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.
15. Reduce the susceptible (sensitive) signal loop area and wiring length to reduce interference.
16. The small signal traces are far away from the large dv/dt signal lines (such as the C pole or D pole of the switch tube, the buffer (snubber) and the clamp network) to reduce coupling, and the ground (or power supply, in short) Potential signal) to further reduce the coupling, and the ground should be in good contact with the ground plane. At the same time, small signal traces should be as far away as possible from large di/dt signal lines to prevent inductive crosstalk. It is better not to go under the large dv/dt signal when the small signal traces. If the back of the small signal trace can be grounded (the same ground), the noise signal coupled to it can also be reduced.
17. It is better to lay the ground around and on the back of these large dv/dt and di/dt signal traces (including the C/D poles of the switching devices and the switch tube radiator), and use the upper and lower layers of ground Via hole connection, and connect this ground to a common ground point (usually the E/S pole of the switch tube, or sampling resistor) with a low impedance trace. This can reduce radiated EMI. It should be noted that the small signal ground must not be connected to this shielding ground, otherwise it will introduce greater interference. Large dv/dt traces usually couple interference to the radiator and nearby ground through mutual capacitance. It is best to connect the switch tube radiator to the shielding ground. The use of surface-mount switching devices will also reduce the mutual capacitance, thereby reducing coupling.
18. It is best not to use vias for traces that are prone to interference, as it will interfere with all layers that the via passes through.
19. Shielding can reduce radiated EMI, but due to increased capacitance to ground, conducted EMI (common mode, or extrinsic differential mode) will increase, but as long as the shielding layer is properly grounded, it will not increase much . It can be considered in the actual design.
20. To prevent common impedance interference, use one point grounding and power supply from one point.
21. Switching power supplies usually have three grounds: input power high current ground, output power high current ground, and small signal control ground. The ground connection method is shown in the following diagram:
22. When grounding, first judge the nature of the ground before connecting. The ground for sampling and error amplification should usually be connected to the negative pole of the output capacitor, and the sampling signal should usually be taken out from the positive pole of the output capacitor. The small signal control ground and drive ground should usually be connected to the E/S pole or sampling resistor of the switch tube respectively to prevent Common impedance interference. Usually the control ground and drive ground of the IC are not led out separately. At this time, the lead impedance from the sampling resistor to the above ground must be as small as possible to minimize common impedance interference and improve the accuracy of current sampling.
23. The output voltage sampling network is best to be close to the error amplifier rather than to the output. This is because low impedance signals are less susceptible to interference than high impedance signals. The sampling traces should be as close as possible to each other to reduce the noise picked up.
24. Pay attention to the layout of inductors to be far away and perpendicular to each other to reduce mutual inductance, especially energy storage inductors and filter inductors.
25. Pay attention to the layout when the high-frequency capacitor and the low-frequency capacitor are used in parallel, the high-frequency capacitor is close to the user.
26. Low-frequency interference is generally differential mode (below 1M), and high-frequency interference is generally common mode, usually coupled by radiation.
27. If the high frequency signal is coupled to the input lead, it is easy to form EMI (common mode). You can put a magnetic ring on the input lead close to the power supply. If the EMI is reduced, it indicates this problem. The solution to this problem is to reduce the coupling or reduce the EMI of the circuit. If the high-frequency noise is not filtered clean and conducted to the input lead, EMI (differential mode) will also be formed. At this time, the magnetic ring cannot solve the problem. String two high-frequency inductors (symmetrical) where the input lead is close to the power supply. A decrease indicates that this problem exists. The solution to this problem is to improve filtering, or to reduce the generation of high-frequency noise by buffering, clamping and other means.
28. Measurement of differential mode and common mode current:
29. The EMI filter should be as close to the incoming line as possible, and the wiring of the incoming line should be as short as possible to minimize the coupling between the front and rear stages of the EMI filter. The incoming wire is best shielded with the chassis ground (the method is as described above). The output EMI filter should be treated similarly. Try to increase the distance between the incoming line and the high dv/dt signal trace, and consider it in the layout.