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In any switching power supply design, the physical design of the PCB is the last link. If the design method is improper, the PCB may radiate too much electromagnetic interference, causing the power supply to be unstable. The following points should be noted for each step. analysis:
First, from the schematic diagram to the PCB design process to establish component parameters -> input principle net list -> design parameter settings -> manual layout -> manual wiring -> verification design -> review -> CAM output.
Second, the parameters set adjacent conductor spacing must be able to meet electrical safety requirements, and in order to facilitate handling and production, the spacing should be as wide as possible. The minimum spacing should be at least suitable for the voltage to withstand. When the wiring density is low, the spacing of the signal lines can be appropriately increased. The signal lines with high and low levels should be as short as possible and the spacing should be increased. Under normal circumstances Set the trace spacing to 8 mils.
The interval between the edge of the pad and the edge of the printed board is greater than 1mm, which can avoid the defect of the pad during processing. When the traces connected to the pads are thinner, the connection between the pads and the traces is designed to be drop-shaped. This has the advantage that the pads are not easily peeled off, but the traces are not easily disconnected from the pads.
Third, the component layout practice confirmed that even if the circuit schematic design is correct, the printed circuit board design is improper, it will also have an adverse impact on the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and the reflection noise will be formed at the end of the transmission line; the interference caused by the inconsistency of the power supply and the ground line will cause the product to be Performance is degraded, so when designing a printed circuit board, you should pay attention to the correct method. Each switching power supply has four current loops:
(1). Power switch AC circuit
(2). Output rectifier AC circuit
(3). Input signal source current loop
(4). Output load current loop input loop charges the input capacitor through an approximate DC current. The filter capacitor mainly acts as a broadband energy storage; similarly, the output filter capacitor is also used to store the high frequency from the output rectifier. Energy while eliminating the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should be connected only from the terminals of the filter capacitor to the power supply; if the connection between the input/output loop and the power switch/rectifier loop It cannot be directly connected to the terminals of the capacitor, and the AC energy will be transmitted to or output from the filter capacitor and radiated into the environment. The AC circuit of the AC circuit of the power switch and the rectifier contain high-profile trapezoidal currents. The harmonic components of these currents are very high, and the frequency is much larger than the fundamental frequency of the switch. The peak amplitude can be up to 5 times the amplitude of the continuous input/output DC current. Usually about 50ns. These two loops are the most susceptible to electromagnetic interference, so these AC loops must be placed before other traces in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, inductors or transformers. Place them adjacently and adjust the position of the components so that the current path between them is as short as possible. The best way to establish a switching power supply layout is similar to its electrical design. The best design flow is as follows:
Place transformer design power switch current loop design output rectifier current loop connected to AC power circuit control circuit design input current source loop and input filter design output load loop and output filter according to the functional unit of the circuit, all the elements of the circuit When the device is laid out, the following principles must be met:
(1) First consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is reduced, and the cost is also increased; if the size is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. The optimal shape of the board is rectangular, with an aspect ratio of 3:2 or 4:3. The components on the edge of the board are generally not less than 2mm from the edge of the board.
(2) Consider the future soldering when placing the device, not too dense.

(3) Center around the core components of each functional circuit and arrange it around it. The components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components. The coupling capacitor is as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to weld and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(6) The first principle of the layout is to ensure the wiring rate of the wiring. When moving the device, pay attention to the connection of the flying wires and put the devices with the connection relationship together.
(7) Reduce the loop area as much as possible to suppress radiated interference from the switching power supply.
4. The wiring switching power supply contains high-frequency signals. Any printed circuit on the PCB can function as an antenna. The length and width of the printed circuit will affect its impedance and inductance, thus affecting the frequency response. Even a printed line that passes a DC signal is coupled to the RF signal from an adjacent printed line and causes a circuit problem (even radiating an interference signal again). Therefore, all traces through alternating current should be designed to be as short and wide as possible, which means that all components connected to the trace and connected to other power lines must be placed in close proximity. The length of the trace is proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength of the response of the printed line. The longer the length, the lower the frequency at which the printed line can transmit and receive electromagnetic waves, and it radiates more RF energy. According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, the direction of the power line and the ground line are consistent with the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply. It plays an important role as a common reference point for the circuit. It is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout, and mixing various groundings will cause the power supply to be unstable. Pay attention to the following points in the ground line design:
1. Correct selection of single-point grounding Generally, the common side of the filter capacitor should be the only connection point where other grounding points are coupled to the AC ground of large current. The grounding point of the same-stage circuit should be as close as possible, and the power supply filter capacitor of this stage circuit is also It should be connected to the grounding point of this stage, mainly considering that the current flowing back to the ground of each part of the circuit changes, because the impedance of the actual flowing line will lead to the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and the inductance between the devices have less influence, and the circulating current formed by the grounding circuit has a great influence on the interference, so a little grounding is used, that is, the power switching current loop (the grounding of several devices in the device) Connected to the grounding pin, the ground of several devices of the output rectifier current loop is also connected to the grounding pin of the corresponding filter capacitor, so that the power supply works stably and is not easy to self-excitation. Connect two diodes or a small resistor, it can be connected to a relatively concentrated piece of copper foil.
2. If the grounding wire is as thick as possible, the grounding potential will change with the current, which will cause the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, ensure the grounding of each large current. Use as short and wide as possible of the printed line, try to widen the power supply, ground line width, preferably the ground line is wider than the power line, their relationship is: ground line > power line > signal line, if possible, ground line The width should be greater than 3mm. It can also be used as a ground wire with a large area of copper. The places that are not used on the printed board are connected to the ground as ground. When performing global routing, the following principles must also be observed:
(1). Wiring direction: From the welding surface, the arrangement orientation of the components should be kept as consistent as possible with the schematic diagram. The wiring direction is preferably consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the welding surface during the production process. The detection, so it is convenient to check, debug and inspect during production (Note: refers to the conditions that meet the circuit performance and machine installation and panel layout requirements).
(3) Center around the core components of each functional circuit and arrange it around it. The components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components. The coupling capacitor is as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to weld and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(6) The first principle of the layout is to ensure the wiring rate of the wiring. When moving the device, pay attention to the connection of the flying wires and put the devices with the connection relationship together.
(7) Reduce the loop area as much as possible to suppress radiated interference from the switching power supply.
4. The wiring switching power supply contains high-frequency signals. Any printed circuit on the PCB can function as an antenna. The length and width of the printed circuit will affect its impedance and inductance, thus affecting the frequency response. Even a printed line that passes a DC signal is coupled to the RF signal from an adjacent printed line and causes a circuit problem (even radiating an interference signal again). Therefore, all traces through alternating current should be designed to be as short and wide as possible, which means that all components connected to the trace and connected to other power lines must be placed in close proximity. The length of the trace is proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength of the response of the printed line. The longer the length, the lower the frequency at which the printed line can transmit and receive electromagnetic waves, and it radiates more RF energy. According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, the direction of the power line and the ground line are consistent with the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply. It plays an important role as a common reference point for the circuit. It is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout, and mixing various groundings will cause the power supply to be unstable. Pay attention to the following points in the ground line design:
1. Correct selection of single-point grounding Generally, the common side of the filter capacitor should be the only connection point where other grounding points are coupled to the AC ground of large current. The grounding point of the same-stage circuit should be as close as possible, and the power supply filter capacitor of this stage circuit is also It should be connected to the grounding point of this stage, mainly considering that the current flowing back to the ground of each part of the circuit changes, because the impedance of the actual flowing line will lead to the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and the inductance between the devices have less influence, and the circulating current formed by the grounding circuit has a great influence on the interference, so a little grounding is used, that is, the power switching current loop (the grounding of several devices in the device) Connected to the grounding pin, the ground of several devices of the output rectifier current loop is also connected to the grounding pin of the corresponding filter capacitor, so that the power supply works stably and is not easy to self-excitation. Connect two diodes or a small resistor, it can be connected to a relatively concentrated piece of copper foil.
2. If the grounding wire is as thick as possible, the grounding potential will change with the current, which will cause the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, ensure the grounding of each large current. Use as short and wide as possible of the printed line, try to widen the power supply, ground line width, preferably the ground line is wider than the power line, their relationship is: ground line > power line > signal line, if possible, ground line The width should be greater than 3mm. It can also be used as a ground wire with a large area of copper. The places that are not used on the printed board are connected to the ground as ground. When performing global routing, the following principles must also be observed:
(1). Wiring direction: From the welding surface, the arrangement orientation of the components should be kept as consistent as possible with the schematic diagram. The wiring direction is preferably consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the welding surface during the production process. The detection, so it is convenient to check, debug and inspect during production (Note: refers to the conditions that meet the circuit performance and machine installation and panel layout requirements).

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