AC/DC PWM method flyback converter design method: substrate layout example

The switching power supply uses the ON/OFF of the switch to control the voltage, but it is still an analog circuit. In extreme terms, although it emits high-frequency noise, it has a feedback loop and is very sensitive to noise. In other words, the path of the switching power supply circuit must be divided into a path that generates noise after a large current is turned on/off, and a control signal path that is very sensitive to noise. The wiring layout of the substrate, the large current path should avoid noise, and the control signal path should avoid the influence of noise. Of course, this kind of noise is radiated noise and will also affect EMC, so the layout must be able to avoid noise as much as possible.

After completing the circuit drawing, enter the stage of actual planning and installation of the layout of the substrate. This item will explain examples of substrate layout, layout principles or key points.

Key points of substrate layout

The switching power supply uses the ON/OFF of the switch to control the voltage, but it is still an analog circuit. In extreme terms, although it emits high-frequency noise, it has a feedback loop and is very sensitive to noise. In other words, the path of the switching power supply circuit must be divided into a path that generates noise after a large current is turned on/off, and a control signal path that is very sensitive to noise. The wiring layout of the substrate, the large current path should avoid noise, and the control signal path should avoid the influence of noise. Of course, this kind of noise is radiated noise and will also affect EMC, so the layout must be able to avoid noise as much as possible.

A good layout requires considerable experience. Improper layout may cause the power supply to fail to start normally, the S/N of the entire system may deteriorate, and the component parts and power supply IC may be damaged in the worst case. Although it is a work that relies heavily on experience, most of them will provide basic board wiring layout examples that can be completed as long as the power supply IC’s technical specifications and additional data are used for users’ reference. Some companies even provide values ​​that can be used directly, such as gerber files.

These best results completed by experienced engineers are worthy of more use.

The following are examples and precautions for the board layout.

AC/DC PWM method flyback converter design method: substrate layout example

The above figure shows the high current path and the control signal path on some simple circuit drawings.

• The red line is the path of large current, which is the main cause of ringing or loss. It should be as thick as possible and shortened as much as possible
• In addition, minimize the loop of the red line
• The orange line on the secondary side is the same as the red line as thick as possible, and the loop as narrow as possible
• The brown wire is the path for the current to flow to the VCC pin. Current flows when switching, so it must be wired separately
• The bottom of the transformer will be affected by the magnetic flux, so it cannot pass through the IC control signal line
• The GND of the red wire, brown wire, blue wire and v color wire should be grounded at one point
• The v color wire is the path through which the surge on the secondary side flows to the primary side, and a large current will flow instantaneously, so separate wiring outside of the red wire and blue wire
• The blue wire is the GND wire for IC control signals, which does not flow large currents, but is easily affected by noise, so it must be wired separately in addition to the red wire, green wire, and brown wire

The following figure is an example of the board layout after considering the above precautions. Mark the range of the red, orange, and brown loops in the figure above.

AC/DC PWM method flyback converter design method: substrate layout example

The photo below is a schematic diagram after installation. It is slightly different from the circuit above, but uses the same power IC, and the component structure is almost the same. Compared with the drawings, you can see what the actual items are.

AC/DC PWM method flyback converter design method: substrate layout example

AC/DC PWM method flyback converter design method: substrate layout example

In the actual design, due to the physical and mechanical factors such as the length and width of the available substrate, the ideal layout may not be designed. However, as mentioned above, in addition to being used as a power source, it may also cause adverse effects on the entire system. In addition, although it is a prototype substrate, there is no difference between a major revision and a redesign, so doing the best at the beginning as much as possible will help save time and costs later.

When designing the power supply, first determine the rough system specifications, many of which are designed at the end of the project. In any case, I hope to design a system that is easy to adjust and easy to operate.

The Links:   2SB1202T-E NL2432HC22-30B