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In the selection discussion of wireless charging receivers, magnetic chips are usually selected according to "one frequency band": based on their μ 'and loss at 100-205 kHz, or their μ' and Q values at 13.56 MHz. However, in the actual structure of mobile phones, the wireless charging coil and NFC antenna often share the same back area of the body, and the magnetic plate needs to face two frequency bands simultaneously in the same physical space.
The difference between the two frequency bands is about two orders of magnitude, and the requirements for magnetic plates are not consistent. In the low frequency range, magnetic plates are required to provide sufficient magnetic flux guidance capability to confine magnetic field lines within the coupling region of the coil; The high-frequency band requires the magnetic chip itself to be sufficiently 'quiet', without introducing additional losses and mismatches in the NFC signal path. If the same material falls within the available window on both frequency bands, the selection becomes a dual band compromise problem.
Two frequency bands, two sets of evaluation indicators
In the wireless charging frequency range of 100-205 kHz, the core task of the magnet is magnetic flux guidance. The evaluation criteria are whether the μ 'is high enough, whether the saturation magnetic induction covers the target power, and whether the loss in this frequency band is controllable. The magnetic field energy of wireless charging coils is relatively abundant, and the μ "of the magnetic sheet is usually not the primary contradiction in this frequency band - as long as it does not generate significant heat, a certain amount of loss is acceptable.
Evaluation logic for 13.56 MHz NFC frequency band
The magnetic field energy of NFC antenna itself is very limited, and the microgrid directly consumes the signal budget. Whether the evaluation indicators become sufficiently low, whether tan δ is controlled within 0.05, and whether the Q value falls within the available range. At this point, the height of μ 'takes a back seat, and the material with μ'55-80 may be more suitable for NFC scenarios than the material with μ'300, as the latter often has a higher μ ".
The same material parameter has completely different weights in two frequency bands. This is the root of the difficulty in selecting materials in dual frequency coexistence scenarios: materials optimized for one frequency band may cross boundaries in another frequency band.
Ferrite: Both frequency bands have available windows, but the upper limit of μ 'is a common constraint
The advantage of ferrite in dual frequency scenarios is that it has relatively "clean" loss characteristics in both frequency bands. The resistivity of NiZn ferrite is extremely high, and eddy current losses can be ignored at 13.56 MHz. The main sources of μ "are hysteresis losses and residual losses, which can be reduced to lower levels through formulation and sintering processes. In the low frequency range of 100-205 kHz, the hysteresis loss of ferrite is also controllable, and μ 'can reach the order of hundreds.
But the upper limit of the μ 'of ferrite is constrained by the intrinsic properties of the material. At 13.56 MHz, the μ 'of ferrite typically does not exceed 250-300; At 100-205 kHz, although μ 'can be higher, the saturation magnetic inductance Bs is still in the range of 0.4-0.5 T, and the saturation margin under high-power wireless charging is limited. When dual frequency coexists, ferrite is not the "strongest" choice in both frequency bands, but it will not have major problems in either frequency band - this "two band enough" characteristic makes ferrite a low-risk starting point in dual frequency scenarios.
Another engineering advantage of ferrite is its hardness, brittleness, and dimensional stability. In the shared area of NFC and wireless charging, the planar size and position of the magnetic sheet need to be accurately matched with the projection of the antenna and coil, and the dimensional tolerance and flatness of the ferrite sheet are relatively easy to control. The cost is the limitation of surface mounting, which requires additional structural design in models that require surface fitting.
Nanocrystals: Abundant in low frequency range, requiring 'process compensation' in high frequency range
The advantage of nanocrystals in the low frequency range is obvious: Bs is about 1.2-1.25 T, which is more than twice that of ferrite, and the saturation margin is more abundant under 25W wireless charging; Under the same efficiency, the thickness can reach 0.11 mm, leaving room for stacking space.
But at 13.56 MHz, nanocrystals face a structural obstacle: extremely low resistivity, significant eddy current losses in untreated continuous strips, high μ ", and direct use in NFC can lead to Q-value collapse. The prerequisite for nanocrystals to enter the dual frequency scene is that the demagnetization process compresses the eddy current loss at 13.56 MHz to a level where the Q value is available. The effective μ 'of the finished product after magnetic fragmentation usually falls within the range of 150-800, and the control level of μ' depends on the fragment size, gap filling method, and layer design. This means that the performance of nanocrystals in dual frequency scenarios is highly dependent on the supplier's magnetic fragmentation process capability. A poorly designed nanocrystalline magnetic sheet with broken magnetic parameters may perform well in the low frequency range, but at 13.56 MHz, it hinders NFC due to insufficient suppression of μ ". If only the low-frequency range of μ 'and Bs are considered during selection, without confirming the μ "and Q values at 13.56 MHz, the dual band compatibility is actually unverified.
Three engineering paths for dual frequency coexistence
There are three common organizational methods in engineering to meet the different requirements of two frequency bands.
The first type is a single-layer broadband magnetic sheet
Using the same material to serve two frequency bands simultaneously, balancing the loss characteristics of the material itself in both frequency bands. Ferrite is a typical choice for this pathway because it has available loss windows in both frequency bands. The cost is that neither frequency band is optimal, and a compromise is needed between the magnetic flux capacity in the low frequency band and the low loss in the high frequency band.
The second type is hierarchical structure
Stacking two layers of magnetic sheets with different characteristics in the thickness direction: the layer facing the coil focuses on low-frequency magnetic flux guidance, and the layer facing the NFC antenna focuses on high-frequency low loss. A layered structure can allocate the requirements of two frequency bands to different physical layers, reducing compromises within a single layer of material. The cost is an increase in total thickness, as well as additional magnetic resistance and process complexity introduced by interlayer interfaces.
The third type is partition layout
Divide the magnetic sheet into different areas in the plane: the wireless charging coil projection area uses high magnetic flux capacity materials, and the NFC antenna projection area uses low loss materials, which are staggered or connected through a transition area in the plane. The challenge of partition layout lies in the boundary: the sudden change in magnetic resistance in the transition zone may cause local saturation or detuning, requiring additional simulation and experimental verification. The choice between the three paths depends on the internal space budget of the aircraft, the priority of the two frequency bands, and the supplier's technological capabilities. There is no one path that dominates in all scenarios.
Selection and judgment of magnetic chips at the receiving end based on "frequency band priority x spatial layout"
Firstly, confirm the spatial relationship between the two frequency bands inside the aircraft before discussing the materials
If the wireless charging coil and NFC antenna completely overlap in the plane, a single-layer broadband magnetic sheet or layered structure is a more realistic choice; If the two are partially offset in the plane, the partition layout can use spatial separation to reduce mutual interference between frequency bands. The spatial relationship determines which engineering paths are feasible, and material selection is carried out within this constraint.
Secondly, in dual band scenarios, consider the μ "at 13.56 MHz as an independent filtering criterion, rather than an accessory for low-frequency performance
The μ 'and Bs in the wireless charging frequency band cannot predict the performance of the NFC frequency band. When selecting, the supplier should be required to provide both μ 'and μ' data in the frequency bands of 100-205 kHz and 13.56 MHz, and confirm whether the tan δ at 13.56 MHz falls within the available range of 0.05. If the supplier can only provide low-frequency data, dual band compatibility cannot be determined.
Thirdly, distinguish between "intrinsic dual frequency characteristics of materials" and "dual frequency performance of finished products after bonding"
The frequency band characteristics of ferrite and nanocrystals are data at the material level, but the actual performance of the two frequency bands after the finished magnetic sheet is mounted between the coil and NFC antenna still depends on the coverage area, interlayer interface, and surrounding metal structure. When selecting, suppliers should be required to provide dual frequency evaluation data that fits the actual antenna structure, rather than just providing magnetic spectra of material samples.
Fourthly, in layered or partitioned schemes, consider "interface" as an independent validation item
The interlayer interfaces of layered structures and the planar boundaries of partition structures are both risk points for magnetic resistance mutations and local saturation. When selecting, it should be confirmed whether the supplier can provide magnetic flux distribution or local loss data at the interface, as well as the stability of the transition zone under different offset conditions.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging receiver magnetic chip product line covers ferrite composite chips and nanocrystalline composite chips, and can provide data in two frequency bands of 100-205 kHz and 13.56 MHz for the μ 'and μ "frequency bands.
The flat mounting requirements of ferrite composite sheets for dual frequency coexistence scenarios; The nanocrystalline composite sheet controls the eddy current loss at 13.56 MHz through magnetic fragmentation and insulation treatment, and the effective μ 'of the finished product can be adjusted within the range of 150-800, supporting customized die-cutting of layered and partitioned layouts. For more technical information, please visithttp://www.avanzado.cn/.