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Switching Power Supply PCB Design Skills
In any switching power supply design, physical design of printed circuit board is last link. If design method is not correct, printed circuit board may emit too much electromagnetic interference, causing power supply to become unstable. is attention required for each step element are analyzed.
1. Design process from schematic to PCB
Set component parameters -> input connections list -> project settings -> manual linking -> manual connection -> design check -> view -> CAM output.
2. Parameter Settings
The distance between adjacent wires must comply with electrical safety requirements, and for ease of operation and production, distance should be as large as possible. The minimum distance should be according to withstand voltage When wiring density is low, distance between signal lines can be increased accordingly. For high and low level signal lines, distance should be as short as possible, and distance should be increased. Set track spacing to 8mil. The distance from edge of inner hole of contact pad to edge of printed circuit board must be greater than 1 mm, which makes it possible to avoid defects in contact pad during processing. When tracks connected to pads are thin, connection between pads and tracks should be made in form of a drop. The advantage of this is that pads are not easy to peel off, but tracks and pads are not easy to separate. Disable.
3. Location of components
Practice has shown that even with correct design of circuit diagram, printed circuit board is designed incorrectly, which negatively affects reliability of electronic equipment. For example, if two thin parallel PCB lines are close to each other, there will be waveform delay and reflection noise at end of transmission line; interference caused by inattentive power considerations Power and ground wires will interfere with product. Therefore, when designing a printed circuit board, care should be taken to select correct method. Each switching power supply has four current loops:
◆ Power switch AC circuit
◆ Rectifier AC Output Circuit
◆ Input source current loop
◆ Output load current loop input
The input capacitor is charged with an approximate DC current, and filter capacitor basically plays role of a broadband energy storage; similarly, output filter capacitor is also used to store high-frequency energy from output rectifier, while at same time eliminating DC energy of load output circuit. Therefore, terminals of input and output capacitors of filter are very important, and input and output current loops should be connected to power supply.power source only from terminals of filter capacitors; if connection between input/output circuit and power switch/rectifier circuit cannot be connected to capacitor.
The AC circuit of power switch and AC circuit of rectifier contain high amplitude trapezoidal currents. The harmonic content of these currents is very high and their frequency is much higher than fundamental switching frequency. The transition time is typically about 50 ns. These two circuits are most likely to generate electromagnetic interference, so these AC circuits must be run before any other printed lines are connected in power supply. The three main components of each circuit, filter capacitors, power switches or rectifiers, inductors or transformers , must be connected to each other. When placing components side by side, arrange them so that current path between them is as short as possible.
The best way to build a switching power supply circuit is similar to its electrical circuit. The best way to design is like this:
1. Place transformer
2. Design a circuit breaker current loop
3. Design output rectifier current loop
4. Control circuit connected to AC circuit
Design input current source circuit and input filter.Design output load circuit and output filter.According to functional block of circuit, following principles should be observed when placing all circuit components:< /p>
● The first thing to pay attention to is PCB size. When PCB size is too large, printed lines will be long, impedance will increase, noise reduction capability will decrease, and cost will also increase; if size is too small, heat dissipation will be poor and adjacent lines will be easily broken. The best PCB shape is rectangular, with an aspect ratio of 3:2 or 4:3, and components located on edge of PCB are usually at least 2mm from edge of PCB.
● Consider future soldering when placing devices, not too tight
● Take main components of each functional diagram as center and create a layout around it. Components should be placed on PCB evenly, neatly and compactly, pins and connections between components should be minimized and shortened, and decoupling capacitors should be as close as possible to VCC of device
● For circuits operating at high frequencies, distribution parameters between components must be taken into account. As a general rule, circuit should place components in parallel as much as possible. Thus, it is not only beautiful, but also easy to assemble and weld, as well as mass-produced.
● Position each functional unitinitialize circuit according to flow of circuit, make layout convenient for signal transmission, and keep signal in same direction whenever possible
● The first layout principle is to ensure speed of wiring, paying attention to connection of twisted wire when moving device, and connecting devices with connection interconnection
● Minimize loop area as much as possible to suppress radiation interference from switching power supply
The switching power supply contains high frequency signals. Any printed line on a printed circuit board can act as an antenna. The length and width of a printed line will affect its impedance and inductance, thereby affecting frequency response. Even traces carrying DC signals can couple with RF signals from adjacent traces and cause problems in circuit (or even emit unwanted signals again). Therefore, all printed lines that carry AC should be as short and wide as possible, which means that all components connected to printed lines and connected to other power lines must be very close.
The length of a printed line is proportional to its inductance and impedance, and width is inversely proportional to inductance and impedance of a printed line. The length reflects wavelength to which printed line responds. The longer length, lower frequency at which printed line can transmit and receive electromagnetic waves, and more RF energy it can radiate. According to current of circuit board, try to increase width of power line to reduce loop resistance. At same time, direction of power line and ground line should match direction of current, which helps to improve ability to withstand noise. Grounding is lower branch of four current loops of switching power supply, it plays a very important role as a common reference point for circuit and is an important method of noise control. Therefore, placement of ground wires must be carefully considered in circuit, and mixing different grounds will result in unstable operation of power supply.
After completing wiring design, it is necessary to carefully check whether wiring design complies with rules formulated by designer, and it is also necessary to confirm that established rules comply with requirements of printed circuit board manufacturing process. Whether distance between pads, lines and through holes, pads of components and through holes, as well as through-holes and through-holes, and whether it meets manufacturing requirements. Is width of power line and ground line suitable, and is there a place on PCB where ground line can be extended. Note: some errors can be ignored, for example, Outline part of somex connectors are placed outside board frame, and errors will occur when checking gap, in addition, copper must be refilled each time tracks and vias are modified. .
The overview is based on PCB Checklist, which includes design rules, layer definitions, line widths, spacing, pads, and via settings. It is also important to check rationality of device layout, power wiring and terrestrial networks, routing and shielding of high-speed clock network, placement and connection of decoupling capacitors, etc.
6. Imprint Design
Notes on exporting gerber files:
● Layers to be output include wiring layer (bottom layer), screen printing layer (including silk screen printing of top layer, screen printing of bottom layer), solder mask layer (solder mask of bottom layer), drilling layer (bottom layer ) and drill file (NC Drill)
● When setting up silkscreen layer, do not select Part Type, but select top layer (bottom layer) and Outline, Text, and Line of silkscreen layer
● When setting Layer of each layer, select Board Outline, when setting Layer of silkscreen layer, do not select Part Type, select top layer (bottom layer) and silkscreen layer Contour, Text, Line
● When creating a detail file, use default PowerPCB settings, do not make any changes
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