PCB Design for Switching Power Supply

For the research and development of switch mode power supplies,PCBDesign occupies a very important position. A bad onePCB，EMCPoor performance, high output noise, weak anti-interference ability, and even defects in basic functions.

Compared to other hardware circuitsPCBSlightly different, switch power supplyPCBThere are some of its own characteristics. This article will briefly discuss switch mode power supplies based on engineering experiencePCBSome basic principles of wiring.

1, spacing

For high-voltage products, line spacing must be considered. The spacing that can meet the corresponding safety regulations is of course, but often for products that do not require certification or cannot meet certification requirements, the spacing is determined by experience. What is the appropriate spacing width? It is necessary to consider whether the production can ensure the cleanliness of the board surface, environmental humidity, and other pollution.

For mains power input, even if the board surface can be kept clean and sealed,MOSClose proximity between tube drain and source600V, less than1mmIn fact, it's also quite dangerous!

2Components at the edge of the board

InPCBEdge mounted capacitors or other easily damaged devices must be considered when placing themPCBThe direction of the board, as shown in the figure, is the comparison of the stress levels experienced by the device during various placement methods.

image1 Comparison of stress on components during board splitting

From this, it can be seen that the device should be kept away from and parallel to the edge of the board, otherwise it may be affectedPCBSplitting the board resulted in damage to the components.

3. Key routing

becausedi/dtTo achieve this, it is necessary to reduce the inductance at the dynamic nodes, otherwise a strong electromagnetic field will be generated. To reduce inductance, it is mainly necessary to reduce the length of wiring, and increasing the width has a smaller effect.

4. Loop area

Whether it is input or output, power loop or signal loop, it should be as small as possible. The power loop emits electromagnetic fields, which will result in poor performanceEMICharacteristics or significant output noise; Meanwhile, if 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 peak of drain noise.

5Signal line

For the entire control section, wiring should consider keeping it away from the power section. If the two are too close due to other restrictions, the control line should not be parallel to the power line, otherwise it may cause abnormal power supply operation and oscillation.

6Laying copper

Sometimes laying copper is unnecessary and should even be avoided. If the copper surface area is large enough and its voltage constantly changes, on the one hand, it may serve as an antenna,Radiate electromagnetic waves to the surroundings; On the other hand, it is easy to pick up noise.

Usually only copper is allowed to be laid on static nodes, such as the output terminal“land”Laying copper on nodes can effectively increase output capacitance and filter out some noise signals.

7. Output rectifier diode

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

8. Mapping

For a circuit, it can bePCBLay copper on one side, it will be based onPCBThe wiring on the other side is automatically mapped to minimize the impedance of this circuit. This is like a group 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, it is possible to connect wires on one side of the control circuit and on the other side“land”Copper is laid on the nodes, and the two faces are connected through vias.

9Ground wire

The wiring of the ground wire must be very careful, otherwise it may causeEMSTheEMIPerformance and other performance have deteriorated. For switch mode power supplyPCBof“land”At least achieve the following two points:（1）Power ground and signal ground should be connected at a single point; （2）There should be no existing loop.

10TheYcapacitance

Input and output are often connectedYCapacitors, sometimes due to certain reasons, may not be able to be hung on the input capacitor ground. In this case, remember to connect them to a static node, such as the high voltage end.

11Other

Actual power supplyPCBWhen designing, other issues may also need to be considered, such as“The varistor should be closely attached to the protected circuit”、“Common mode electrical induction increases discharge teeth”、“chipVCCCeramic capacitors should be added to the power supply area”and so on. Additionally, is special treatment required, such as copper foil, shielding, etcPCBThe design phase also needs to be considered.

Sometimes there are situations where multiple principles conflict with each other, and meeting one of them cannot meet the others. This requires engineers to apply their existing experience and determine the most suitable wiring based on actual project requirements!