PCB Technical - Circuit board design specification of switching power supply

Circuit board design specification of switching power supply

In any switching power supply design, the physical design of the PCB circuit board is the last link. If the design method is improper, the circuit board may radiate excessive electromagnetic interference and cause the power supply to work unstable. The following is the attention required in each step The matters are analyzed:
1. Design flow from schematic to circuit board 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 have an adverse effect on the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, it will cause the delay of the signal waveform and the reflection noise 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 input circuit charges the input capacitor through an approximate DC current, and the filter capacitor mainly serves as a 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 of the output load circuit. . Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should only be connected to the power supply from the terminals of the filter capacitor respectively; if the connection between the input/output loop and the power switch/rectifier loop cannot be connected to the capacitor The terminal is directly connected, and the AC energy will be radiated into 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 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 size of the circuit board. When the size of the circuit board 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 size of the circuit board is too small, the heat dissipation will not be good, and the 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 arranged evenly, neatly and compactly on the circuit board, 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.
(6) The first principle of the layout is to ensure the wiring rate, pay attention to the connection of the flying leads when moving the device, and put the devices with the connection relationship together.
(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply