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The relationship between layout and PCB board-switching power supply
PCB Blog
The relationship between layout and PCB board-switching power supply

The relationship between layout and PCB board-switching power supply

2022-01-05
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Author:pcb

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, an electromagnetic compatibility engineer, or a PCB board layout engineer, you must understand the cause of the electromagnetic compatibility problem and take measures, especially Layout engineers, need to understand how to avoid the expansion of dirty spots, this article mainly introduces

The main points of the power supply PCB board design are discussed.
29 basic relationships between layout and PCB board
1. Several basic principles: any wire has impedance; current always automatically selects the path of 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 principles of reducing EMI and enhancing anti-interference ability are 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: the width of the trace Try to be as large as possible, but remove the excess part), and try to walk in a straight line, reduce its hidden enclosed 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 decrease the mutual capacitance with these signals (the main way is to reduce the effective coupling area and increase the distance. The mutual capacitance decreases as the distance increases. Faster) is more critical.
5. Use the principle of loop cancellation as much as possible to route, and further reduce the area of large di/dt loops (similar to twisted pair, use the principle of loop cancellation to improve anti-interference ability and increase transmission distance).

PCB board

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 design the return path of each trace.
8. When multiple PCB boards are connected through connectors, it is also necessary to consider making the loop area reach, especially for large di/dt signals, high frequency signals or sensitive signals. A signal wire corresponds to a ground wire, and the two wires are as close as possible. If necessary, twisted pair wires can be used to connect (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 from the motherboard to the front panel is connected with a twisted pair cable. It can be seen that the twisted pair connection is important 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 they are not handled properly.
11. When wiring, first consider high-current traces and traces that are prone to radiation.
12. Switching power supplies usually have 4 current loops: input, output, switch, and freewheeling. 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.
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.
15. Reduce the susceptible (sensitive) signal loop area and trace length to reduce interference.
16. Small signal traces are far away from large dv/dt signal lines (such as the C pole or D pole of the switch tube, buffer (snubber) and clamp network) to reduce coupling, and ground (or power supply, in short, often 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. Do not trace the small signal under the large dv/dt signal. If the back of the small signal traces can be grounded (the same ground), the noise signal coupled to it can also be reduced.
17. A better approach is 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 heat sink), and use the upper and lower layers for grounding. Connect via holes, 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. 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. Do not 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 weighed and considered in the actual design.
20. To prevent common impedance interference, use one point grounding and power supply from one point.

PCB board

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, the nature of the ground should be judged first, and then the connection should be made. The ground for sampling and error amplification should usually be connected to the negative pole of the output capacitor. The sampling signal should usually be taken out from the positive pole of the output capacitor. Common impedance interference. Usually the control ground and drive ground of the IC are not led out separately. At this time, the lead impedance of the sampling resistor to the above ground must be as small as possible to reduce common impedance interference and improve the accuracy of current sampling.
23 The output voltage sampling network is close to the error amplifier instead of 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 inductances to be far away and perpendicular to each other to reduce mutual inductance, especially energy storage inductance and filter inductance.
25. Pay attention to the layout when high-frequency capacitors and low-frequency capacitors are used in parallel, and the high-frequency capacitors are close to the user.
26. Low-frequency interference is generally differential mode (below 1M), and high-frequency interference is generally common mode, usually coupled through 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 way to solve 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 as possible to the incoming line, and the routing 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 line is 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 incoming lines and high dv/dt signal traces, which should be considered in the PCB board layout.