PCB design skills of switching power supply

In any switching power supply design, the physical design of the PCB board is the last step. If the design method is improper, the PCB Design may radiate too much electromagnetic interference, causing the power supply to work unstably. The following is the analysis of the matters needing attention in each step.

1. PCB Design process from schematic diagram to PCB
Create component parameters ->Input schematic net list ->Design parameter setting ->Manual layout ->Manual wiring ->Validate design ->Review ->CAM output.

2. Parameter setting
The spacing of adjacent conductors must meet the electrical safety requirements, and the spacing should be as wide as possible in order to facilitate operation and production. The minimum spacing should be at least suitable for the voltage to be borne. When the wiring density is low, the spacing of signal lines can be appropriately increased. For signal lines with high and low level differences, the spacing should be as short as possible and increased. Generally, the routing spacing should be set as 8mil. The distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, so as to avoid the pad defect during processing. When the wiring connected to the pad is thin, the connection between the pad and the wiring should be designed as water droplets. This has the advantage that the pad is not easy to peel, but the wiring is not easy to disconnect from the pad.

3. Component layout
Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is inappropriate, it will have a negative impact on the reliability of electronic equipment. For example, if two thin parallel lines of the printed circuit 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 improper consideration of power supply and ground wire will degrade the performance of the product. Therefore, when designing the printed circuit board, attention should be paid to the correct method. Each switching power supply has four current circuits:
◆ AC circuit of power switch
◆ Output rectifier AC circuit
◆ Input signal source current circuit
◆ Output load current circuit input circuit

The input capacitor is charged by an approximate DC current, and the filter capacitor mainly plays a role of broadband energy storage; Similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier and eliminate the DC energy from the output load circuit. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current circuits should only be connected to the power supply from the terminals of the filter capacitor; If the connection between the input/output circuit and the power switch/rectifier circuit cannot be directly connected to the terminal of the capacitor, the AC energy will be radiated to the environment by the input or output filter capacitor.

The AC circuit of the power switch and the AC circuit of the rectifier contain high-amplitude trapezoidal current. The harmonic component of these currents is very high, and its frequency is far greater than the switching fundamental frequency. The peak amplitude can be up to 5 times of the amplitude of the continuous input/output DC current, and the transition time is usually about 50ns. These two circuits are most likely to generate electromagnetic interference. Therefore, these AC circuits must be laid before wiring other printed lines in the power supply. The three main components of each circuit, such as filter capacitor, power switch or rectifier, inductor or transformer, should be placed adjacent to each other. Adjust the position of the components to make the current path between them as short as possible.

The best way to establish the switch power supply layout is similar to its electrical design. The best design process is as follows:
1. Place transformer
2. Design power switch current circuit
3. Design output rectifier current circuit
4. Control circuit connected to AC power circuit

When designing the input current source circuit and the input filter, designing the output load circuit and the output filter according to the functional unit of the circuit, the layout of all components of the circuit should comply with the following principles:
● PCB size should be considered first. When the PCB size is too large, the printed line is long, the impedance increases, the anti-noise ability decreases, and the cost also increases; If it 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, with a length-width ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm from the edge of the circuit board
● When placing components, consider future welding, not too dense
● Take the core components of each functional circuit as the center and lay out around it. Components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the lead and connection between components, and the decoupling capacitance should be as close as possible to the VCC of components
● For circuits operating at high frequency, the distribution parameters between components shall be considered. In general circuits, components shall be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass production
● Arrange the position of each functional circuit unit according to the circuit flow, make the layout convenient for signal flow, and keep the signal in the same direction as far as possible
● The primary principle of the layout is to ensure the routing rate of the wiring, pay attention to the connection of the flying wire when moving the components, and put the components with connection relationship together
● Reduce the loop area as much as possible to suppress the radiated interference of switching power supply

4. Wiring
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, thus affecting the frequency response. Even the printed wire passing through the DC signal will be coupled to the RF signal from the adjacent printed wire and cause circuit problems (even radiate the interference signal again). Therefore, all printed lines passing through AC current should be designed as short and wide as possible, which means that all components connected to printed lines and other power lines must be placed close.
The length of the printed wire is directly proportional to the inductance and impedance of the printed wire, while the width is inversely proportional to the inductance and impedance of the printed wire. The length reflects the response wavelength of the printed line. The longer the length is, the lower the frequency of the printed line can send and receive electromagnetic waves, and it can radiate more RF energy. According to the current of the printed circuit board, the power line width shall be increased as much as possible to reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of current, which will help to enhance the anti-noise ability. Grounding is the bottom branch of the four current circuits of the switching power supply, which plays a very 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 power supply operation.

5. Inspection
After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and at the same time, it is also necessary to confirm whether the rules formulated conform to the requirements of the printed board production process. 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, and whether it meets the production requirements. Whether the width of the power line and ground wire is appropriate, and whether there is any place in the PCB where the ground wire can be widened. Note: Some errors can be ignored. For example, part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; In addition, each time the wiring and vias are modified, the copper shall be coated again.
According to the “PCB checklist”, the review includes the design rules, layer definition, line width, spacing, pad and via settings, and also focuses on the rationality of device layout, the routing of power supply and ground wire network, the routing and shielding of high-speed clock network, and the placement and connection of decoupling capacitors.

6. PCB Design output
Precautions for outputting photo files:
● The layers to be output include wiring layer (bottom layer), silk screen layer (including top layer silk screen and bottom layer silk screen), solder mask layer (bottom layer solder mask), and drilling layer (bottom layer). In addition, a drilling file (NC Drill) should be generated
● When setting the Layer of the screen layer, do not select Part Type, but select the Outline, Text and Line of the top layer (bottom layer) and the screen layer
● When setting the Layer of each layer, select the Board Outline. When setting the Layer of the silk screen layer, do not select the Part Type. Select the Outline, Text, Line of the top layer (bottom layer) and the silk screen layer
● When generating drilling files, use the default settings of PowerPC B and do not make any changes

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