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In the discussion of selecting magnetic separators for mobile phones, ferrite and nanocrystals are often compared in a table: whose magnetic permeability is higher, whose saturation magnetization is higher, and who is thinner. These parameters are certainly important, but when the charging power is pushed from 15W to 25W and the thickness budget is pressed around 0.1 mm, the true determinant of whether the plan can be implemented is often not the "high or low" of a certain parameter, but the boundary exposed by the material system when power and thickness are pressed simultaneously.
The boundary position between ferrite and nanocrystals is different, and the failure mode after crossing the boundary is also different.
15W is the comfort zone of ferrite, and 25W is approaching the boundary
In the 15W Qi wireless charging module, ferrite magnetic separators have long been a mature and economical choice. It has high resistivity, negligible eddy current losses, and controllable hysteresis losses in the frequency range of 110-205 kHz. The magnetic permeability of ferrite sheets for wireless charging can reach 200-500 at 100-200 kHz, with a common thickness range of 0.1-0.26 mm.
But the saturation magnetization Bs of ferrite is only 0.35-0.45 T. When the power is increased to 25W, the absolute value of the working magnetic flux density synchronously increases, and the working point of the ferrite separator begins to approach the saturation zone. Saturation itself is not a "sudden failure", but a gradual process: the magnetic permeability decreases as the magnetic flux density increases, and more and more magnetic lines penetrate the magnetic separator into the metal backplate, resulting in increased eddy current losses and local temperature rise. In a 15W module, the eddy current loss of the metal backplate can reduce efficiency by 18% -25% without using magnetic separators; After adopting high-performance nanocrystalline magnetic separators, eddy current losses can be reduced to below 3%, overall efficiency can be improved by 8-12 percentage points, and the back temperature rise can be reduced by 6-10 ℃. This set of data will be further magnified in the 25W scenario because the working magnetic flux density is higher, leaving a narrower margin for the "linear region" of ferrite.
The Bs of nanocrystals are about 1.2-1.25 T, which is about three times that of ferrites. This means that at the same magnetic flux density, the working point of the nanocrystals is further away from the saturation region. When the magnetic permeability is the same as 600, nanocrystals only need 0.11 mm to achieve the same magnetic conductivity as 0.27 mm ferrite; Under the same testing conditions, the charging efficiency of the nanocrystalline scheme is about 2.5 percentage points higher than that of ferrite, and the surface temperature rise of the equipment is 7-8 ℃ lower. The engineering significance of this set of data is not "nanocrystals are better overall", but rather that under the cross constraint of thin and high-power, the saturation boundary of ferrite is reached before nanocrystals.
The thickness compression changes the failure sequence, not just the numerical value
The thickness is compressed from 0.27 mm to 0.11 mm, and the cross-sectional area decreases by about 60%. The magnetic flux that can be carried by the same material system decreases synchronously. For ferrites, this compression directly compresses the saturation margin: the working point, which was originally barely in the safe zone at 15W, may enter the nonlinear zone at 25W. For nanocrystals, high Bs provide a wider saturation margin starting point, but when the thickness is compressed below 0.08 mm, the reduction in cross-sectional area still narrows the margin. Even when Bs reaches 1.25 T, high power or coil offset may still enter the nonlinear region.
This means that the failure sequence of the two types of materials in thinning is different. The first thing ferrite usually encounters is a decrease in magnetic flux guidance ability caused by saturation, manifested as an increase in leakage magnetic flux, an increase in eddy currents in the metal backplate, and a deterioration in temperature rise. The first thing that nanocrystals may encounter is the effective magnetic permeability loss caused by the demagnetization process - in order to control eddy currents in the thin layer, the strip must be demagnetized, and the equivalent air gap formed by the fragment gap will reduce the effective μ '. The effective magnetic permeability of the finished nanocrystalline magnetic separator usually falls in the range of 150-2000, far below the initial value of the strip. When the thickness is further compressed, the fault-tolerant space of the broken magnetic parameters and layer design synchronously narrows, and the influence of the batch fluctuation of μ 'on resonance matching is amplified.
The division of loss composition: the "cleanliness" of ferrite and the "process dependence" of nanocrystals
The loss of ferrite in the NFC and wireless charging frequency bands is relatively 'clean'. High resistivity, negligible eddy current loss, and the main sources of μ "are hysteresis loss and residual loss. At 13.56 MHz, the magnetic permeability of ferrite for NFC is about 130-150, and the μ "can be controlled at a lower level. This is the fundamental reason why ferrite is difficult to replace in NFC isolation: it's not how high the μ 'is, but because the μ' is low enough to not consume the limited magnetic field energy of the NFC antenna.
The loss composition of nanocrystals is highly dependent on magnetic fragmentation and insulation processes. Continuous tapes without broken magnets exhibit significant eddy current losses in the frequency range of 13.56 MHz and above, with a high value of μ ". Direct use in NFC can lead to Q value collapse. After cutting off the eddy current path through the broken magnetic field, the angle of μ 'decreases, but the gap between the fragments also changes the effective angle of μ'. A poorly designed nano chip with broken magnetic parameters may have a strip size of up to tens of thousands, but the final product may not be compressed to a usable level in the target frequency band. Therefore, the selection of nanocrystalline magnetic separators cannot be limited to "tape μ '" or "nominal μ'", but must confirm the finished product state, the μ "and Q value after bonding with the antenna structure.
Temperature rise is not the result of a single material parameter
The temperature rise of nanocrystalline magnetic separators is 7-8 ℃ lower, which is often cited, but it is not solely explained by "lower losses". The difference in temperature rise comes from the superposition of three factors: lower hysteresis and eddy current losses reduce the strength of the heat source; The thinner magnetic layer shortens the conduction path from the heat source to the metal shell; The thermal diffusion ability of the nanocrystalline metal based structure itself is superior to that of the oxide ceramic structure of ferrite. Ferrite has lower thermal conductivity, making it easier for heat to accumulate locally and form hotspots.
In the 25W scenario, the temperature rise constraint is tighter than in the 15W scenario. Qi2.2 strengthens thermal management requirements while increasing power, and the system needs to maintain an acceptable surface temperature at higher power. This means that the selection of magnetic separators cannot only compare "whose loss is low", but also needs to confirm where the loss occurs and whether the heat can be effectively exported. If the magnetic separator is mounted with a low thermal conductivity backing adhesive, even if the intrinsic material loss is low, heat may still accumulate between the magnetic separator and the metal backing plate, offsetting the temperature rise advantage at the material level.
Selection judgment based on the cross constraint of power and thickness
Firstly, verify the saturation margin based on the target power level, rather than using the thickness conclusion from the 15W era
The working magnetic flux density in the 25W scenario is higher than 15W. Even if the alignment accuracy is improved and the offset tolerance is reduced, the magnetic layer thickness and effective area of the magnetic separator still need to be reconfirmed according to the actual working point. Require suppliers to provide the proportion of operating magnetic flux density relative to Bs under target power and actual coil conditions, rather than just labeling static Bs. If the operating point is close to 70% -80% of Bs, the fault-tolerant space for thickness reduction will be very limited.
Secondly, distinguish between "magnetic permeability" and "effective magnetic permeability", and investigate the composition of losses according to the material system
The selection of ferrite should pay attention to the target frequency band's μ "and tan δ, and confirm whether its hysteresis loss is still controllable at high power. The selection of nanocrystals should require effective μ 'and μ "data of the finished product state, and inquire about the fragmentation magnetic parameters, fragment gap filling method, and layer design. The initial μ 'of the strip cannot be used as a basis for judgment.
Thirdly, understand the temperature rise data under actual mounting conditions
The temperature rise difference of 7-8 ℃ was obtained under specific testing conditions. When selecting, it is necessary to confirm the tested power level, coil spacing, adhesive type, and heat dissipation boundary. For thin modules of 25W and above, the thermal path design of the magnetic separator is equally important as the intrinsic loss of the material.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of magnetic isolation sheets for mobile phones covers ferrite composite sheets and nanocrystalline composite sheets. The effective magnetic permeability of the nanocrystalline series products can be customized from 150-2000, with a saturation magnetic inductance of about 1.2 T and a thickness coverage of 0.03-0.3 mm. It can provide saturation margin verification data and loss frequency band decomposition at target power.
The product supports thermal conductive adhesive and multi-layer composite structure, providing material system matching for wireless charging scenarios of mobile phones with 15W, 25W and higher power. For more technical information, please visithttp://www.avanzado.cn/.