Switching power supply PCB design experience sharing

Every electronics engineer must know that PCB design occupies a very important position when designing. Taking power supply as an example, PCB design will directly affect the EMC performance, output noise, anti-interference ability, and even basic functions of the power supply. The PCB layout of the power section is slightly different from other hardware. How do I design it?


Spacing


For high voltage products, line spacing must be considered. Of course, 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 certification, the spacing is determined by experience. What is the proper spacing? It must be considered whether the production can ensure the cleanliness of the surface, environmental humidity, and other pollution.


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


Board edge components


SMD capacitors or other vulnerable devices on the edge of the PCB must consider the direction of the PCB sub-board when placing, as shown in the comparison of the stress on the device when the various placement methods are shown.

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It can be seen that the device should be far away and parallel to the edge of the sub-board, otherwise the components may be damaged due to the PCB sub-board.


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 result in poor EMI characteristics or large output noise; at the same time, if it is received by the control loop, it may 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.


Key trace


Due to the di / dt effect, the inductance at the dynamic node must be reduced, otherwise a strong electromagnetic field will be generated. If you want to reduce the inductance, it is mainly to reduce the length of the wiring, and increasing the width has less effect.


Signal line


For the entire control section, consider routing it away from the power section. If the two are close together due to other restrictions, the control line and the power line should not be paralleled, otherwise the power supply may work abnormally and oscillate.


In addition, if the control line is very long, you should place a pair of back and forth wires close to each other, or place the two on the two sides of the PCB and face them directly to reduce the loop area and avoid interference from the electromagnetic field of the power part. Figure 2 illustrates the correct and incorrect signal line wiring methods between points A and B.

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Of course, there should be as few vias as possible on the signal lines!


Copper


Sometimes copper laying is completely unnecessary and should even be avoided. If the copper area is large enough and its voltage is constantly changing, it may act as an antenna to radiate electromagnetic waves to the surroundings; on the other hand, it may easily pick up noise.


Usually only copper is allowed to be laid on the static nodes, for example, copper on the output terminal "ground" node can equivalently increase the output capacitance and filter out some noise signals.


Map


For a circuit, 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 circuit. This is like a set of impedances with different impedance values connected in parallel, and the current will automatically choose the path with the smallest impedance to flow through.


In fact, you can wire on one side of the control circuit, and copper on the other side of the "ground" node. The two sides are connected through vias.


Output rectifier diode


If the output rectifier diode is close to the output terminal, it should not be placed in parallel with the output. Otherwise, the electromagnetic field generated by the diode will penetrate the loop formed by the power output and the external load, which will increase the measured output noise.

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Ground


The wiring of the ground wire must be very careful, otherwise EMS, EMI performance and other performance may be deteriorated. For the "ground" of the switching power supply PCB, at least the following two points must be achieved: (1) the power ground and the signal ground should be connected at a single point; (2) there should be no ground loop.


Y capacitor


The input and output are often connected to Y capacitors. Sometimes, for some reasons, it may not be possible to hang them on the input capacitor ground. At this time, remember that they must be connected to a static node, such as a high-voltage terminal.


Other


In actual power PCB design, there may be some other issues to consider, such as "the varistor should be close to the protected circuit", "common mode electric induction to increase the discharge teeth", "the chip VCC power supply should increase the ceramic capacitor", etc. . In addition, whether special treatment is needed, such as copper foil, shielding, etc., needs to be considered at the PCB design stage.


Sometimes there are situations where multiple principles conflict with each other. Meeting one of them will not meet the other. This requires the engineer to apply the existing experience and determine the most suitable wiring according to the actual project requirements!


To sum up


In order to create a highly stable product, there are many design details required in the hardware design. This article is only to introduce the most common power supply design in hardware. In order to make the overall product or system have a stable and reliable power supply, most engineers choose a power module as the basis for system power supply.