PCB Technical - 3W principle, 20h principle and 55 principle in PCB design

Switching regulators for voltage conversion use inductors to temporarily store energy. These inductors are usually very large in size and must be positioned in the printed circuit board (PCB) layout of the switching regulator. The current will not change instantaneously, but it is not difficult to change the inductance through. Changes can only be continuous, usually relatively slow.

The switching regulator switches the current back and forth between two different paths. The switching speed depends on the duration of the edge. The circuit through which switching current flows is called thermal loop or AC current path, which conducts current in one switch state and does not conduct current in another switch state. In PCB layout, the thermal circuit area should be small and the path should be short, so as to reduce the parasitic inductance in these traces. Parasitic line inductors will produce useless voltage imbalance and cause electromagnetic interference (EMI).

Switching regulator for step down switching (with critical thermal circuit as shown in dotted line)

Figure 1 shows a step-down regulator with the key thermal circuit shown as a dashed line. It can be seen that coil L1 is not part of the thermal circuit. Therefore, it can be assumed that the position of the inductor is not important. Therefore, the location of the secondary circuit is the correct position of the inductor. However, some rules should be followed.

Sensitive control wiring must not be placed under the inductor (not on or under the PCB surface), in the inner layer or on the back of the PCB. Affected by the current flow, the coil will produce a magnetic field, which will affect the weak signal in the signal path. In switching regulators, a key signal path is the feedback path, which connects the output voltage to the switching regulator IC or resistor divider.

It should also be noted that the actual coil has both capacitive and inductive effects. The coil windings are directly connected to the switch node of the step-down 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 switch node. Due to the short switching time and high input voltage in the circuit, there will be considerable coupling effect on other paths on PCB. Therefore, sensitive wiring should be away from the coil.

Example circuit of adp2360 step down converter with coil position

Figure 2 shows an example layout of adp2360. In this figure, the important heat return roadmap in Figure 1 is green. As can be seen from the figure, the Yellow feedback path has a certain distance from the offline loop L1. It is located in the inner layer of PCB.

Some circuit designers don't even want any copper in the PCB under the coil. For example, they will provide a cut-out below the inductance, even in the ground plane layer. The goal is to prevent eddy current in the grounding plane under the coil due to the coil magnetic field. There is no mistake in this method, but it is also argued that the grounding plane should be consistent and should not be interrupted

1. The ground plane used for shielding is effective without interruption.

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

3. Even if eddy current is generated, these currents can only flow locally, causing little loss and hardly affecting the function of the grounding plane.

Therefore, it is agreed that the grounding plane layer, even under the coil, should be kept intact.

In conclusion, we can conclude that although the coil of the switching regulator is not part of the critical thermal loop, it is advisable not to place sensitive control wiring under or near the coil. Various planes on a PCB - for example, a ground plane or a VDD plane (supply voltage) - can be constructed continuously without cuts.