Due to the switching characteristics of the switching power supply, it is easy to cause the switching power supply to produce great electromagnetic compatibility interference. As a power supply engineer, electromagnetic compatibility engineer, or a PCB layout engineer, you must understand the causes of electromagnetic compatibility problems and have resolved measures, especially layout Engineers need to know how to avoid the expansion of dirty spots. This article mainly introduces the main points of power supply PCB design.

1. Several basic principles: any wire has impedance; current always automatically selects the path with the least impedance; radiation intensity is related to current, frequency, and loop area; common mode interference is related to the mutual capacitance of large dv/dt signals to ground ; The principle of reducing EMI and enhancing anti-interference ability is similar.

2. The layout should be partitioned according to power supply, analog, high-speed digital and each functional block.

3. Minimize the area of ​​the large di/dt loop and reduce the length (or area, width of the large dv/dt signal line). The increase in the trace area will increase the distributed capacitance. The general approach is: trace width Try to be as large as possible, but remove the excess part), and try to walk in a straight line to reduce the hidden area to reduce radiation.

4. Inductive crosstalk is mainly caused by the large di/dt loop (loop antenna), and the induction intensity is proportional to the mutual inductance, so it is more important to reduce the mutual inductance with these signals (the main way is to reduce the loop area and increase the distance); Sexual crosstalk is mainly generated by large dv/dt signals, and the induction intensity is proportional to the mutual capacitance. All the mutual capacitances with these signals are reduced (the main way is to reduce the effective coupling area and increase the distance. The mutual capacitance decreases with the increase of distance. Faster) is more critical.

5. Try to use the principle of loop cancellation to further reduce the area of ​​the large di/dt loop, as shown in Figure 1 (similar to twisted pair
Use the principle of loop cancellation to improve the anti-interference ability and increase the transmission distance):

Figure 1, Loop cancellation (freewheeling loop of boost circuit)

6. Reducing the loop area not only reduces the radiation, but also reduces the loop inductance, making the circuit performance better.

7. Reducing the loop area requires us to accurately design the return path of each trace.

8. When multiple PCBs are connected via connectors, it is also necessary to consider minimizing the loop area, especially for large di/dt signals, high frequency signals or sensitive signals. It is best that one signal wire corresponds to one ground wire, and the two wires are as close as possible. If necessary, twisted pair wires can be used for connection (the length of each twisted pair wire corresponds to an integer multiple of the noise half-wavelength). If you open the computer case, you can see that the USB interface between the motherboard and the front panel is connected with a twisted pair, which shows the importance of the twisted pair connection for anti-interference and reducing radiation.

9. For the data cable, try to arrange more ground wires in the cable, and make these ground wires evenly distributed in the cable, which can effectively reduce the loop area.

10. Although some inter-board connection lines are low-frequency signals, because these low-frequency signals contain a lot of high-frequency noise (through conduction and radiation), it is easy to radiate these noises if not handled properly.

11. When wiring, first consider large current traces and traces that are prone to radiation.

12. Switching power supplies usually have 4 current loops: input, output, switch, freewheeling, (Figure 2). Among them, the input and output current loops are almost direct current, almost no emi is generated, but they are easily disturbed; the switching and freewheeling current loops have larger di/dt, which needs attention.
Figure 2, Current loop of Buck circuit

13. The gate drive circuit of the mos (igbt) tube usually also contains a large di/dt.

14. Do not place small signal circuits, such as control and analog circuits, inside large current, high frequency and high voltage circuits to avoid interference.

To be continued…..