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In the design of smartphones and smartwatches, NFC antennas and wireless charging coils are often placed in the same area. The two share the limited space on the back of the fuselage and the same set of magnetic shielding structures. On the surface, this appears to be a space saving design choice. But from an electromagnetic perspective, these two functions have almost opposite requirements for magnetic separators.
The wireless charging receiver requires the magnetic separator to have the highest possible magnetic permeability. The higher the magnetic permeability, the stronger the ability of magnetic field lines to be constrained within the coupling region of the coil, the less leakage magnetic field, and the higher the charging efficiency. When the NFC antenna operates at a frequency of 13.56 MHz, the high magnetic permeability of the magnetic separator can actually cause problems: high magnetic permeability materials can reduce the resonant frequency of the NFC antenna, leading to antenna detuning, shortened reading range, and even inability to communicate normally. A study has pointed out that if the magnetic separator is not selected properly, NFC card swiping failure and touchscreen flickering problems will significantly increase during wireless charging operation.
This contradiction means that when NFC and wireless charging share a magnetic separator, engineers are not faced with a "material selection" problem, but a "material design" problem.
Where does the conflict of magnetic permeability requirements between two functions come from
NFC works at 13.56 MHz, and wireless charging Qi standard works at 110-205 kHz. The frequency difference has two levels of impact.
At the material level, the complex magnetic permeability of the magnetic separator varies with frequency. In the Qi frequency range of 110-205 kHz, the real part of the magnetic permeability of nanocrystalline or ferrite separators is at a high level, which can effectively guide magnetic field lines. But at 13.56 MHz, the real part of the magnetic permeability of the same material usually begins to decrease, and the tangent of the magnetic loss angle increases. If the magnetic permeability of the material is still too high at 13.56 MHz, the inductance value of the NFC antenna will exceed the matching range and the resonant frequency will shift.
At the antenna design level, the inductance of the NFC antenna coil is directly related to the effective magnetic permeability of the magnetic separator. When the magnetic separator is tightly attached to the NFC antenna, the inductance of the antenna will increase due to the loading of the magnetic material. If the magnetic permeability of the magnetic separator is too high and the antenna inductance exceeds the design value, the resonant capacitor needs to be re matched. In the design of shared antenna coils for NFC and wireless charging, the difficulty of matching further increases - wireless charging requires higher inductance to achieve efficient coupling, while NFC requires lower Q values to ensure sufficient bandwidth. It is difficult to meet two requirements simultaneously on the same coil.
Gradient permeability stack: a feasible solution
One of the technical routes to solve this contradiction is the gradient permeability stack structure. The core idea is to make the different layers of the magnetic isolation sheet have different magnetic permeability. A low magnetic permeability shielding sheet is used on the side close to the NFC antenna, and a high magnetic permeability shielding sheet is used on the side far away from the NFC antenna.
A magnetic shielding laminated structure applied to wireless charging and NFC integration is disclosed in the patent literature. This structure is located below the NFC antenna and includes at least two layers of stacked shielding sheets. The magnetic permeability of the shielding sheets increases by 8% -40% from the side closer to the NFC antenna to the side farther away from the NFC antenna. The low magnetic permeability layer near the NFC antenna reduces the sensitivity of the NFC antenna to magnetic material loading, avoiding antenna detuning; The high magnetic permeability layer far away from the NFC antenna provides sufficient magnetic flux guidance capability to ensure wireless charging efficiency.
Another engineering advantage of this gradient design is its adjustable performance. By adjusting the magnetic permeability ratio and number of layers, different balance points can be found between NFC performance and wireless charging efficiency. For applications with higher NFC reading distance requirements (such as mobile payments), the thickness ratio of the low magnetic permeability layer can be increased; For applications that require higher charging efficiency, such as high-power fast charging, the contribution of the high permeability layer can be increased.
The role transformation of magnetic separators in multifunctional integration
When the magnetic separator serves both NFC and wireless charging functions, its role changes from a "single function magnetic flux guide" to a "multi band electromagnetic management device". This shift means that the selection logic needs to shift from "parameter comparison" to "system matching".
The traditional selection process for magnetic separators is to determine the operating frequency, select materials with appropriate magnetic permeability, and confirm that the thickness meets spatial constraints. But in the multifunctional integration scenario, the selection needs to consider the material response at both frequencies simultaneously. Engineers need to confirm the real and imaginary part data of the magnetic permeability of the isolator in the frequency bands of 110-205 kHz (Qi) and 13.56 MHz (NFC), rather than just focusing on the nominal value at a single frequency.
Another overlooked factor is the performance drift of the magnetic separator under different functional working states. When wireless charging and NFC work simultaneously, the high-frequency alternating magnetic field generated by the wireless charging coil may generate additional noise in the NFC frequency band, affecting the quality of NFC communication. Magnetic separators need to have sufficient loss and shielding capabilities in both frequency bands to isolate this cross interference.
Judging the selection of multifunctional magnetic separators based on system requirements
For devices that require support for both NFC and wireless charging, the selection of magnetic separators can be based on the following three dimensions to establish a judgment logic.
Firstly, confirm whether the magnetic permeability of the magnetic separator in the NFC frequency band is within a reasonable range
The design of NFC antennas typically operates at a frequency of 13.56 MHz, and the effective magnetic permeability of the isolator at this frequency needs to match the inductance design of the antenna. Too high can cause disharmony, while too low can result in insufficient shielding effect. When selecting, the supplier should be required to provide the effective magnetic permeability data of the magnetic separator at 13.56 MHz, rather than only providing data in the Qi frequency band.
Secondly, evaluate whether the magnetic permeability distribution of gradient stacked structures can be customized
The priority of NFC reading distance and wireless charging efficiency varies among different devices. The mobile payment scenario requires higher stability in NFC reading distance and lower surface magnetic permeability; The high-power charging scenario requires higher efficiency and a stronger contribution from the high permeability layer. When selecting, it should be confirmed whether the supplier supports customized magnetic permeability distribution for gradient stacked structures, and whether the adjustable range covers the target requirements.
Thirdly, verify the level of cross interference when two functions work simultaneously
In the scenario where NFC communication and wireless charging are carried out simultaneously, the magnetic isolation sheet needs to simultaneously isolate the interference of the wireless charging magnetic field on the NFC antenna, as well as the impact of NFC signals on the charging coil. When selecting, it is necessary to confirm the cross coupling data of the material in two frequency bands, or verify whether the NFC card swiping success rate and charging efficiency meet the standards simultaneously through actual joint debugging testing.
Advanced Institute (Shenzhen) Technology Co., LtdThe high permeability and low loss magnetic isolation sheet adopts a composite structure of nanocrystalline tape and ferrite, supporting multi-layer gradient stacking design, and can adjust the magnetic permeability distribution of each layer according to the priority of NFC and wireless charging.
The effective magnetic permeability of the finished product can be customized from 150-2000, with a saturation magnetic inductance of about 1.2 T, a loss of ≤ 10 W/kg @ 20kHz/0.5T, a thickness range of 0.02-0.20mm, and a temperature resistance of -40 ° C to 150 ° C. The product supports roll, sheet, shaped die-cutting, and multi-layer composite, and can provide collaborative development from simulation, selection to NFC debugging. For more technical information, please visithttp://www.avanzado.cn/.