Where should the inductor be placed on the power supply PCB? A "guide" is given to you~

Switching regulators for voltage conversion use inductors to temporarily store energy. These inductors are usually very large and must be located in the printed circuit board (PCB) layout of the switching regulator. This task is not difficult because the current through the inductor can change, but not instantaneously. The change can only be continuous and usually relatively slowly.

A switching regulator switches current back and forth between two different paths. This switching is very fast, and the specific switching speed depends on the duration of the switching edge. The traces through which the switching current flows are called hot loops or AC current paths, which conduct current in one switching state and do not conduct current in the other switching state. In PCB layout, the hot loop area should be small and the paths should be short to minimize the parasitic inductance in these traces. Parasitic trace inductance will produce unwanted voltage offsets and cause electromagnetic interference (EMI).

Figure 1. Switching regulator for step-down conversion with critical hot loops shown as dashed lines.

Figure 1 shows a buck regulator with the critical hot loop shown as a dotted line. It can be seen that coil L1 is not part of the hot loop. Therefore, it can be assumed that the placement of this inductor is not important. It is correct to place the inductor outside the hot loop - so in the first example, placement is secondary. However, there are some rules that should be followed.

Do not route sensitive control traces under the inductor (either on the PCB surface or below), on inner layers, or on the backside of the PCB. The coil generates a magnetic field due to the current flow, which can affect small signals in the signal path. In a switching regulator, a critical signal path is the feedback path, which connects the output voltage to the switching regulator IC or a resistor divider.

It should also be noted that real coils have both capacitive and inductive effects. The first coil winding is directly connected to the switch node of the buck switching regulator, as shown in Figure 1. As a result, the voltage in the coil changes as strongly and quickly as the voltage at the switch node. Due to the very short switching times in the circuit and the high input voltage, considerable coupling effects occur on other paths on the PCB. Therefore, sensitive traces should be kept away from the coil.

Figure 2. Example circuit for the ADP2360 buck converter with coil placement.

Figure 2 shows an example layout of the ADP2360. In this figure, the important hot loop in Figure 1 is marked in green. It can be seen from the figure that the yellow feedback path is some distance away from the coil L1. It is located on the inner layer of the PCB.

Some circuit designers do not even want any copper layer in the PCB under the coil. For example, they provide a cutout under the inductor, even in the ground plane layer. The goal is to prevent eddy currents in the ground plane under the coil due to the coil's magnetic field. There is nothing wrong with this approach, but there are arguments that the ground plane should be consistent and should not be interrupted:

• Ground planes used for shielding work best when they are uninterrupted.

• The more copper a PCB has, the better the heat dissipation.

• Even if eddy currents are generated, these currents flow only locally, cause only small losses, and have little impact on the functionality of the ground plane.

Therefore, it is agreed that the ground plane layer, even underneath the coil, should also remain intact.

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