High permeability and low loss magnetic isolation sheet: frequency weight and material system matching logic composed of wireless charging receiver losses

In the design review of wireless charging receivers, the selection of magnetic separators is often simplified as a parameter comparison: whoever has a higher magnetic permeability is "better". But when engineers attach two magnetic separators with similar nominal magnetic permeability to the same receiving board for testing, the temperature rise may differ by 8-12 ° C, and the charging efficiency may differ by 3-6 percentage points.

The root of the difference lies not in the magnetic permeability itself, but in the losses - more precisely, in the proportion of eddy current losses and hysteresis losses.

Frequency weight difference between two loss mechanisms

The magnetic separator operates in an alternating magnetic field, and its losses are composed of hysteresis losses, eddy current losses, and residual losses. Hysteresis loss comes from the irreversible movement of magnetic domain walls, which is proportional to the coercive force Hc of the material. Eddy current loss comes from the eddy current induced inside a conductive material by an alternating magnetic field, which is inversely proportional to the material's resistivity and proportional to the square of the frequency.

The dependence direction of these two types of losses on material parameters is not consistent. Reducing hysteresis loss requires reducing coercive force, while reducing eddy current loss requires increasing resistivity or reducing material unit size. In ferrite materials, the resistivity is naturally high, eddy current losses can be ignored, and hysteresis losses are the main contradiction. However, in nanocrystalline and amorphous alloys, the resistivity is only one millionth of that of ferrite, and eddy current losses dominate - especially at operating frequencies above 100 kHz, where eddy current losses can account for over 70% of the total losses.

Broken magnetic patterning: cutting off the eddy current path from the source

The core method for controlling eddy current losses in nanocrystalline magnetic separators is magnetic fragmentation treatment - dividing continuous nanocrystalline strips into small, mutually insulating fragments to cut off the closed path of eddy currents over a large area.

The pattern design of broken magnets directly affects the suppression effect of eddy current losses. The magnetic crushing process disclosed in the patent literature adopts a magnetic crushing steel roller with a square pattern on the surface, with a side length of 0.5-1.5 mm and a roller pressure of 100-300 kg. The gaps between the fragment units are filled with air, and the electrical resistivity of air is much higher than that of nanocrystalline alloys, thus establishing an effective insulation barrier between the fragments. 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 filling method of fragment gaps is equally crucial. If the fragments are completely filled with adhesive layer between them, the viscosity and curing shrinkage of the adhesive layer will generate stress on the fragments, causing them to change orientation, widen gaps, and ultimately reduce magnetic permeability. 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. Advanced Institute Technology adopts an air gap filling scheme in the magnetic fragmentation process, combined with thermal conductivity or heat dissipation coatings to prevent double-sided tape from entering the debris gap, while maintaining the stability of magnetic permeability while controlling eddy current losses.

Suppression of Hysteresis Loss: Reducing Coercivity from the Material Source

After the eddy current loss is resolved, hysteresis loss becomes the main component of residual loss. The hysteresis loss is directly proportional to the coercive force Hc, which is highly sensitive to the heat treatment state of the material.

During the preparation process of nanocrystalline tapes, amorphous precursors undergo heat treatment to form nanocrystalline structures. When the grain size is controlled within the range of 10-20 nm, the material exhibits excellent soft magnetic properties - the coercivity Hc can be reduced to below 1.0 A/m, far lower than the coercivity level of traditional ferrites. This low coercivity is directly converted into low hysteresis loss, making the loss advantage of nanocrystalline magnetic separators more obvious under high current conditions.

However, in high-power wireless charging scenarios, hysteresis loss is also constrained by saturation magnetization Bs. The saturation magnetization of ferrite is about 0.4 T, while the saturation magnetization of nanocrystalline materials can reach over 1.2 T. A higher Bs means that the material only enters the saturation zone at a greater magnetic field strength, and the hysteresis loop area is smaller and the hysteresis loss is lower when the operating point is far away from the saturation zone. In scenarios of car mounted wireless charging or industrial wireless power supply above 50W, this difference directly determines whether the magnetic separator will experience saturation anomalies and local overheating during operation.

Priority of loss control at different frequencies

The working frequency of Qi standard is 110-205 kHz, and the working frequency of AirFuel resonant standard is 6.78 MHz. The difference in frequency changes the weight relationship between the two types of losses.

In the low frequency range of 110-205 kHz, although eddy current losses still exist in nanocrystalline materials, the proportion of hysteresis losses relatively increases. At this point, the design focus of the fragmented magnetic pattern is to ensure that the magnetic permeability does not decrease excessively due to fragmentation. In the high frequency range of 6.78 MHz, eddy current losses are directly proportional to the square of the frequency, and their proportion rapidly increases. The refinement of the fragmented magnetic pattern becomes a key variable for loss control. The selection of magnetic separators in the AirFuel scenario should prioritize confirming the eddy current loss data of the material at 6.78 MHz, rather than solely focusing on the magnetic permeability values at low frequencies.

The following table summarizes the control strategy differences of two loss mechanisms at different frequencies:

Comparison of material systems between ferrite and nanocrystals

Ferrite and nanocrystals are currently the two mainstream material systems for wireless charging receiver magnetic separators, and there are systematic differences between the two in terms of loss characteristics, thickness capability, and power adaptability.

The advantages of ferrite magnetic separators are high resistivity and low eddy current losses, which have a mature engineering application foundation in the mid to low frequency range. But its saturation magnetization is only 0.4-0.5 T, which is prone to saturation in high-power scenarios and is hard, brittle, and fragile, making it difficult to adapt to curved coils. The saturation magnetic induction of nanocrystalline magnetic separators reaches 1.2-1.25 T, and at the same efficiency, the thickness is only about 40% of ferrite - a typical thickness of 0.11 mm corresponds to 0.27 mm of ferrite. The flexibility of nanocrystals allows them to adhere to curved coils, which has irreplaceable advantages in wearable devices and ultra-thin smartphones. However, the resistivity of nanocrystals is extremely low, and eddy current losses must be controlled through demagnetization and insulation treatment, with a higher process complexity than ferrite.

Three engineering judgments in selection

For the selection of magnetic separators for wireless charging receivers, simply focusing on the magnetic permeability value is no longer sufficient to distinguish the actual performance differences of materials. The validation data from the following three dimensions can help engineers make more accurate judgments.