Key points and electrical requirements for PCB design of switching power supply
In any switching power supply design, the physical design of the PCB board is the last link. If the design method is not appropriate, the PCB may radiate too much electromagnetic interference, causing unstable power supply operation. The following is an analysis of the matters that need to be paid attention to in each step.
1.Design process from schematic diagram to PCB
Establish component parameters->Enter principle netlist->Design parameter setting->Manual layout->Manual wiring->Verify design->Review->CAM output.
2.Parameter setting
The spacing between adjacent wires must meet the electrical safety requirements, and for ease of operation and production, the spacing should be as wide as possible. The minimum spacing must at least be suitable for the voltage to be tolerated. When the wiring density is low, the spacing between signal lines can be appropriately increased. The signal lines with high and low level differences should be as short as possible and the spacing should be increased. In general, the routing spacing is set to 8mil. The distance from the edge of the inner hole of the pad to the edge of the printed circuit board should be greater than 1mm, so as to avoid pad defects during processing. When the trace connected to the pad is thin, the connection between the pad and the trace should be designed into a water drop shape. The advantage of this is that the pad is not easy to peel, but the trace and the pad are not easy to disconnect.
3.Component layout
Practice has proved that even if the circuit schematic is designed correctly, improper printed circuit board design will have an adverse effect on the reliability of electronic equipment. For example, if two thin parallel lines on the printed circuit board are close to each other, it will form a delay in the signal waveform and form reflected noise at the terminal of the transmission line; interference caused by inconsiderate consideration of power supply and ground wires will reduce the performance of the product. Therefore, when designing printed circuit boards, you should pay attention to using the correct method.
Each switching power supply has four current loops:
(1) Power switch AC loop
(2) Output rectifier AC loop
(3) Input signal source current loop
(4) Output load current loop The input loop charges the input capacitor with a current close to DC
The best way to establish the layout of a switching power supply is similar to its electrical design. The best design process is as follows:
- Place the transformer
- Design the switching power supply current loop
- Design the output rectifier current loop
- Connect to the control circuit of the AC power supply circuit
Design the input current source loop and input filter Design the output load loop and output filter When laying out all the components of the circuit according to the functional units of the circuit, the following principles must be followed:
(1) First, consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation is poor, and the adjacent lines are easily interfered. The best shape of the circuit board is a rectangle with an aspect ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.
(2) When placing components, consider the subsequent soldering and do not place them too densely.
(3) The core components of each functional circuit are the center and the layout is carried out around them. The components should be arranged evenly, neatly and compactly on the PCB. The leads and connections between the components should be minimized and shortened as much as possible, and the decoupling capacitor should be as close to the VCC of the device as possible.
(4) For circuits working at high frequencies, the distribution parameters between components should be considered. In general, the components should be arranged in parallel as much as possible in the circuit. This is not only beautiful, but also easy to assemble and solder, and easy to mass produce.
(5) The positions of each functional circuit unit are arranged according to the flow of the circuit, so that the layout is convenient for signal flow and the signal keeps the same direction as much as possible.
(6) The first principle of layout is to ensure the wiring pass rate. When moving the device, pay attention to the connection of the flying wire and put the devices with connection relationship together.
(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply.
