Do you need to lay copper under the inductor?

The inductor has an alternating current, and copper laying on the bottom of the inductor will generate eddy current on the ground plane. The eddy current effect will affect the inductance of the power inductor, and the eddy current will also increase the loss of the system. At the same time, the noise generated by the alternating current will increase the noise of the ground plane. Will affect the stability of other signals.

From the perspective of EMC, laying copper on the bottom of the inductor and laying copper on the complete ground plane is conducive to the design of EMI; the current production process of the inductor is upgraded, the inductor adopts a shielded inductor, and there are few leaked magnetic induction lines, which is harmful to the inductor. The volume has little effect, and it is also conducive to heat dissipation.

How to choose in actual engineering？

How to choose in engineering, we must first understand the structure of the inductor. Our commonly used inductors are unshielded I-shaped inductors, semi-shielded inductors, and integrated inductors. What are their characteristics?

For unshielded I-shaped inductors, the magnetic circuit is composed of a magnetic core and air, and its magnetic induction line will be completely exposed to the air without any magnetic shielding.

The semi-shielded inductor can be seen from its skeleton structure. On the basis of the I-shaped inductor, a magnetic shielding material is added around the inductor. Because the magnetic resistance of the magnetic shielding material is small, the magnetic induction line is basically locked in the magnetic permeable material, and only a small part of the magnetic field will leak out from the air gap, so it can play a certain shielding effect.

The one-piece inductor is made by casting the winding and the magnetically permeable material at one time during the production of the inductor. There is only a small air gap inside to prevent the inductor from being saturated, so there is basically no overflow of the magnetic induction line of this inductor.

Experimental verification

In the experiment, we use a buck chip evaluation board from MPS to conduct an experiment. To simulate placing a copper layer under the inductor, we placed a grounded copper patch near the inductor and then measured the inductor current ripple to evaluate the effect of placing copper under the inductor.

Experimental results show that when the copper sheet is placed close to the unshielded inductor, the peak inductor current increases by about 8% (see Figure 2). When using other types of inductors, the peak-to-peak value of the inductor current remains almost the same?

This experiment proves that laying copper on the bottom of the inductor has only a small effect on the inductance of the unshielded inductor, and has almost no effect on the inductance of the shielded inductor.

Does the copper at the bottom of the inductor affect the power supply?

Before thinking about this question, first review and understand the eddy current effect. The magnetic induction line is from N to S level. When there is an alternating magnetic field passing through the surface of the conductor, it can be known from the law of electromagnetic induction that an induced current is formed on the surface of the conductor. The direction of the magnetic field generated by the induced current will always play a role in weakening the size of the original magnetic field.

The following is the situation of the current loop of the Boost DC/DC circuit, let’s talk about the influence of copper on the bottom of the inductor on the design of the power supply.

When the Boost is working normally, the load current flows through the inductor to form a loop. Due to the existence of the switch tube, the current changes dynamically, which can form the magnetic induction line of the inductor. The magnetic induction line on the surface of the conductor forms a closed magnetic circuit, and part of the magnetic circuit will form a magnetic flux leakage and overflow into the air. If there is no copper at the bottom of the inductor, the magnetic induction lines overflowing from the inductor will exist in the entire power system, leaving the system without a relatively quiet space, which will cause EMI performance degradation.

If the bottom of the inductor is covered with complete copper, an eddy current effect will be generated on the bottom plane of the inductor, and the eddy current will offset the magnetic field generated by part of the leakage inductance, weakening the original magnetic leakage induction line. The bottom of the inductor is coated with copper, and the eddy current generated is like an electromagnetic shield, which blocks the downward propagation of the magnetic induction line, so that the high-frequency magnetic field generated by the inductor can be shielded on one side of the conductor, which greatly reduces the impact of the high-frequency magnetic field on the space. influence of other components.

From two perspectives, from the perspective of EMI, it is recommended to apply copper; from the perspective of inductance, shielded inductance has no effect, so copper is also recommended, and only the bottom of the I-shaped inductor is applied to the inductance. There is a little influence, so it depends on the actual project.

In the actual PCB layout, the filter output from the switch is placed on the PCB plane opposite to the inductor, which is more conducive to avoiding high-frequency interference with filter components and preventing high-frequency interference from being transmitted through the line.