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In the design review of NFC antennas, the permeability of the magnetic separator is usually treated as a "higher, better" parameter. But at the frequency of 13.56 MHz, this intuition will fail.
A wireless charging magnetic separator that performs well at 100 kHz may not work at all in NFC scenarios. The reason is not that its μ 'is not high enough, but that its μ' is too high. High μ "means that the material itself consumes a large amount of energy in an alternating magnetic field, and the magnetic flux radiated by the antenna is absorbed by the magnetic separator in the form of heat before it reaches the card reader. At the same time, the presence of μ "will change the equivalent inductance and resonance conditions of the antenna, causing the LC circuit, which was originally tuned to 13.56 MHz, to deviate from the operating frequency point.
The core engineering problem of NFC magnetic separators can therefore be summarized as finding a decoupled window between sufficiently high μ 'and sufficiently low μ' '.
13.56 MHz loss sensitivity: why the "absolute value" of μ "is more important than the ratio
In the low frequency range (such as 110-205 kHz in Qi), engineers typically evaluate the loss level of materials using the loss tangent tan δ=μ "/μ '. If the ratio is low enough, the loss is considered acceptable. But at 13.56 MHz, this habit needs to be adjusted.
The magnetic field strength of NFC antennas is inherently weak. When the magnetic field generated by the card reader attenuates in space and reaches the mobile phone end, the energy available to excite NFC antenna resonance is already very limited. If the absolute value of the magnetic separator's μ "is too high, even if the ratio of tan δ does not appear high, the magnetic field energy absorbed by the material is still sufficient to significantly shorten the communication distance. A patent for NFC magnetic sheets explicitly states that shielding sheets suitable for 13.56 MHz need to maintain tan δ not exceeding 0.05 within the range of 100-250 μ '. TDK's IFQ06 series controls the μ 'at a level of approximately 2, with a μ' of 56, corresponding to a quality factor Q=μ '/μ' ≈ 28.
The engineering meaning here is that the selection of NFC magnetic separators should be based on the absolute value of μ "as the first sieve, rather than the height of μ 'as the starting point. A nanocrystalline scheme with μ '=500 but μ'=50 may not perform as well as a thin magnetic sheet with μ '=56 but μ'=2 in the NFC frequency band. 50 μ ″ means that a large amount of energy is dissipated by the material per cycle, and the NFC system does not have sufficient power budget to withstand this dissipation.
Where does the loss advantage of ferrite in NFC frequency band come from
MnZn and NiZn ferrites dominate in NFC separators not because of their high μ ', but because of their sufficiently high electrical resistivity, and eddy current losses can be ignored at 13.56 MHz.
The resistivity of ferrite is usually above 10 Ω· cm, and the eddy current induced by alternating magnetic field inside the material is extremely small. The main sources of μ "are hysteresis loss and residual loss, rather than eddy current loss. This enables ferrite to achieve a level of μ 'in the range of 100-250 and tan δ below 0.05 at 13.56 MHz. For NFC antennas, this combination means that the material provides sufficient magnetic flux guidance capability without introducing significant attenuation in the signal path.
But the upper limit of μ 'for ferrite also comes from the same set of physical constraints. The saturation magnetization Bs of ferrite is only 0.4-0.5 T, and the μ 'typically does not exceed 250 at 13.56 MHz. If NFC antennas require higher magnetic flux convergence capability to improve read distance, the μ 'of ferrite is already approaching the upper limit of its material system. This is the structural boundary of ferrite in NFC scenarios: loss is controllable, but the adjustment space of μ 'is limited.
The prerequisite for nanocrystals to enter the NFC scene is to first suppress eddy current losses
The μ 'of nanocrystalline materials can be much higher than that of ferrites, but their resistivity is extremely low (about 130 μ Ω· cm). If left untreated at 13.56 MHz, eddy current loss will become the dominant term for μ'. A patent on NFC module magnetic separators points out that nanocrystalline tapes are prone to significant eddy current losses at medium to high frequencies in NFC, which can affect the normal use of NFC modules.
Therefore, the prerequisite for nanocrystals to enter the NFC scene is not that "μ 'is high enough", but that "eddy current losses are first compressed to a sufficiently low level". The engineering methods include magnetic fragmentation, which divides the continuous strip into small insulated fragments and cuts off the closed path of eddy currents over a large area. The air gap resistivity between fragments is much higher than that of the alloy itself, and eddy currents can only form small loops inside individual fragments, resulting in a decrease in μ ".
But breaking the magnetic field not only reduces eddy current losses, but also changes the effective μ '. The gap between fragments constitutes the equivalent air gap in the magnetic circuit, and the magnetic field lines increase the magnetic resistance when jumping between fragments. The effective μ 'of the finished nanocrystalline magnetic separator usually falls in the range of 150-2000, much lower than the initial magnetic permeability of the strip. For NFC scenarios, engineers need to match the fragmentation parameters (fragment size, gap filling method) with the target μ ': fragments are too small, μ' loss is too large, and magnetic field convergence ability is insufficient; The fragments are too large, and the vortex path has not been fully cut off, resulting in a still high μ ".
Qi and NFC share: Gradient stacking solves the problem of "opposite requirements on both sides"
The contradiction is most acute when NFC and wireless charging share a magnetic separator. The wireless charging end hopes that the side of the magnetic separator facing the coil, μ ', will be as high as possible to constrain magnetic flux and reduce leakage; However, NFC antennas require the contact surface μ 'to be kept at a moderate level to avoid excessive increase in antenna inductance and resonance frequency deviation from 13.56 MHz due to loading effects.
The gradient permeability stacked structure is the mainstream engineering approach to address this contradiction. The solution disclosed in the patent literature is that the shielding sheet is composed of at least two layers stacked together, and the magnetic permeability increases by 8% -40% from the side closer to the NFC antenna to the side farther away from the NFC antenna. The low μ 'layer near the NFC antenna reduces the sensitivity of the antenna to magnetic material loading, avoiding detuning; The high μ 'layer away from the NFC antenna is responsible for magnetic flux guidance, ensuring wireless charging efficiency. This design assigns the two requirements of "NFC friendly" and "efficient charging" to different physical layers, rather than forcing a compromise within the same material layer.
For devices that retain magnetic attraction structures, the situation will be more complex. The magnet itself will locally change the magnetic field distribution, and the local saturation behavior and μ 'distribution of the magnetic separator near the magnet need to be checked separately, and cannot be replaced by the average μ' in the non-magnetic state.
Selection and judgment of NFC magnetic separators based on μ "
For the selection of magnetic separators in NFC scenarios, the traditional approach of "looking at the height of μ '" needs to be adjusted to a judgment logic starting from loss.
Firstly, using μ "and tan δ at 13.56 MHz as the first screening criteria
Require suppliers to provide data on the target frequency points of μ ', μ', and tan δ, rather than just providing magnetic permeability at low frequencies (such as 100 kHz). For NFC applications, the absolute value of μ "directly determines the degree of signal attenuation. The referenceable boundaries are: tan δ should not exceed 0.05, and the absolute value of μ "should match the power budget of the NFC system. A scheme with μ '=200 and μ'=4 may outperform the scheme with μ '=500 and μ'=40 in terms of signal attenuation.
Secondly, distinguish between "strip material μ '" and "finished product effective μ'"
The initial μ 'of nanocrystalline magnetic separators may be as high as tens of thousands, but the effective μ' of the finished product after fragmentation and lamination is usually only 150-2000. The inductance design of NFC antennas relies on the actual loading effect after the finished product is attached to the antenna, rather than the initial value of the strip. When selecting, the supplier should be required to provide the finished product status, effective μ 'and Q value change data after fitting the NFC antenna structure.
Thirdly, in the Qi/NFC shared scenario, confirm whether the μ 'distribution of the gradient stack can be customized
Different devices have different priorities for NFC reading distance and charging efficiency. Mobile payments require higher stability of NFC reading distance and lower antenna side μ '; High power charging requires a stronger contribution from the high μ 'layer. When selecting, it should be confirmed whether the supplier supports customization of stacked magnetic permeability gradient and whether the adjustable range covers the target requirements.
Advanced Institute (Shenzhen) Technology Co., LtdThe NFC magnetic isolation sheet covers two routes: ferrite composite sheet and nanocrystalline composite sheet. Ferrite composite sheets can achieve a microsecond ≥ 120 at 13.56 MHz with a thickness of 0.1 mm, balancing thinning and magnetic permeability; The nanocrystalline composite sheet can achieve a thickness of over 500 μ 'at a thickness of 0.05 mm, and the eddy current loss is controlled by the magnetic fragmentation process, making it suitable for flagship models that are sensitive to space and have high reading distance requirements.
The product supports customized gradient magnetic permeability stack structure for NFC and wireless charging shared scenarios. For more technical information, please visithttp://www.avanzado.cn/.