After the improvement of Qi2 alignment accuracy: Why has the competition focus of wireless charging magnetic separators shifted to "intrinsic loss"

The selection logic of wireless charging magnetic separators has been deeply influenced by a variable in the past few years: coil alignment deviation.

In the early days of the Qi standard, it was common for users to have the coil laterally offset by a few millimeters when placing their phone on a charging pad. The design of the magnetic separator requires sufficient reserved magnetic flux coverage area to ensure that the magnetic field lines are still constrained within the coil coupling region under maximum offset conditions. This leads to the size and shape of the magnetic separator often being larger than the ideal alignment state, and the distribution of magnetic flux density at the working point is also more uneven - when offset, the magnetic flux density on one side increases and the other side decreases, and the saturation risk and local temperature rise are amplified in the offset direction.

The changes introduced by Qi2 v2.1 in 2025 are changing this premise. The Wireless Charging Alliance (WPC) adopted active alignment technology based on mobile coils in version 2.1: the coil on the charger side can automatically detect the position of the coil on the device side and move for alignment, achieving positioning accuracy comparable to magnetic suction without a magnetic suction ring on the device. At the same time, Qi2.2 has increased its maximum power from 15W to 25W and strengthened its thermal management requirements.

These two changes combined have a structural impact on the design constraints of the magnetic separator.

After the improvement of alignment accuracy, offset tolerance is no longer the primary design contradiction of magnetic separators

In designs that require tolerance for significant offset, the magnetic flux capacity of the magnetic separator must be checked according to the "worst-case offset condition". This means that the effective area and thickness of the magnetic layer are both increased by the offset scenario, and a portion of the material capability is used to cover alignment uncertainty rather than to improve efficiency under ideal alignment conditions.

When actively aligning and controlling the coil offset within a smaller range, the distribution of operating points of the magnetic separator becomes more uniform, and the peak magnetic flux density under worst-case conditions decreases. The direct consequence is that the pressure of saturation margin is reduced, and the magnetic separator can operate safely at thinner magnetic layer thicknesses without having to reserve excessive magnetic flux capacity for offset.

The weight of intrinsic loss increases, and the material gap is amplified

After the uncertainty of alignment is absorbed by system level means, the performance differences between magnetic separators are more exposed in the "intrinsic losses". At the same alignment accuracy and power level, the differences in efficiency, temperature rise, and thickness between magnetic separators of different material systems are no longer masked by offset tolerances.

In a typical 15W wireless charging module, the eddy current loss of the metal backplate can reduce charging efficiency by 18% -25% without using a magnetic separator; 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 ℃. The meaning of this set of data is that the primary value of magnetic separators is always to "block eddy currents" rather than "compensate for offsets". After the improvement of alignment accuracy, the proportion of this primary value in system efficiency becomes more prominent.

The gap between ferrite and nanocrystals is more easily perceived in this context. At the same efficiency, the typical thickness of nanocrystalline magnetic separators is 0.11 mm, while ferrite requires 0.27 mm. The charging efficiency of the nanocrystalline scheme is about 2.5 percentage points higher than that of ferrite, and the temperature rise is 7-8 ℃ lower. In the stage where offset tolerance dominates design, engineers can balance it with "thicker ferrite but lower cost"; As the alignment accuracy improves and the weight of thickness and temperature rise increases, the thickness and brittleness constraints of ferrite become more difficult to bypass in ultra-thin devices.

The verification method of saturation margin needs to be adjusted synchronously

The improvement in alignment accuracy changes the offset distribution, not the absolute value of the working magnetic flux density. Qi2.2 increases the power limit to 25W, which means that under the same coil area and coupling coefficient, the magnetic flux density borne by the magnetic separator is higher than that of the 15W era.

Therefore, the saturation margin verification of magnetic separators cannot simply derive "margin automatically improves" from "offset tolerance reduction". The correct way to make a judgment is to re measure the magnetic flux density distribution at the working point of the magnetic separator under the target power level and actual alignment accuracy, and confirm whether its peak still falls within the linear region of the material. For the 25W scenario, even with improved alignment accuracy, nanocrystalline magnetic separators may still need to maintain saturation margin by moderately increasing the thickness of the magnetic layer or using multi-layer stacking, while the upper limit of Bs for ferrites (0.4-0.5 T) faces more direct saturation pressure at this power level.

The alignment technology route itself also affects the shape design of the magnetic separator

The mobile coil scheme of Qi2 v2.1 is mainly aimed at the charger side, and the device side can still be designed without a magnetic suction ring. This means that the shape and opening design of the receiving end magnetic separator no longer need to be arranged around the geometric constraints of the magnetic attraction ring, and the effective magnetic area of the magnetic separator can more fully utilize the space behind the coil. For devices that pursue extreme thickness, this degree of freedom can be translated into an increase in magnetic layer area or a decrease in number of layers, without having to make room for the magnetic structure.

But another route - magnetic alignment - still exists in a large number of devices. For the receiving end that supports both magnetic attraction and Qi2, the magnetic separator needs to maintain sufficient magnetic flux guidance ability around the magnet, as the presence of the magnet itself will locally alter the magnetic field distribution. When selecting, it is necessary to confirm the local saturation behavior of the magnetic separator near the magnet, rather than just looking at the average magnetic permeability in the absence of a magnet.

A new judgment on the selection of magnetic separators based on alignment accuracy

The changes in alignment technology do not alter the basic physical function of the magnetic separator, but which parameters have been altered should be given priority attention.