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 steps are performed for each step. analysis.
- Design flow from schematic to PCB
Create component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verification design -> review -> CAM output.
- Parameter settings
Adjacent wire spacing must meet electrical safety requirements, and spacing should be as wide as possible for ease of operation and production. The minimum spacing must be suitable for the voltage to withstand at least. 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 increase the spacing. Under normal circumstances Set the track pitch to 8mil. The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, so as to avoid the defect of the pad during processing. When the traces connected to the pads are thin, the connection between the pads and the traces is designed to be drop-shaped. This has the advantage that the pads are not easily stripped, but the traces are not easily disconnected from the pads.
- Component layout
The practice has proved that even if the circuit schematic design is correct, improper design of the printed circuit board will also have an adverse effect on the reliability of electronic equipment. For example, if the two fine parallel lines of the printed board are in close proximity, 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 inconsiderate consideration of the power source and the ground line will make the product The performance is degraded. Therefore, when designing a printed circuit board, care should be taken to use the correct method. Each switching power supply has four current loops:
◆ Power switch AC circuit
◆ Output rectifier circuit
◆ Input signal source current loop
◆ Output load current loop input circuit
By charging the input capacitor with an approximately dc current, the filter capacitor acts primarily as a broadband energy storage; similarly, the output filter capacitor is also used to store the high-frequency energy from the output rectifier while eliminating the dc energy from the output load circuit. Therefore, the input and output filter capacitor terminals are very important, the input and output current loops should only be connected to the power supply from the filter capacitor terminals respectively; if the connection between the input/output circuit and the power switch/rectifier circuit cannot be connected to the capacitor. The terminals are directly connected and the AC energy will be radiated into the environment by the input or output filter capacitors.
The AC circuit of the power switch and the AC circuit of the rectifier contains high-amplitude trapezoidal currents. The harmonic components of these currents are very high, their frequency is much greater than the switching fundamental frequency, and the peak amplitude can be up to 5 times the continuous input/output DC current amplitude. The transition time is usually About 50ns. These two loops are the most susceptible to electromagnetic interference. Therefore, these AC loops must be routed before the other traces in the power supply. The three main components of each loop filter capacitor, power switch or rectifier, inductor or transformer should be Placed adjacently, adjust the position of the components so that the current path between them is as short as possible.
The best way to create a switching power supply layout is similar to its electrical design. The best design flow is as follows:
Place the transformer
- Design power switch current loop
- Design output rectifier current loop
- A control circuit connected to AC power circuit
Designing the input current source circuit and input filter Designing the output load circuit and the output filter According to the functional unit of the circuit, when laying out all the components of the circuit, the following principles must be met:
- The size of the PCB must be considered first. 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 best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. The components on the edge of the circuit board are generally not less than 2mm from the edge of the circuit board.
- Consider the future soldering when placing the device. Do not be too dense
- Center around the core components of each functional circuit and layout around it. The components should be arranged evenly, neatly and compactly on the PCB to minimize and shorten the leads and connections between the components. The decoupling capacitors should be as close as possible to the VCC of the device.
- Circuits that operate at high frequencies must consider the distribution parameters between components. General circuits should arrange parallel components as much as possible. In this way, not only beautiful but also easy to weld, easy mass production
- Arrange the positions of each functional circuit unit according to the flow of the circuit so that the layout is convenient for signal circulation and the signal is kept in the same direction as possible
- The first principle of the layout is to ensure the routing of the wiring, pay attention to the connection of the flying line when moving the device, put the device with the connection relationship together
- Reduce the loop area as much as possible to suppress the radiated interference of the switching power supply
The switching power supply contains high-frequency signals. Any printed wire on the PCB can act as an antenna. The length and width of the printed wire will affect its impedance and inductive reactance, which will affect the frequency response. Even a printed wire passing through a DC signal may couple from the adjacent printed wire to the RF signal and cause circuit problems (even re-radiate an interference signal). Therefore, all printed lines with alternating current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and connected to other power lines must be placed in close proximity.
The length of the trace is directly proportional to the amount of 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 send and receive electromagnetic waves, it can radiate more RF energy. According to the size of the printed circuit board current, try to increase the width of the power cord to reduce loop resistance. At the same time, the direction of the power line and the ground line and the direction of the current are the same, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply and serves as a common reference point for the circuit. It is an important method of controlling interference. Therefore, careful consideration should be given to the placement of the grounding wire in the layout. Mixing various groundings will cause unstable power supply operation.
After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules established by the designer. It is also necessary to confirm whether the established rules are in accordance with the requirements of the PCB production process. Generally, the inspection line and the line, the line and the component pads, and the line Whether the distance between the through hole, the component pad and the through hole, the through the hole and the through hole is reasonable, and whether the production requirement is satisfied. Whether the width of the power line and the ground line is appropriate, and whether there is a place where the ground line can be widened in the PCB. Note: Some errors can be ignored. For example, some of the outlines of the connectors are placed outside the frame and errors occur when checking the spacing. After each modification of the traces and vias, the copper must be renewed once.
According to the “PCB Inspection Checklist”, the review includes design rules, layer definitions, line widths, pitches, pads, and vias settings, as well as a review of the rationality of device layout, power supply and ground network routing, and high-speed clock networking. The wiring and shielding, the placement and connection of decoupling capacitors, etc.
- Design output
Precautions for outputting light files:
- The layers to be exported include wiring layer (bottom layer), silkscreen layer (including top screen printing, bottom screen printing), solder mask (underlayer solder mask), drilling layer (bottom layer), and drill file (NC Drill)
- When setting the layer of silkscreen layer, do not select Part Type, select the top layer (bottom layer) and silkscreen layer Outline, Text, Line
- When setting the layer for each layer, select BoardOutline. When setting the layer of the silkscreen layer, do not select Part Type, select the top layer (bottom layer), and select Outline, Text, and Line of the silkscreen layer.