How should the inductor of the switching power supply be placed correctly in PCB design?

Typically a switching regulator's coil is not part of the critical thermal loop, but it is wise not to run sensitive control traces under or near the coil. Various planes on the PCB - for example, the ground plane or the VDD plane (supply voltage) - can be constructed continuously without cutouts. First, the question arises: where should the coil be placed?

Switching regulators used for voltage conversion use inductors to temporarily store energy. These inductors are often very large in size and must be positioned within the switching regulator's printed circuit board (PCB) layout. This task is not difficult because the current through the inductor may change, but not instantaneously. Change can only be continuous and usually relatively slow.

A switching regulator switches current back and forth between two different paths. This switching is very fast, depending on the duration of the switching edge. The traces through which switching current flows are called hot loops or AC current paths, which conduct current in one switching state and not in another switching state. In PCB layout, keep hot loop areas small and paths short to minimize parasitic inductance in these traces. Parasitic trace inductance can create unwanted voltage offsets and cause electromagnetic interference (EMI).

Figure 1. Switching regulator for buck conversion (with critical thermal loop shown as dashed line)

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

Do not route sensitive control traces under the inductor (neither on the PCB surface nor underneath it), in inner layers, or on the back of the PCB. Affected by the flow of current, the coil generates a magnetic field, which affects weak 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 resistive divider.

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

Figure 2. Example circuit of ADP2360 buck converter with coil placement.

Figure 2 shows an example layout for the ADP2360. In this diagram, the important thermal loops in Figure 1 are marked green. As can be seen from the figure, the yellow feedback path is a certain distance away from coil L1. It is located on the inner layer of the PCB.

Some circuit designers don't even want any copper layer in the PCB under the coil. For example, they provide cutouts under the inductor, even in the ground plane layer. The goal is to prevent eddy currents from forming in the ground plane below the coil due to the coil's magnetic field. There is nothing wrong with this approach, but there is an argument to be made that the ground plane should remain consistent and should not be interrupted:

1. The ground plane used for shielding works best when it is not interrupted.

2. The more copper the PCB has, the better the heat dissipation.

3. Even if eddy currents are generated, these currents can only flow locally, causing only small losses, and will hardly affect the function of the ground plane.

So agree with the point that the ground plane layer, even underneath the coil, should remain intact.