A mobile phone was placed on a wireless charging dock, and the charging speed was slower than expected, with obvious heating on the back of the body - engineers checked the coil alignment, protocol compatibility, and power adapter, but did not find any problems. The real culprit is often hidden on the back of the coil: a soft magnetic thin sheet with a thickness of less than 0.1 millimeters.

Wireless charging relies on magnetic coupling between the transmitting and receiving coils to transfer energy. The receiving coil is tightly attached to the battery, metal frame, or circuit board - when an alternating magnetic field penetrates the coil and encounters these metal components, eddy currents are generated in the metal. The direction of eddy currents is opposite to the original magnetic field of the antenna, forming a counteracting reverse magnetic field that severely weakens the effective magnetic flux. The result is a sharp drop in charging efficiency, and energy is dissipated in the form of heat in the metal backplate.

2、 The three engineering dimensions of magnetic separators: efficiency, temperature rise, and thickness

In the design of wireless charging receivers, engineers' demands for magnetic separators can be summarized into three engineering dimensions:

Engineering trade-offs:There are engineering constraints between these three dimensions - increasing magnetic permeability may increase losses, reducing losses may sacrifice saturation magnetization, and thinning thickness may reduce magnetic focusing ability. The selection of magnetic separators is essentially about finding the optimal balance between these three dimensions.

3、 Ferrite vs Nanocrystals: A Comparison of Technological Pathways for Two Materials

The mainstream material systems for current wireless charging receiver magnetic separators are ferrite and nanocrystalline alloys, which have significant differences in key performance indicators:

Data source: Actual measurement by the Advanced Institute of Science and Technology Laboratory and industry public information

The ferrite magnetic separator technology is mature and cost-effective, suitable for both transmitting and low-power receiving ends. However, its saturation magnetization is only 0.4-0.5T, making it highly susceptible to magnetic saturation in high-power scenarios. The material is brittle and hard, with a large thickness (usually above 0.2mm), making it difficult to meet the design requirements of ultra-thin mobile phones.

The nanocrystalline magnetic separator is formed by crystallization annealing of amorphous alloy to form an ultrafine grain structure of 10-20 nanometers, which combines the high saturation magnetic inductance (≥ 1.2T) of iron-based amorphous alloy and the high magnetic permeability (20000-80000) and low loss characteristics of cobalt based amorphous alloy. The magnetic permeability of nanocrystalline materials can still maintain over 80% at a frequency of 1MHz, which is suitable for the 15W-50W power requirements of mainstream wireless fast charging protocols such as Qi 1.3. Its flexible characteristics can effectively avoid the risk of magnetic leakage caused by the fragmentation of the magnetic separator.

4、 Thickness design: thicker is not better

The thickness of the magnetic separator is an engineering variable that requires precise balancing. The initial increase in thickness can significantly improve the magnetic flux diversion ability, but the benefits decrease after exceeding a certain value. When the thickness of the nanocrystalline magnetic separator reaches about 0.1mm, it is basically saturated. Continuing to increase the thickness has very limited effect on improving efficiency, and instead increases material costs and stacking height.

Advanced Institute of Technology's NanoShield ™ The nanocrystalline magnetic separator series offers three thickness specifications of 0.08mm, 0.11mm, and 0.15mm, covering different space requirements from ultra-thin mobile phones to car charging. The thickness tolerance is controlled within ± 3 μ m, suitable for ultra-thin stacking design, to avoid assembly stress.

5、 Key Performance Parameters and Selection Points

According to the official website of the Advanced Institute of Technology, the key selection parameters for the magnetic isolation sheet of the wireless charging receiver are as follows:

Data source: Advanced Institute Technology's own laboratory and third-party testing reports

Engineers should pay attention to the following core dimensions when selecting:

• Working frequency matching:The main frequency range for wireless charging is 100kHz-300kHz (Qi standard), and the magnetic separator needs to have high magnetic permeability and low loss in this frequency band. Meanwhile, if the device needs to be compatible with NFC (13.56MHz), the performance of the magnetic separator in this frequency band also needs to be evaluated. Advanced Institute Technology's nanocrystalline magnetic isolation sheet can simultaneously meet the three functions of WPC wireless charging, MST magnetic secure transmission, and NFC near-field communication without interfering with each other.
• High power anti saturation:The higher the charging power, the greater the magnetic field strength that the magnetic separator can withstand. The higher the Bs value, the stronger the anti magnetic saturation ability. The Bs of nanocrystalline materials is ≥ 1.25T, far superior to the 0.4-0.5T of ferrites, and has significant advantages in high-power fast charging scenarios.
• Die cutting accuracy and assembly adaptation:The magnetic separator needs to be molded into a precise shape that matches the coil. Advanced Institute Technology supports high-precision die-cutting with a minimum aperture of 0.8mm and a minimum line spacing of 0.5mm, with a die-cutting accuracy of ± 0.05mm, and provides a variety of adhesive options and customized backing adhesives.

6、 Typical application scenarios

7、 Conclusion

The essence of the wireless charging receiver's magnetic isolation sheet is to use high magnetic permeability materials to build a low magnetic resistance "highway" for magnetic field lines - allowing the magnetic field to propagate along the interior of the isolation sheet, bypass the metal back plate, and concentrate in the coil coupling area, while minimizing energy loss with low loss characteristics.

The evolution of materials from ferrite to nanocrystals reflects the demand upgrade of wireless charging technology from "rechargeable" to "fast charging" and then to "lightweight fast charging". Nanocrystalline materials are becoming the mainstream choice for wireless charging receivers in mobile phones, wearable devices, and cars due to their initial magnetic permeability of 20000-80000, saturation magnetic induction of ≥ 1.25T, thickness as low as 0.08mm, and bendable flexibility. Understanding the triangular game between efficiency, temperature rise, and thickness, and understanding the performance boundaries of different material systems - these are the key to upgrading magnetic separators from "a single magnetic material" to "engineering decisions".