Important points of board layout
A switching power supply controls a voltage by switching operations, but is itself purely an analog circuit. To exaggerate a bit, a switching power supply generates high-frequency noise, but has a feedback loop and so is sensitive to noise. In other words, it must be kept in mind that a switching power supply circuit has a circuit path that generates noise as a large current is turned on and off, and a control signal path that is sensitive to noise. In board layout, efforts are made to ensure that the large-current path emits as little noise as possible, in order that the control signal path remains unaffected by noise. Of course noise also affects EMC as radiated noise, and so it is important that the layout be designed so as to generate as little noise as possible.
A considerable amount of experience is required to be able to produce a good board layout. An inappropriate layout not only impedes normal operation of the power supply, but may also worsen the S/N of the system as a whole, and in the worst case may destroy constituent components and the power supply IC. This is a task where experience makes a big difference, but in many cases, examples of basic board wiring layout are provided in the power supply IC data sheet and supplemental materials. In not a few cases, data that can be utilized immediately, such as Gerber format files, are provided.
These are top-quality resources created by engineers with plentiful experience, and so maximum use should be made of them.
Below, an example of a board layout and points to keep in mind are described.
The above diagram shows a large-current path and a control signal path in a partially abbreviated circuit diagram.
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The red line is the large-current path, and is a cause of ringing and losses, and therefore is made as wide and as short as possible.
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Moreover, the loop of the red line is made as small as possible.
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The secondary-side orange line is, similarly to the red line, designed to be wide and short, forming a small loop.
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The brown line is the path of the current flowing to the VCC pin; current flows upon switching, and so this path is placed in separate wiring.
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Anything immediately below the transformer is affected by magnetic flux, and so the IC control signal line is not placed in this area.
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It is desirable that the ground for the red, brown, blue, and green lines be a single-point ground.
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The green line is a path over which secondary-side surges escape to the primary side; large currents flow instantaneously, and so wiring separate from that for the red line and blue line is used.
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The blue line is a ground line for IC control signals, and does not pass large currents; but wiring separate from that for the red line, green line and brown line is used so as to reduce the effects of noise.
The following image is an example of a board layout that takes these matters into consideration. The red, orange, and brown loops of the above diagram are shown.
The following photos are images of the board after mounting. There are some differences with the above circuit, but the same power supply IC is used, and the component configuration is substantially the same. The relation of the actual circuit components to the diagrams above should be clear.
In actual design, due to physical and mechanical constraints such as the vertical and horizontal dimensions of boards that can be used, the ideal layout may not be attainable. However, as explained above, there is the possibility of adverse influence not only on the performance of the power supply, but on the system as a whole. Further, even when fabricating a trial board, major corrections are not different from a do-over, and so striving for the optimum design from the very start is effective in holding down cost and time overruns later on.
It often happens that the general system specs have been decided, and design of the power supply starts in the final stages of the project. That calls for skilled adjustments culminating in a design that requires no follow-up
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