Q: First throw problems: Where should the coil be placed?
Switch regulators for voltage conversion Storage energy when inductive. These inductors are usually very large, and the position must be arranged in the printed circuit board (PCB) layout of the switching regulator. This task is not difficult because the current of the inductor may change, but it is not instantaneous. Changes may only be continuous, usually relatively slow.
The switching regulator switches back to the current between two different paths. This switching is very fast, and the specific switching speed depends on the duration of the switching edge. The switch current flows through the circle referred to as a heat loop or AC current path, which conduct current in one switch state, and does not conduct current in another switch state. In the PCB layout, the hot circuit area should be small and the path is short to minimize the parasitic inductance in these traces. The parasitic trace inductor generates useless voltage disorders and results in electromagnetic interference (EMI).
Figure 1. Switch regulator for buck converted (a key heat loop as shown by dashed line).
Figure 1 shows a buck regulator, where the key heat loop is shown as a dashed line. It can be seen that the coil L1 is not part of the hot circuit. Therefore, it can be assumed that the placement position of the inductor is not important. It is true that the inductor is located in the hot circuit - thus in the first example, the position is secondary. However, some rules should be followed.
Do not reach the inductance (not below the surface of the PCB), in the inner layer or a sensitive control circle on the back of the PCB. Affected by current flows, the coil generates a magnetic field, and the result will affect weak signals in the signal path. In the switching regulator, a critical signal path is a feedback path that connects the output voltage to the switching regulator IC or a resistor voltage divider.
It should also be noted that the actual coil has both capacitive effects and inductive effects. The first coil winding is directly connected to the switching node of the buck switch stretcher, as shown in Figure 1. As a result, the voltage variation in the coil is as strong as the voltage at the switch node. Since the switching time in the circuit is very short and the input voltage is high, a considerable coupling effect is generated on the other path on the PCB. Therefore, sensitive traces should be away from coils.
Figure 2. Example circuit of an ADP2360 buck converter with a coil placed position.
Figure 2 shows an example layout of the ADP2360. In this figure, the important hot circuit lane in Figure 1 is green. As can be seen from the figure, the yellow feedback path is a certain distance from the coil L1. It is located in the inner layer of the PCB.
Some circuit designers do not necessarily want any copper layers in the PCB under the coil. For example, they provide a slit below inductance, even in the ground plane layer. Its goal is to prevent the ground plane under the coil to form eddy currents due to the coil magnetic field. This method is not wrong, but it also believes that the ground plane should be consistent and should not be interrupted:
The ground plane used to shield is best in good performance when it is not interrupted.
The more copper of the PCB, the better heat dissipation.
Even if eddy current is generated, these currents can only be partially flowing, only small losses, and hardly affect the function of the ground plane.
Therefore, agree to the ground plane layer, even under the coil, should also maintain a complete point of view.
In summary, we can conclude that although the coil of the switch stator is not part of a critical heat loop, it is wise to control the coil or near the coil. Various planes on the PCB, for example, ground plane or VDD plane (power supply voltage) - can be constructed continuously without a slit.
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