The alternating current in the inductor will produce eddy currents on the ground plane, which will affect the inductance of the power inductor, increase the system loss, and also produce noise, affecting the stability of other signals. However, from an EMC point of view, laying copper on the bottom of the inductor can be beneficial for EMI (electromagnetic interference) control, especially when designing a complete ground plane. However, with the advancement of inductor production technology, shielded inductors have gradually become popular. These inductors, through effective shielding design, reduce the leakage of magnetic induction lines, reduce the influence on the inductance, and also improve the heat dissipation performance.
How to choose in the actual project?
How to choose in the project, we must first understand the structure of inductance. Our commonly used inductors are unshielded I-inductors, semi-shielded inductors, and integrated molded inductors. So what are their characteristics?

Unshielded I-inductor

Magnetic circuit structure: The magnetic circuit of the unshielded I-inductor is composed of a magnetic core and air, and its magnetic induction line is completely exposed to the air without any magnetic shielding. This means that its magnetic field tends to spread outward, potentially interfering with surrounding circuits.

Experimental phenomenon: In the experiment, when the bottom of the unshielded I-inductor is laid with copper, the inductance decreases significantly. This is due to the interaction of the external magnetic field with the copper sheet, resulting in a eddy current effect, resulting in a reduction in inductance.

Semi-shielded inductance

Magnetic circuit structure: The semi-shielded inductor adds a magnetic shielding material on the basis of the I-inductor. Due to the small magnetic resistance of the magnetic shielding material, the magnetic inductance line is mostly locked in the magnetic conductive material, and only a small amount of magnetic field will leak out from the air gap, playing a partial shielding role.

Shielding effect: Compared with unshielded inductors, semi-shielded inductors reduce leakage of external magnetic fields, but may still be partially affected by external magnetic fields, depending on the design and application environment.

Integrated inductor

Magnetic circuit structure: In the production process, the winding and the magnetic conductive material are cast once, and there is only a small air gap inside, which can effectively prevent inductance saturation. This structure makes the inductor’s magnetic line basically not overflow, with excellent shielding effect.

Experimental phenomenon: In the experiment, the inductance of the integrated forming inductor almost does not change in the presence of copper, indicating that its shielding structure can effectively prevent the interference of external magnetic field and the influence of eddy current effect.

What effect does copper at the bottom of the inductor have on the power supply?
Before discussing the influence of copper on the power supply, we first review the basic concept of eddy current effect.
When the magnetic inductance line passes through the conductor surface from the N pole to the S pole, if there is an alternating magnetic field, the conductor surface will generate an induced current according to the law of electromagnetic induction. This induced current is called eddy current. The direction of the magnetic field generated by the vortex is always opposite to the direction of the original magnetic field, trying to weaken the strength of the original magnetic field. This phenomenon not only affects the strength of the magnetic field, but also leads to energy loss.

Here is a look at the current loop of a Boost DC/DC circuit to talk about the effect of copper on the bottom of the inductor on the power supply design.

When the Boost boost circuit is working normally, the load current flowing through the inductor forms a loop. Due to the presence of the switching tube, the current changes dynamically, resulting in a magnetic induction line inside the inductor. Some of these magnetic inductance lines form a closed magnetic loop on the conductor surface, and the other part may form magnetic leakage and spill into the air.

If the bottom of the inductor is not coated with copper, the spilled magnetic inductance line will spread throughout the power system, increasing electromagnetic interference (EMI) within the system, thereby reducing the EMI performance of the system, resulting in the lack of a relatively quiet electromagnetic environment for the power system.

If a complete copper layer is laid at the bottom of the inductor, eddy current effects will occur at the bottom of the inductor. The eddy current will partially offset the magnetic field generated by the leakage induction, thus weakening the original magnetic leakage effect. The bottom copper coating acts like an electromagnetic shield, blocking the downward propagation of the magnetic induction line, effectively limiting the high-frequency magnetic field generated by the inductance to one side of the conductor. This greatly reduces the impact of the high-frequency magnetic field on other components around it, helping to improve the electromagnetic compatibility (EMC) of the system.

From two perspectives:

EMI Angle: In order to suppress electromagnetic interference, it is recommended to apply copper to the bottom of the inductor. The eddy current effect of the copper layer can shield the high-frequency magnetic field, reduce the influence on the surrounding circuit, and help improve the electromagnetic compatibility (EMC) of the system.

Inductance Angle: For shielded inductors, applying copper has little effect on its inductance, so it is also recommended to apply copper. For the I-inductance, copper coating will have a certain impact on the inductance, so it is necessary to weigh and choose according to the specific needs in the actual project.

In the actual PCB layout, the filter output by the switch is usually placed on the PCB plane relative to the inductor, which can more effectively avoid high-frequency interference affecting the filter components and prevent high-frequency interference from being transmitted out through the line.