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    Problems needing attention in every link of PCB board in switching power supply design

    Author:xiaozhi-zhan Views:28 Release time:2019-05-29 15:51:38

    In the electrical age, switching power supply is particularly important, especially in PCB design, GENITEC full-automatic PCB splitter share: PCB board in the design of switching power supply should pay attention to the issues, mainly from the design, layout, circuit voltage conversion, wiring and so on. The information is arranged by GENITEC PCB. If you have any questions, please contact Xiaozhi.


    In the design of switching power supply, the physical design of PCB board is the last step. If the design method is improper, PCB may radiate too much electromagnetic interference, resulting in instability of power supply. The following points for attention in each step are analyzed:


    Design flow from schematic diagram to PCB

    Establishing Component Parameters - "Input Principle Network Table -"Design Parameters Settings - "Manual Layout -"Manual Wiring - "Verification Design -" Review - "CAM Output.


    Problems needing attention in every link of PCB board in switching power supply design


    Component layout


    Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, the reliability of electronic equipment will be adversely affected. For example, if the two thin parallel lines of PCB are close to each other, the delay of signal waveform will be formed, and the reflection noise will be formed at the terminal of transmission line. The interference caused by improper consideration of power supply and ground wire will degrade the performance of the product. Therefore, the correct method should be adopted in the design of PCB. Each switching power supply has four current loops:


    (1) AC circuit of power switch

    (2) Output Rectifier AC Circuit

    (3) Current loop of input signal source

    (4) Output Load Current Circuit Input Circuit


    By charging the input capacitor with an approximate DC current, the filter capacitor mainly acts as a broadband energy storage. Similarly, the output filter capacitor is used to store the high-frequency energy from the output rectifier, while eliminating the direct current energy from the output load loop. Therefore, the wiring terminals of input and output filter capacitors are very important. The input and output current loops should be connected to the power supply only from the wiring terminals of filter capacitors. If the connection between input/output loops and power switch/rectifier loops is not directly connected to the wiring terminals of capacitors, the AC energy will be radiated from the input or output filter capacitors to the environment.


    The AC circuit of power switch and rectifier contains high amplitude trapezoidal current. The harmonic component of these currents is very high. Its frequency is much larger than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times of the continuous input/output DC current, and the transition time is usually about 50 n/s.


    These two circuits are most prone to electromagnetic interference. Therefore, these AC circuits must be laid before other printed wiring in the power supply. The three main components of each circuit, filter capacitors, power switches or rectifiers, inductors or transformers, should be placed adjacent to each other to adjust the position of components so that the current path between them is as short as possible. The best way to establish the layout of switching power supply is similar to its electrical design. The optimal design process is as follows:


    Place transformer

    Design of Switching Current Circuit of Power Supply

    Design of Current Circuit of Output Rectifier

    Control Circuit Connected to AC Power Circuit


    When designing the input current source circuit and input filter, designing the output load circuit and output filter, according to the functional unit of the circuit, the layout of all components of the circuit should conform to the following principles:


    (1) The size of PCB should be considered first. When the size of PCB is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases. If the size of PCB is too small, the heat dissipation is not good, and the adjacent lines are easily disturbed. The best shape of the circuit board is rectangular, the ratio of length to width is 3:2 or 4:3. Components located at the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board.


    (2) When placing the device, we should consider the future welding, not too dense.


    (3) Around the core components of each functional circuit, the layout is carried out. Components should be evenly, neatly and compactly arranged on PCB to minimize and shorten the lead and connection between components, and decoupling capacitors should be as close as possible to the VCC of the device.


    (4) Circuits operating at high frequencies should consider the distribution parameters between components. General circuits should arrange components in parallel as far as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass production.


    (5) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout can facilitate the signal flow and keep the signal in the same direction as possible.


    (6) The primary principle of layout is to ensure the routing rate, pay attention to the connection of flying wires when moving devices, and put the connected devices together.


    (7) Reduce the area of the ring road as much as possible to restrain the radiation interference of switching power supply.


    Parameter setting

    The distance between adjacent conductors must meet the requirements of electrical safety, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum spacing should be at least suitable for withstanding voltage. When the wiring density is low, the spacing of signal lines can be increased appropriately. The signal lines with large disparities between high and low levels should be as short as possible and the spacing should be increased. In general, the spacing of routing lines should be set to 8 mil.


    The distance between the inner hole edge of the pad and the edge of the printed circuit board is more than 1m, which can avoid the pad defect during processing. When the connection with the pad is thin, the connection between the pad and the pad should be designed as water droplets. The advantage is that the pad is not easy to peel, but the pad is not easy to disconnect.


    wiring


    Switching power supply contains high frequency signal. Any printed line on PCB can act as an antenna. The length and width of printed line will affect its impedance and inductance, thus affecting the frequency response. Even the printed line through the DC signal will be coupled from the adjacent printed line to the radio frequency signal and cause circuit problems (even re-radiating interference signals). Therefore, all printed wires passing through AC current should be designed as short and wide as possible, which means that all components connected to printed wires and other power lines must be placed very close.


    The length of the printed line is proportional to its inductance and impedance, while the width is inversely proportional to its inductance and impedance. The length reflects the wavelength of the response of the printed line. The longer the length, the lower the frequency of the electromagnetic wave that the printed line can transmit and receive, the more radio frequency energy it can radiate. According to the current of printed circuit board, the width of power supply line should be increased as far as possible to reduce loop resistance. At the same time, the direction of the power supply and ground wires is consistent with the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of four current loops of switching power supply. It plays an important role as the common reference point of the circuit and is an important method to control interference.


    Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing various grounding wires will cause unstable operation of power supply.


    The following points should be paid attention to in the design of the ground line:


    1. Proper selection of single-point grounding usually means that the common end of the filter capacitor should be the only connection point coupled to the AC ground where the current is high. The grounding point of the same stage circuit should be as close as possible, and the power supply filter capacitor of this stage circuit should also be connected to the grounding point of this stage. The main consideration is that the current flowing back to the ground from each part of the circuit is variable, because the impedance of the actual circuit flowing through is changed. It will lead to the change of the potential of each part of the circuit and lead to interference. In this switching power supply, its wiring and inductance between devices have little influence, and the circulation current formed by grounding circuit has great influence on interference. Therefore, one-point grounding is adopted, that is, the ground wires of switching current circuit of power supply (several of the devices are connected to the grounding foot, and the ground wires of several devices of output rectifier current circuit are also connected to the grounding foot of the corresponding filter capacitor. The power supply works stably and is not easy to self-excite. When a single point can not be achieved, two diodes or a small resistance can be joined together, in fact, it can be joined at a relatively concentrated piece of copper foil.


    2. If the grounding wire is as thin as possible, the grounding potential will change with the change of current, resulting in unstable timing signal level of electronic equipment and deterioration of anti-noise performance. Therefore, it is necessary to ensure that each grounding end of large current adopts printed lines as short and wide as possible, and widen the width of power supply and grounding wire as far as possible, preferably the grounding wire is wider than the power supply line. Their relationship is ground wire power supply. Wire: Signal line, if possible, the width of grounding wire should be more than 3mm. Large area copper layer can also be used as grounding wire. In PCB, all unused places are connected with the ground as grounding wire. The following principles should also be followed when conducting global wiring:


    (1) Wiring direction: From the view of welding surface, the arrangement orientation of components should be consistent with the principle diagram as far as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram. Because various parameters of the welding surface need to be tested in the production process, it is convenient for inspection, debugging and maintenance in production. On the premise of the request.


    (2) When designing the wiring diagram, the routes should turn as little as possible, the width of the printed arc should not change abruptly, and the corners of the conductors should be more than 90 degrees, so as to make the lines simple and clear.


    (3) There is no crossing circuit in printed circuit. For possible crossing lines, two methods can be used: drilling and winding. That is to say, let a lead "drill" through the gap at the foot of other resistors, capacitors and transistors, or "circle" from one end of a lead that may be crossed. In special cases, how to make the circuit very complex, in order to simplify the design, it is also allowed to use conductors to cross-connect to solve the problem of crossed circuits. Because of the single panel, the direct insertion device is located on top surface and the table-mounted device is located on bottom surface, the direct insertion device can overlap with the table-mounted device in the layout, but the overlap of the pad should be avoided.



    3. For DC-DC with low voltage in the local switching power supply of input and output, if the output voltage is to be fed back to the primary stage of transformer, the two sides of the circuit should have a common reference ground, so after copper is laid on the ground wires of both sides, they should be connected together to form a common ground.


    inspect


    After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer. At the same time, it is necessary to confirm whether the rules formulated conform to the requirements of the production process of printed circuit boards. Generally, it is necessary to check whether the distance between wire and wire, wire and component pad, wire and through hole, component pad and through hole, through hole and through hole is reasonable or not, and whether it meets the production requirements. Whether the widths of the power and ground wires are appropriate, and whether there is room for widening the ground wires in the PCB. Note: Some errors can be ignored, for example, some plug-in Outline parts are placed outside the board frame, checking the spacing will make errors; in addition, every time after modifying the routing and through the hole, it must be copper again.


    Review according to "PCB Checklist"


    The contents include design rules, layer definition, line width, spacing, pad and hole setting, and also focus on reviewing the rationality of device layout, the routing and shielding of power supply and ground network, the routing and shielding of high-speed clock network, the placement and connection of decoupling capacitors, etc.


    Notices for designing output optical drawing documents:


    A. The layers that need to be output are wiring layer (bottom layer), screen printing layer (including top layer, bottom layer), solder resistance layer (bottom layer), drilling layer (bottom layer), and drilling file (NCD rill).


    B. When setting Layer of screen printing layer, do not select Part Type, choose Outline, Text and Line of top layer (bottom layer) and screen printing layer.


    C. When setting Layer for each layer, select Board Outline. When setting Layer for screen printing layer, do not select Part Type. Select Outline, Text and Line for top layer (bottom layer) and screen printing layer.


    D. When generating drilling files, use the default settings of Power PCB and do not make any changes.


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