Switch Mode Power Supply Board Layout Example

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Optimizing the circuit board layout is an important aspect when designing a switch-mode power supply. Proper layout ensures that the switching regulator maintains stable operation and minimizes radiated and conducted interference (EMI). Electronics developers are well aware of this. However, what is not known is what the optimized circuit board layout for a switch-mode power supply should look like.

Figure 1 shows the LT8640S evaluation board circuit. This is a step-down switching regulator that supports input voltages up to 42 V and can deliver up to 6 A of output current. All components are packed tightly together. It is generally recommended to pack components as tightly as possible on a board. This is not wrong, but it may not be appropriate if the goal is to achieve an optimized board layout.

In Figure 1, there are several (11) passive components surrounding the switching regulator IC. When deploying these passive components, which ones should be prioritized and why?

Figure 1. The circuit board for the LT8640S switching regulator has tight component placement, resulting in a very compact board layout.

The most important principle in switching regulator PCB design is to keep the traces that carry high switching currents as short as possible. If this principle can be successfully implemented, a large portion of the switching regulator circuit board can be laid out properly.

How can you easily implement this golden rule in your board layout? The first step is to identify the critical paths in the switching regulator topology. In these critical paths, the current changes as the switch switches. Figure 2 shows a typical circuit for a step-down converter (buck topology). The critical paths are shown in red. These connecting lines may or may not carry full current, depending on the state of the power switch. The shorter these paths are, the better. In a step-down converter, the input capacitor should be as close as possible to the VIN pin and the GND pin of the switching regulator IC.

Figure 2. Schematic of a step-down switching regulator, with the paths where current changes rapidly are shown in red.

Figure 3 shows the basic schematic of a boost topology circuit. This circuit converts a low voltage to a higher voltage. Again, the current path where the current changes as the power switch switches is shown in red. One thing to note is that the placement of the input capacitor is not important at all. The placement of the output capacitor is more critical. It must be placed as close as possible to the flyback diode (or high-side switch) and the ground connection of the low-side switch.

Figure 3. Schematic of a boost switching regulator, with the paths where current changes rapidly are shown in red.

Then, any other switching regulator topology can be examined to see how the current changes when the power switch is toggled. The traditional method is to print out the circuit and then draw the current paths with three different colored pencils. Use the first color to mark the current path during the on period, that is, the current path when the power switch is turned on. Use the second color to mark the current path during the off period, that is, the current path when the power switch is closed. Finally, use the third color to mark all the current paths that were previously marked only in the first color and only in the second color. In this way, it is clear that the critical paths where the current changes as the power switch is toggled can be seen.

For the inexperienced circuit designer, the layout of a switching regulator board can seem like a black art. The core rule is to keep the trace paths where current flows as the switch switches on and off as short and tight as possible. This is a simple and logical explanation and is the basis for an optimized board layout in a switch-mode power supply design.

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For the inexperienced circuit designer, the layout of a switching regulator board can seem like a black art. The core rule is to keep the trace paths where current flows as the switch switches on and off as short and tight as possible. This is a simple and logical explanation and is the basis for an optimized board layout in a switch-mode power supply design.

Thanks for sharing your experience!

bigbat

There is another explanation: a "ring" should not be formed around components such as transformers, because a ring will become an inductor with only one circle, and there is an iron core living in the middle of the inductor, which makes it difficult not to interfere. Therefore, try to make it a line as much as possible.

xutong

You can also refer to linear - an139f