Electronic Design - Design and Analysis of Switching Power Supply PCB Board

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

1. The design process from schematic to PCB. Establish component parameters-"input principle netlist-"design parameter settings -" manual layout-"manual wiring-"verify design -" review-"CAM output.

2. Parameter setting The distance between adjacent wires must be able to meet the electrical safety requirements, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum spacing must be at least suitable for the voltage tolerated. When the wiring density is low, the spacing of the signal lines can be increased appropriately. For signal lines with a large gap between high and low levels, the spacing should be as short as possible and the spacing should be increased. Generally, Set the trace spacing 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, which can avoid the defects of the pad during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed into a drop shape. The advantage of this is that the pads are not easy to peel, but the traces and the pads are not easily disconnected.

Third, the component layout practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, the signal waveform will be delayed and reflected noise will be formed at the terminal of the transmission line. The performance drops, so when designing the printed circuit board, you should pay attention to adopting 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

The AC circuit of the power switch and the AC circuit of the rectifier contain high-amplitude trapezoidal currents. The harmonic components of these currents are very high. The frequency is much greater than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times the amplitude of the continuous input/output DC current. The transition time is usually About 50ns. These two loops are the most prone to electromagnetic interference, so these AC loops must be laid out before the other printed lines in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, inductors or transformers. Place them next to each other and adjust the position of the components to make the current path between them as short as possible. The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

Place the transformer

Design the power switch current loop

Design output rectifier current loop

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:

(1) 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, and the aspect ratio is 3:2 or 4:3. The components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.

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

(3) Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged 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.

(4) For circuits operating at high frequencies, the distributed parameters between components should 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.

(5) 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.

4. Wiring The switching power supply contains high-frequency signals. Any printed line on the PCB can act as an antenna. The length and width of the printed line will affect its impedance and inductance, thereby affecting the frequency response. Even printed lines that pass DC signals can couple to radio frequency signals from adjacent printed lines and cause circuit problems (and even radiate interfering 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 following points should be paid attention to in the ground wire design:

1. Correctly choose single-point grounding. Generally, the common end of the filter capacitor should be the only connection point for other grounding points to couple to the AC ground of high current. It should be connected to the grounding point of this level. The main consideration is that the current returning to the ground in each part of the circuit is changed. The impedance of the actual flowing line will cause the change of the ground potential of each part of the circuit and introduce interference.

2. Thicken the grounding wire as much as possible. If the grounding wire is very thin, the ground potential will change with the current change, which will cause the timing signal level of the electronic equipment to be unstable, and the anti-noise performance will deteriorate. Therefore, ensure that each high current ground terminal Use printed lines as short and wide as possible, and widen the width of the power and ground lines as much as possible. It is better to make the ground line 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, and a large area copper layer can also be used as a ground wire. Connect the unused places on the printed circuit board as a ground wire.

5. After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. At the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed board production process. Generally check the wire and wire, wire and component welding Whether the distance between the disk, the wire and the through hole, the component pad and the through hole, and the distance between 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, when a part of the outline of some connectors is placed outside the board frame, errors will occur when checking the spacing; in addition, each time the traces and vias are modified, the copper must be re-plated. 6. Review According to the "PCB checklist", the content includes design rules, layer definitions, line widths, spacing, pads, and via settings. It should also focus on reviewing the rationality of the device layout, the routing of power and ground networks, and high-speed The routing and shielding of the clock network, the placement and connection of decoupling capacitors, etc.