This article will explain how to place the inductance of the switching power supply on the PCB design. First, let's look at this question: Where should the coil be placed?
Switching regulators for voltage conversion use inductors to temporarily store energy. These inductors are typically very large in size and must be placed in the printed circuit board (PCB) layout of the switching regulator. This task is not difficult, because the current through the inductor may change, but not instantaneously. Changes can only be continuous and usually relatively slow.
The switching regulator switches the 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 trace through which the switching current flows is called a thermal loop or an alternating current path, which conducts current in one switching state and does not conduct current in the other switching state.
In the PCB layout, the thermal loop area should be small and the path should be short to minimize parasitic inductance in these traces. Parasitic trace inductance can cause unwanted voltage offsets and cause electromagnetic interference (EMI).
(Figure 1 Switching Regulator for Buck Conversion (with a critical thermal circuit as shown by the dashed line))
Figure 1 shows a buck regulator in which the critical thermal circuit is shown as a dashed line. It can be seen that the coil L1 is not part of the thermal circuit. Therefore, it can be assumed that the placement position of the inductor is not important. It is correct to have the inductor outside the thermal loop - so in the first example, the placement position is secondary. However, some rules should be followed.
Do not place sensitive control traces under the inductor (not on the surface of the PCB or below), in the inner layer, or on the back of the PCB. Under the influence of current flow, the coil generates a magnetic field, which affects the weak signal in the signal path. In a switching regulator, a critical signal path is the feedback path that connects the output voltage to a switching regulator IC or a resistor divider.
It should also be noted that the actual coil has both a capacitive effect and an inductive effect. The first coil winding is directly connected to the switching node of the buck switching regulator, as shown in Figure 1. As a result, the voltage change in the coil is as strong and rapid as the voltage at the switch node. Due to the very short switching time 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 ADP2360 buck converter with coil placement)
Figure 2 shows an example layout of the ADP2360. In this figure, the important thermal circuit in Figure 1 is marked in green. As can be seen from the figure, the yellow feedback path is at a certain distance from the coil L1. It is located on the inner layer of the PCB.
Some circuit designers don't even want any copper layers in the PCB under the coil. For example, they provide a cut under the inductor, even in a ground plane layer. The goal is to prevent the ground plane below the coil from forming eddy currents due to the coil magnetic field. There is nothing wrong with this method, but there is also controversy that the ground plane should be consistent and should not be interrupted:
The ground plane for shielding works best when not interrupted.
The more copper the PCB has, the better the heat dissipation.
Even if eddy currents are generated, these currents can only flow locally, causing only a small loss and hardly affecting the function of the ground plane.
Therefore, agree to the ground plane layer, even under the coil, should also maintain a complete view.
In summary, we can conclude that although the coil of the switching regulator is not part of the critical thermal loop, it is sensible not to place sensitive control traces under or near the coil. The various planes on the PCB—for example, the ground plane or the VDD plane (supply voltage)—can be constructed continuously without the need for a cut.
