Talk about some of the most basic principles of switching power supply PCB wiring.
1, Spacing
For high-voltage products must take into account the spacing between the wires. A spacing that meets the appropriate safety requirements is of course preferable, but in many cases for products that do not require certification, or cannot meet certification, the spacing is determined by experience. What is the appropriate spacing? Consideration must be given to whether or not the production can guarantee a clean board surface, ambient humidity, other contaminants, etc.
For utility input, even if you can ensure that the board is clean, sealed, MOS tube leakage between the poles close to 600V, less than 1mm is in fact more dangerous!
2, board edge components
PCB edge of the chip capacitors or other easily damaged devices, must be considered when placing the PCB sub-panel direction, such as the figure is a variety of placement methods, the device is subject to the size of the stress comparison.
3, loop area
Whether input or output, power loop or signal loop, should be as small as possible. The power loop emits electromagnetic fields, which will result in poor EMI characteristics or large 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 Alignment
Due to the di/dt effect, the inductance at the dynamic node must be reduced, otherwise a strong electromagnetic field will be generated. To reduce inductance, mainly to reduce the length of the wiring, increase the width of the role of small.
5, signal lines
For the entire control section, the wiring should be considered away from the power section. If the two are close to each other due to other constraints, the control line should not be parallel to the power line, otherwise it may lead to abnormal power operation and vibration.
In addition, if the control line is very long, it should be close to the pair of lines back and forth, or the two are placed on two sides of the PCB and directly opposite each other, thus reducing its loop area and avoiding interference by the electromagnetic field of the power section. As Figure 2 illustrates the A, B between the two points, the right and wrong signal line wiring method.
6, laying copper
Sometimes laying copper is completely unnecessary, or even should be avoided. If the copper area is large enough and its voltage is constantly changing, on the one hand, it may be used as an antenna to radiate electromagnetic waves to the surrounding; on the other hand, it is easy to pick up the noise.
Usually only allowed to lay copper in the static nodes, such as the output side of the "ground" node laying copper, can be equivalent to increase the output capacitance, filter out some noise signals.
7, Mapping
For a circuit, you can lay copper on one side of the PCB, it will be automatically mapped according to the wiring on the other side of the PCB to minimise the impedance of the circuit. This is like a set of different impedance values of the impedance in parallel, the current will automatically choose the impedance of the smallest path to flow through the same.
In fact, you can control the part of the circuit on one side of the wiring, and on the other side of the "ground" node laying copper, the two sides of the connection through the hole.
8, output rectifier diode
If the output rectifier diode is close to the output, it should not be placed parallel to the output. Otherwise, the electromagnetic field generated at the diode will penetrate the power supply output and external load loop, so that the measured output noise increases.
9, ground
Ground wiring must be very careful, otherwise it may cause EMS, EMI performance and other performance deterioration. For switching power supply PCB "ground", at least the following two points: (1) power ground and signal ground, should be connected to a single point; (2) there should be no ground loop.
10, Y capacitance
Input and output will often access the Y capacitor, sometimes for some reason, it may not be able to hang on the input capacitor ground, then remember, must be connected to the static node, such as high-voltage end.
11, Other
The actual power supply PCB design, you may also want to consider some other issues, such as "varistors should be close to the protected circuit", "common mode inductance to increase the discharge teeth", "chip VCC power supply should be added to the porcelain capacitors "and so on. In addition, whether the need for special treatment, such as copper foil, shielding, etc., in the PCB design stage also needs to be considered.
