU should be connected to the power supply or ground through the corresponding matching resistance, or defined as the output end. The terminals of the integrated circuit that should be connected to the power supply and the ground must be connected, and should not be left floating.

5) Do not leave the input terminal of the gate circuit that is not in use, but connect to the power supply or ground through the corresponding matching resistor. The positive input terminal of the unused operational amplifier is grounded, and the negative input terminal is connected to the output terminal.

6) Set up a high-frequency decoupling capacitor for each integrated circuit. A small high-frequency bypass capacitor must be added to each electrolytic capacitor.

7) Use large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as charging and discharging energy storage capacitors on the circuit board. When using tubular capacitors, the case should be grounded.

4 Conclusion

With the increasing development of science and technology, the miniaturization and intelligence of various electronic devices have become the mainstream trend. At the same time, the operating environment of electronic products or equipment will become more and more complex. Anti-interference technology and electromagnetic compatibility technology also require continuous development and maturity. PCB designers and circuit board manufacturersmust pay sufficient attention to practical applications.