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Examining PCB layouts for power stages

15 Apr 2016  | Robert Taylor

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One of the significant factors to having a successful power supply design is a correct printed circuit board (PCB) layout. Non-isolated power stages are the basic building blocks of power systems. Understanding where the currents are flowing and how the high-frequency loops are constructed is the most important step.

In this Power Tip, I investigate the buck, boost and single-ended primary inductor converter (SEPIC) power stages.

To start, use a buck converter to make a lower output voltage compared to the input voltage. Figure 1 shows a buck converter schematic and layout.

Figure 1: Included in this simplified schematic are both input and output capacitors, an inductor, a switching transistor and blocking diode.

During the pulse-width modulator (PWM) on time, current flows via the green arrow path from the input capacitors through the switching transistor to the inductor. When the PWM is off, current continues to flow through the inductor via the pink arrow path. This means that the output has a continuously flowing current. The input has a high-frequency current that switches on and off during each cycle. The most important part of the power stage layout is to reduce the high-frequency loop. This is reflected by the blue arrow in the image above. During the transistor turn on period, current briefly flows through the diode D1 to ground. During this time, if the input capacitors are not located close together, this large current surge can cause some design issues.

Make sure you make the power traces or power planes wide enough to handle the power supply current. Generally speaking, the power planes should be as large as possible with the exception of the switching node. The switching node has a high dV/dt signal that can couple to other parts of the PCB layout, so minimising the surface area is good practice. Use vias to connect power planes on different layers. A simple rule of thumb is to not exceed 1A of current per via (10-mil drill hole). If you create one large continuous ground plane the size of the PCB, this can help to reduce noise and high-frequency loops.

Use boost converters are used to create a higher output voltage from a lower input voltage. You can apply the same process you used for the buck converter to the boost converter to identify critical paths and loops.

Figure 2: A boost converter schematic and PCB layout of the power stage.

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