PCB Technical - On a switch mode PCB, inductor placement is appropriate

Inductors on pcb are passive components used to store energy in circuits and are widely used in electronic devices. With the progress of science and technology, the application of inductors on printed circuit boards (PCBs) is becoming increasingly important and has become an indispensable part of modern electronic products.

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

Switching regulators switch current back and forth between two different paths. This switching is very fast, depending on the duration of the switching edge. The lines through which switching current flows are called thermal circuits or ALTERNATING current paths, which conduct current in one switching state and not in the other switching state. In PCB layouts, the thermal loop area should be small and the path short to minimize the parasitic inductance in these routes. Parasitic wire inductance can produce unwanted voltage imbalance and lead to electromagnetic interference (EMI).

Therefore,it can be assumed that the placement of the inductor is not important.It is correct to have the inductor outside the hot loop--so in this case, placement is of secondary importance. However, there are some rules that should be followed. Sensitive control wiring shall not be laid under the inductor (not on or below the PCB surface), in the inner layer or on the back of the PCB. Under the influence of current flow, the coil creates a magnetic field, which can affect weak signals in the signal path. In switching regulators, a critical signal path is the feedback path, which connects the output voltage to the switching regulator IC or resistance divider.

It should also be noted that actual coils have both capacitive and inductive effects.The windings are directly connected to the switch node of the step-down switch regulator.As a result, the voltage in the coil changes as strongly and rapidly as the voltage at the switching node.Because the switching time in the circuit is very short and the input voltage is very high,there is considerable coupling effect on the other paths on the PCB.Therefore,sensitive wiring should be kept away from the coil.As can be seen from the figure,the yellow feedback path is at a certain distance from the coil L1.It is located in the inner layer of the circuit board.

Some circuit designers don't even want any copper layers in the PCB substrate under the coil.For example,they provide a notch below the inductor, even in a grounded 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 it is also argued that the grounding plane should remain consistent and should not be interrupted:

In PCB design, there are mainly the following types of inductors:

Wire-wound inductors: Inductors wound from wire with high power and large inductance values for high frequency applications.

Thin Film Inductors: Manufactured through thin film technology, they are small and can operate at high frequencies and are suitable for devices with limited space.

Ferrite Inductors: Using ferrite material as the core, they provide higher inductance values and are mainly used in power supply and filter designs.

The main functions of an inductor on a PCB include:

Filtering: Inductors can be combined with capacitors to form low-pass or high-pass filters to minimize noise and smooth power supply.

Energy storage: In switching power supplies, inductors are responsible for storing and releasing energy to improve the efficiency of the power supply.

Signal Processing: Inductors can be used in the signal chain to process signals at specific frequencies, suppress unwanted frequencies and enhance signal quality.

The performance of an inductor is affected by a number of factors, especially the operating frequency. Inductors primarily exhibit current hindrance, or inductive impedance, at low frequencies. As the frequency increases, the reactance gradually increases until it reaches the resonant frequency, at which point the inductor exhibits capacitive characteristics . Choosing the right inductor to ensure its stable inductance value is crucial in design.

The following aspects need to be considered when selecting an inductor for a circuit design:

Inductance: Ensure that the selected inductance value meets the circuit requirements to ensure circuit performance .

Operating Frequency: Select the right type of inductor with the desired frequency characteristics to ensure optimal performance.

Size and Power: When space is limited in the PCB layout, it is necessary to select the right inductor size to meet power requirements and optimize routing.

Thermal management: Inductors generate heat during operation, and proper layout to ensure good heat dissipation is also an important design consideration.

The grounding plane used for shielding is effective without interruption.

The more copper a PCB has, the better it dissipates heat.

Even if eddies are generated, these currents flow only locally,cause only small losses,and hardly affect the function of the ground plane.

Therefore, it is agreed that the grounding plane layer, even below the coil, should remain intact.