The cost of flexibility of flexible magnetic separators: how the fragmentation process affects eddy current losses and wireless charging efficiency

In the design review of TWS earphone charging case and smartwatch, the selection criteria for magnetic separators are undergoing a subtle change. A few years ago, the core parameters that engineers focused on were magnetic permeability and saturation magnetization. Today, under assembly conditions where the thickness is compressed to less than 0.5 mm and the mounting surface changes from flat to curved, "flexibility" and "ultra-thin" have replaced "magnetic permeability" as the primary screening criteria.

This change has brought about an easily overlooked engineering problem: flexibility is not free.

Where does flexibility come from: the physical essence of magnetic breaking technology

The magnetic permeability of block shaped nanocrystalline alloy strip is extremely high, but it is hard and brittle. An untreated nanocrystalline strip can experience uncontrollable fracture and fragment scattering during bending, resulting in complete loss of magnetic properties. To make this material "flexible", it requires a process called demagnetization.

The essence of magnetic fragmentation is the controlled division of continuous nanocrystalline strips into tiny, mutually insulating fragments. The gaps between the fragmented units are filled with air or insulating material, thereby establishing a resistive barrier between the fragments and cutting off the closed path of eddy currents over a large area. The selection of fragment size requires a balance between eddy current loss and magnetic permeability: the smaller the fragment, the shorter the eddy current path, and the lower the eddy current loss; But as the gap area between fragments increases, the effective magnetic permeability of the material will decrease.

The reason why the broken magnetic separator can bend without failure is because the bending strain is "absorbed" by the gaps between the fragments. The fragment unit undergoes relative sliding during bending, manifested macroscopically as material flexibility, while each fragment itself does not undergo significant deformation. The adhesive film on both sides fixes the fragments, so even if they are bent to a broken state, the fragments will not scatter and the electromagnetic performance can still be maintained.

The first cost of magnetic fragmentation: a decrease in effective magnetic permeability

The effective magnetic permeability of the fragmented magnetic separator is much lower than the initial magnetic permeability of the original strip. The initial magnetic permeability of the original nanocrystalline strip can reach 30000-80000, but after fragmentation and lamination processes, the effective magnetic permeability of the finished product usually drops to 800-1500. This decline is not a process defect, but an inevitable result of fragmented structures - the gaps between the fragments form magnetic resistance, and electromagnetic waves need to "jump" between the fragments when penetrating the magnetic separator, cutting off the effective magnetic path.

The second cost of magnetic fragmentation: interface debonding and magnetic circuit integrity

Flexible magnetic separators are usually composed of multiple layers of fragmented magnetic layers that are compounded with double-sided tape. The original intention of designing multi-layer structures is to obtain sufficient magnetic flux within a limited thickness, but each layer interface becomes a potential failure point.

During repeated bending or thermal cycling, the relative sliding between fragment units will generate periodic shear stress on the adhesive layer. Under the action of viscosity and curing shrinkage, the adhesive layer will generate additional stress on the fragments, causing them to change orientation, widen gaps, and ultimately reduce magnetic permeability. This process will accelerate when the bending radius is small (such as ≤ 3 mm) or when there are more bending times (such as over 1000 times).

A more subtle issue is the way in which the gaps between fragments are filled. If the fragments are completely filled with adhesive layer, the viscosity of the adhesive layer will generate squeezing force on the fragments, causing deformation and orientation change of the fragments. A better solution is to at least partially fill the gaps between the fragments with air - air is both an insulator and a non-magnetic medium that does not exert additional stress on the fragments. In some process schemes, the insulation layer penetrates into the gaps between fragments to form a buffer support, preventing collisions and micro fragmentation between fragments, which is particularly important under bending conditions.

The third cost of flexibility: the trade-off between ultra-thin and loss

Another design trend for flexible magnetic separators is ultra-thin. At present, the leading single-layer thickness of nanocrystalline magnetic separators has reached 16 μ m, with a typical product thickness range of 0.018-0.1 mm. At the same efficiency, the typical thickness of nanocrystalline magnetic separators is only 0.11 mm, while ferrite requires 0.27 mm. The charging efficiency is about 2.5% higher and the temperature rise is 7-8 ℃ lower.

However, ultra-thin technology also comes with a cost of loss. The magnetic flux carrying capacity of a magnetic separator is directly proportional to its cross-sectional area. When the thickness is compressed from 0.27 mm to 0.11 mm, the magnetic flux that the same material system can carry decreases by about 60%. This means that at the same operating frequency and magnetic flux density, the working point of the ultra-thin magnetic separator is closer to the saturation zone. Once it enters saturation, the magnetic permeability drops sharply, and the magnetic field penetrates the magnetic separator into the metal layer. The deterioration of eddy current temperature rise and charging efficiency occurs simultaneously.

The saturation magnetization Bs of nanocrystalline materials is about 1.2-1.25 T, which is about three times that of ferrites (0.35-0.45 T). This high Bs characteristic provides a physical basis for ultra-thin materials. But in high-power scenarios (such as wireless charging for vehicles above 50W), even nanocrystalline magnetic separators need to be stacked in multiple layers to ensure sufficient magnetic flux carrying capacity.

The following table compares the thickness requirements and loss control focus of flexible magnetic separators in different application scenarios:

Reverse selection logic from flexible constraints

For engineers who require flexible magnetic separators, the selection logic should start from the constraint of "flexibility" and reverse derive the matching relationship between the material system and the process plan.