8 Must-See Switching Power Supply Layout Tips

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8 Must-See Switching Power Supply Layout Tips [Copy link]

For a switching power supply engineer, PCB drawing is actually a crucial part that affects a product. If you cannot do a good layout, the entire power supply may not work properly, and the smallest problem is wave stabilization or EMC.

This is someone else's finished switching power supply module. Below I will talk to you about some of the design highlights of this power supply module.

According to the standard wiring spacing

This is written in the agreement. If you don't follow this, you will definitely fail the withstand voltage test because the high voltage will directly break down the air. Note that the distance between the fuse and the fuse is relatively far, requiring more than 3mm, which is why the fuse is placed at the front end of the circuit.

The second thing to note is that even if the safety regulations do not mention it, if the two traces are too close, they will still break down during normal operation. The outer layer withstand voltage of two PCBs with a spacing of 1mm is 200V, so the trace distance of 220V AC or 310V DC is generally at least 2mm. I usually keep it above 2.5mm.

These devices all have safety requirements. To put it bluntly, there are minimum size requirements for the two devices. Devices that are too small are actually impossible to pass safety regulations. Do you understand? This is why the so-called switching power supply PCB engineers are actually dancing with shackles.

The skeleton of the switching power supply transformer must also be strictly controlled in order to comply with safety regulations. Especially the distance from the primary to the secondary, small power transformers must have flying wires.

The length of the flying wire must also be controlled. If the flying wire is too short, the withstand voltage may be affected. If the flying wire is too long, it may radiate electromagnetic signals to the outside and fail the EMC test. Therefore, it needs to be written in detail in the specification book. When drawing the PCB, you must pay attention to the pads of the flying wire and they cannot be too strange.

Current flow

This is rarely mentioned. The reason is simple: many people don’t pay attention.

Looking at the two designs, why is the routing from the RV1 varistor to the x2 capacitor at the back deliberately done this way instead of directly short-circuiting them all with copper?

Note that after the fuse, the wiring connecting the varistor VR1 and then the x2 capacitor makes a complete detour. Why is this?

The reason is simple, it is to prevent the current from having a way back on the PCB. The current only flows through the part with the smallest impedance. If the copper is directly covered, the components that must be passed may be skipped, so this is not allowed.

Similarly, the electrolytic capacitor here is also used to prevent the current from bypassing the necessary capacitor and flowing directly to the load. Although the drawing method is different, the actual function is the same.

This is a wrong case. The red L live wire is first connected to the common mode inductor and then to the x2 capacitor. The line from the common mode inductor to the x2 capacitor will produce a strange phenomenon. The current flows back and forth, turning into an antenna. When the x2 current is charged, the current inside the wire is positive, and when the capacitor is discharged, the current inside the wire is reverse. If this is not an antenna, then what is it?

Minimum high voltage main current loop

A Switching Power Supply Engineer's Design Notes Log, Part 4 PCB Design

The so-called minimum high-voltage main current loop refers to the loop formed between the last high-voltage filter capacitor and the primary of the transformer and the high-voltage MOS tube. This loop will inevitably produce serious electromagnetic radiation because it has to pass through high-voltage pulse electrical signals. The only way we can improve it is to reduce the loop area. The smaller the loop, the smaller the antenna and the less radiation.

This is the actual wiring layout, you can refer to it. JT1 is a flying wire, which directly introduces the 310V positive voltage into the transformer.

Independent voltage sampling routing

The sampling voltage of the switching power supply must be separated from the high current wiring of the switching power supply. The independent wiring sampling should be done from the very end of the switching power supply output power, so as to avoid the interference of the load current on the sampling line.

The sampling circuit is at the very end. The voltage is directly taken from the load output terminal, and no large current flows on the sampling line. Various sampling errors are avoided.

PCB current carrying capacity

It is well known that the overcurrent capacity of PCB is limited, but how much current can pass through PCB?

The table above can give you a detailed reference. After reading the table, you should know that for low-power switching power supplies, it is not necessary to make the high-voltage side traces very thick, unless it is to provide heat dissipation for the device, otherwise 1mm is generally sufficient, and 2mm at most is sufficient in most cases.

But for the low voltage side, what about the high current?

On the one hand, it increases the line width, and on the other hand, it removes part of the solder mask layer and creates a window in the steel mesh layer to allow the wire to be tinned. The current carrying capacity of the wire will be increased accordingly. Please note that you must open a hole in the Paste steel mesh layer, otherwise the solder will not be applied. Remember this.

PCB via heat dissipation tips

Many times we need to dissipate heat through the PCB circuit board. At this time, we will drill some vias and transfer the heat to the back of the PCB. At this time, there is a little trick, that is, the via plug can increase the efficiency of heat conduction, but the via plug has a common maximum diameter, generally the via diameter is no more than 0.45mm, I usually take 0.4mm diameter to be safe.

Drawing of the discharge tube

Generally, there is a discharge tube between the high-voltage side and the low-voltage side of the switching power supply to discharge static electricity.

Many engineers will draw manually when they are doing PCB layout. My suggestion is to make a package directly, and then call it in conjunction with the PCB, so as not to destroy the linkage of the PCB. It just means that you need to draw two special-shaped packages. It is relatively easy.

Note that you only need to remove the solder mask layer here, and never draw the steel mesh layer in the middle, because tinning is not required here, only the pads need to be tinned.

These are almost all my PCB drawing experiences when designing switching power supplies. To be honest, I have been fooled by others in the design of switching power supplies. There are too many half-baked people here, and many of them are metaphysical. What I said are relatively reliable and verified experiences. These are also the bitter lessons of those switching power supply manufacturers. Many times, of course, they don’t want others to know, and there is no way. I share it today in the hope that fewer people will take these detours and it can help future generations.

Source: Internet compilation. If copyright is involved, please contact us to delete.

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