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EMC design in PCB
Visit:808 Date:2020-08-24
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 communications, household appliances, power frequency electromagnetic fields and high frequency electromagnetic fields of power transmission lines. 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. The concepts of 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 PCB 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 boards is directly related to The quality and performance of electronic equipment. 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 complex wire layout on the PCB, numerous parts and components, and dense installation, which will inevitably cause interference between them More and more serious, so the suppression of 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 board design has a great impact 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, good PCB board design is also a very important factor in electromagnetic compatibility (EMC).
  1.1 Reasonable PCB layer design
  According to the complexity of the circuit, a reasonable choice of the number of PCB layers 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 the multi-layer PCB 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 PCB board is routed horizontally, and the next side is routed vertically, and then connected by vias.
  1.2 Reasonable PCB board size design
   When the PCB board size is too large, it will cause the printed wire to increase, the impedance increases, the anti-noise ability decreases, and the equipment volume increases and the cost increases 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 board, and the wiring layer (Keepout Layer) is prohibited 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 used, and the aspect ratio is 3:2. When the circuit board surface size is larger than 150mm*200mm, the mechanical strength of the PCB board should be considered.
  2, PCB layout
In PCB board design, electronic engineers may only focus on increasing the density, reducing the occupied space, making simple production, or pursuing aesthetics and uniform layout, ignoring the influence of circuit layout on electromagnetic compatibility (EMC), so that a large number of signals are radiated into space Form mutual interference. A poor PCB layout can cause more electromagnetic compatibility (EMC) problems rather than eliminate them.
   The characteristics of the 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 separately or made into a single PCB board. In the layout, special attention should be paid to the distribution of strong and weak signals and the signal transmission direction.
  2.1 Component layout of PCB board
   The layout of PCB components is the same as that of other logic circuits. 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 PCB board should fully consider the problem of anti-electromagnetic interference. One of the principles is to keep the leads between components 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.
   The clock generator, crystal oscillator and the clock input of the CPU are all prone to noise, so they should be closer to each other. Devices that are prone to noise, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, another PCB 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 public circuits.
  3) High, medium and low speed logic circuits need different areas on the PCB board.
  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 elements and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires.
   8) Place the voltage regulator and filter capacitor as close to the rectifier diode as possible.
  9) PCB boards are divided according to frequency and current switching characteristics, and the distance between noise 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, the heating components such as power tubes and transformers should be placed as far as possible to disperse the heat to facilitate heat dissipation. Do not concentrate in one place, and do not have high capacitors too close to avoid Make the electrolyte age prematurely.
  2.2 PCB layout
  The composition of a PCB board is a multilayer structure that uses a series of lamination, wiring and prepreg processing on the vertical stack. In the multi-layer PCB board, in order to facilitate debugging, the signal line will be laid on the outermost layer.
   Under high frequency conditions, the distributed inductance and distributed capacitance of the PCB board, such as traces, vias, resistors, capacitors, and connectors, cannot be ignored. Resistance will produce 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.
   The wire connections of the PCB board are mostly done through vias. One via can bring about 0.5pF distributed capacitance, 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 the PCB has a distributed inductance of 520nH. A dual-in-line 24-pin integrated circuit socket introduces 4-18nH distributed inductance.
  The general requirements that should be followed to avoid the influence of PCB board wiring distribution parameters:
  1) Increase the spacing of traces to reduce crosstalk of capacitive coupling.
2) In double-panel wiring, the wires on both sides should be perpendicular, oblique, or bent 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 ground wire between these wires.
  3) Place sensitive high-frequency lines away from high-noise power lines to reduce mutual coupling; high-frequency digital circuit traces are 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) Address line or data line, the length of the trace should not be too different, otherwise the short-line part must be artificially bent 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. Under normal circumstances, the distance between the board is 2mm at 2kV, and the ratio is added. For example, if you want to withstand the 3kV withstand voltage test, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, to avoid creepage, slots are also slotted between the high and low voltages on the PCB.)
  3. Circuit design in PCB board
  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 (EMC) and electromagnetic interference (EMI) suppression and electromagnetic anti-interference characteristics. In order to achieve the purpose of electromagnetic compatibility when using circuit schematics for PCB layout, necessary measures must be taken, that is, necessary additional circuits should be added to the circuit schematics to improve the electromagnetic compatibility performance of their products. The following circuit measures can be used in the 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 for relays (high-frequency capacitors, reverse diodes, etc.).
  3) The signal entering the PCB board should be filtered, and the signal from the high noise area to the low noise area should also be filtered. At the same time, a series of terminal resistors should be used to reduce signal reflection.
   4) The useless end of MCU should be connected to power or ground through corresponding matching resistance, or defined as output end. The terminals of the integrated circuit that should be connected to the power supply and the ground must be connected, and do not float.
  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 of the unused operational amplifier is grounded, and the negative input is connected to the output.
  6) Set 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 charge and discharge storage capacitors on the PCB. When using tubular capacitors, the case should be grounded.

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