PCB Technical - PCB design switching power supply

The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

1. Place the transformer

2.PCB design power switch current loop

3. Design the output rectifier current loop

4. Control circuit connected to AC power circuit

Design input current source loop and input filter

When designing the output load loop and 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:

. First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. The best shape of the circuit board is rectangular, the aspect ratio is 3: 2 or 4: 3, the components located on the edge of the circuit board, generally not less than 2mm from the edge of the circuit board

. When placing the device, consider the subsequent soldering, not too dense

. Take the core component of each functional circuit as the center and lay out around it. The components should be arranged evenly, neatly and compactly on the PCB, minimize and shorten the leads and connections between the components, and the decoupling capacitor should be as close as possible to the VCC of the device

. For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass produce

. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible

. The first principle of PCB layout is to ensure the routing rate, pay attention to the connection of the flying leads when moving the components, and put the components with the connection relationship together

.Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply

4. Wiring

The switching power supply contains high-frequency signals. Any printed line on the PCB can function as an antenna. The length and width of the printed line will affect its impedance and inductance, thereby affecting the frequency response. Even the printed lines that pass DC signals will couple from adjacent printed lines to

Radio frequency

Signal and cause circuit problems (even radiating interference signals again). Therefore, all printed lines that pass AC current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and other power lines must be placed very close.

The length of the printed line is proportional to its inductance and impedance, and the width is inversely proportional to the inductance and impedance of the printed line. The length reflects the wavelength of the printed line's response. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, and it can radiate more radio frequency energy. According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop

resistance. At the same time, make the direction of the power line and the ground line 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, and it is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout. Mixing various groundings will cause unstable power supply operation.

5. Check

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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 necessary to confirm whether the established rules meet the requirements of the printed board production process. Generally, check the lines and lines, the line and the component pads, and the lines. Whether the distance from the through hole, the component pad and the through hole, the through hole and the through hole is reasonable, and whether it meets the production requirements. Whether the width of the power line and the ground line are appropriate, and whether there is a place to widen the ground line in the PCB. Note: Some errors can be ignored. For example, 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 must be re-coated.

According to the "PCB circuit board checklist", the content includes design rules, layer definitions, line widths, spacing, pads, via settings, and also focus on reviewing the rationality of the device layout, power supply and grounding network routing, high-speed The routing and shielding of the clock network, the placement and connection of decoupling capacitors, etc.

6. Design output

Precautions for exporting Gerber files:

. The layers that need to be output include wiring layer (bottom layer), silk screen layer (including top and bottom screen printing), solder mask (bottom solder mask), drilling layer (bottom layer), and also generate drilling files (NC Drill)

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

. When setting the Layer of each layer, select Board Outline. When setting the Layer of the silk screen layer, do not select Part Type, select the top layer (bottom layer) and the Outline, Text, Line of the silk screen layer

. When generating drilling files, use PowerPCB