For the research and development of switching power supply, PCB design occupies a very important position. A bad PCB has poor EMC performance, high output noise, weak anti-interference ability, and even basic functions are defective.

Slightly different from other hardware PCBS, switching power PCBS have some characteristics of their own. This article will briefly talk about some of the most basic principles of PCB wiring for switching power supply based on engineering experience.

1, spacing

Line spacing must be considered for high voltage products. The spacing that can meet the requirements of the corresponding safety regulations is of course the best, but many times for products that do not need certification, or can not meet the certification, the spacing is determined by experience. What width of spacing is appropriate? Must consider production whether to assure board surface cleanness, environmental humidity, other pollution wait for a circumstance how.

For the mains input, even if the board surface can be guaranteed clean and sealed, MOS tube drain source electrode close to 600V, less than 1mm is actually more dangerous!

2. Components at the edge of the board

For the patch capacitance or other easily damaged devices at the edge of PCB, the PCB splitter direction must be taken into consideration when placing. The figure shows the comparison of the stress on the devices under various placement methods.

FIG. 1 Comparison of stress on the device when the plate is split

It can be seen that the device should be away from and parallel to the edge of the splitter, otherwise the component may be damaged due to the PCB splitter.

3. Loop area

Whether input or output, power loop or signal loop, should be as small as possible. The power loop emits electromagnetic field, which will lead to poor EMI characteristics or large output noise; At the same time, if received by the control ring, it is likely to cause an exception.

On the other hand, if the power loop area is larger, the equivalent parasitic inductance will also increase, which may increase the drain noise peak.

4. Key wiring

Due to the effect of DI/DT, the inductance at the dynamic node must be reduced, otherwise strong electromagnetic field will be generated. If want to reduce inductance, basically want to reduce the length of wiring, increase width action is small.

5. Signal cables

For the entire control section, consideration should be given to wiring 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 parallel, otherwise it may lead to abnormal operation of the power supply, shock.

In addition, if the control line is very long, a pair of back and forth lines should be close to each other, or the two lines should be placed on the two sides of the PCB facing each other, so as to reduce the loop area and avoid interference by the electromagnetic field of the power part. FIG. 2 illustrates the correct and incorrect signal line routing methods between A and B.

Figure 2 Correct and wrong signal cable routing methods.

Of course, the signal line should minimize the connection through holes!

6, copper

Sometimes laying copper is completely unnecessary and should even be avoided. If the copper was large enough and its voltage varied, it might act as an antenna, radiating electromagnetic waves around it. On the other hand, it’s easy to pick up noise.

Generally, copper laying is only allowed on static nodes, such as the “ground” node at the output end, which can effectively increase the output capacitance and filter out some noise signals.

7, mapping,

For a circuit, copper can be laid on one side of the PCB, which automatically maps to the wiring on the other side of the PCB to minimize the impedance of the circuit. It is as if a set of impediments with different impedance values are connected in parallel, and the current will automatically choose the path with the lowest impedance to flow through.

You can actually wire the control part of the circuit on one side, and lay copper on the “ground” node on the other side, and connect the two sides through a 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 into the loop formed by the power output and the external load, so that the measured output noise increases.

FIG. 3 Correct and incorrect placement of diodes

9, ground wire,

The wiring of ground cables must be very careful. Otherwise, EMS, EMI and other performance may deteriorate. For switching power supply PCB “ground”, at least the following two points :(1) power ground and signal ground, should be single point connection; (2) There should be no ground loop.

10. Y capacitance

Input and output are often connected to Y capacitor, sometimes for some reasons, it may not be able to hang on the input capacitor ground, remember at this time, must be connected to a static node, such as high voltage terminal.

11, other

When designing the PCB of the actual power supply, there may be some other issues to consider, such as “the varistor should be close to the protected circuit”, “common mode induction to increase the discharge teeth”, “chip VCC power supply should increase the capacitor” and so on. In addition, the need for special treatment, such as copper foil, shielding, etc., should also be considered in the PCB design stage.

Sometimes often encounter a number of principles conflict with each other, to meet one of them can not meet the other, this is the need for engineers to apply existing experience, according to the actual project needs, determine the most appropriate wiring!