1. Spacing
For high-voltage products, the line spacing must be considered. The spacing that can meet the corresponding safety requirements is of course the best, but in many cases, for products that do not require certification, or cannot meet the certification, the spacing is determined by experience. What is the proper spacing? It must be considered whether the production can guarantee the cleanliness of the board surface, environmental humidity, other pollution, etc.
For the mains input, even if the board surface is clean and sealed, the MOS tube drain and source are close to 600V, less than 1mm is actually more dangerous!
2. Components on the edge of the board
SMD capacitors or other fragile devices on the edge of the PCB must be placed in consideration of the direction of the PCB sub-board. The picture shows the comparison of the stress on the device when various placement methods are used.
3. Loop area
Whether it is input or output, power loop or signal loop, it should be as small as possible. The electromagnetic field emitted by the power loop will cause poor EMI characteristics or larger output noise; at the same time, if it is received by the control loop, it is likely to cause anomalies.
On the other hand, if the power loop area is large, its equivalent parasitic inductance will also increase, which may increase the drain noise spike.
4. Key lines
Due to the effect of di/dt, the inductance at the dynamic node must be reduced, otherwise a strong electromagnetic field will be generated. To reduce the inductance, the main thing is to reduce the length of the wiring, and increasing the width has less effect.
5. Signal line
For the entire control section, consider routing it away from the power section. If the two are close to each other due to other restrictions, the control line and the power line should not be connected in parallel, otherwise the power supply may work abnormally and oscillate.
In addition, if the control line is very long, the back and forth pair of lines should be close, or the two should be placed on two sides of the PCB and facing each other, so as to reduce the loop area and avoid being interfered by the electromagnetic field of the power part.
6, copper plating
Sometimes copper is completely unnecessary and should even be avoided. If the copper area is large enough and its voltage is constantly changing, on the one hand, it may act as an antenna, radiating electromagnetic waves to the surrounding; on the other hand, it can easily pick up noise.
Usually, only copper is allowed to be placed on static nodes, for example, copper is placed on the "ground" node of the output, which can equivalently increase the output capacitance and filter out some noise signals.
7. Mapping
For a loop, copper can be laid on one side of the PCB, and it will be automatically mapped according to the wiring on the other side of the PCB to minimize the impedance of this loop. It is as if a group of impedances with different impedance values are connected in parallel, and the current will automatically choose the path with the least impedance to flow through.
In fact, one side of the control part of the circuit can be wired, and the other side of the "ground" node is copper, and the two sides are connected by vias.
8. Output rectifier diode
If the output rectifier diode is relatively close to the output, it should not be placed in parallel with the output. Otherwise, the electromagnetic field generated at the diode will penetrate into the loop formed by the output of the power supply and the external load, increasing the measured output noise.
9. Ground wire
The routing of the ground wire must be very careful, otherwise it may cause EMS, EMI performance and other performance degradation. For the "ground" of the switching power supply PCB, at least do the following two points: (1) The power ground and the signal ground should be connected at a single point; (2) There should be no ground loop.
10. Y capacitor
The input and output are often connected to the Y capacitor. Sometimes, for some reasons, it may not be possible to hang it on the input capacitor ground. At this time, remember to connect it to a static node, such as a high-voltage terminal.
