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The efficiency data of the wireless charging receiver is usually labeled under the condition of "coil center alignment". The 90% efficiency, 2.5 percentage point advantage, and 7-8 ℃ temperature rise difference on the specification sheet are all based on the premise that the geometric centers of the transmitting and receiving coils coincide.
The conditions under which users actually use it are not like this. Placing the phone on the charging pad with a lateral offset of 3-5 mm is normal; The car mount may experience sustained position drift during vehicle vibration; Although magnetic alignment reduces the offset, not all devices come with magnetic rings. The offset condition is not an "abnormal situation", but a daily condition that the receiving end's magnetic separator must face. Under offset conditions, the selection logic of magnetic separators will change: the scheme with the highest nominal efficiency may not necessarily be the one that is still usable after offset.
How offset changes magnetic flux distribution: saturation on one side, leakage on one side
When the coil is offset, the magnetic flux coupling area between the transmitting coil and the receiving coil is no longer symmetrical. The coil spacing on the offset side decreases, and the magnetic flux density increases; As the distance between the opposite sides increases, the magnetic flux density decreases. The magnetic flux distribution on the magnetic separator changes accordingly: the local magnetic flux density on the offset side may approach or even exceed the saturation magnetization of the material, while the edge magnetic flux on the opposite side is more likely to wrap out of the coverage area of the magnetic separator and enter the metal backplate to excite eddy currents.
This redistribution brings about two simultaneous consequences. The saturation on the offset side leads to a decrease in magnetic permeability in the region, causing magnetic field lines to penetrate the magnetic separator and enter the metal, resulting in local concentration of eddy current heat; The leakage magnetic flux on the opposite side increases, and the unconstrained magnetic flux generates additional eddy current losses in the battery shielding layer or middle frame. When the two are combined, the total loss of the system increases after offset, and the temperature rise is concentrated on one side edge of the magnetic separator, rather than uniformly distributed.
At this point, if the saturation margin of the magnetic separator is designed according to the "ideal alignment" condition, the offset local magnetic flux density may have already crossed the linear region. The nominal efficiency is measured in an unsaturated state, and the actual operating point after offset may have already fallen at the edge of the saturation zone. That's why the plan of "0.5 percentage points higher nominal efficiency" may appear in the hands of users as "slightly biased and disconnected from charging".
The three components of design margin: saturation margin, coverage factor, and edge margin
The availability under offset conditions depends on how much margin the magnetic separator has reserved in three dimensions.
Saturation margin
The most direct item. If the working magnetic flux density at ideal alignment is close to 70% -80% of Bs, and the offset side magnetic flux density increases, the working point can easily enter the nonlinear region. Reserving a larger saturation margin means that the material is not fully utilized during ideal alignment, but in exchange for stability under offset conditions. This is also why in the 25W receiver scenario, even if the alignment accuracy is improved, the thickness and effective area of the magnetic layer still need to be reconfirmed according to the actual working point, rather than using the thickness conclusion of the 15W era.
Coverage coefficient
It determines the amount of magnetic leakage during offset. A coverage rate of ≥ 90% is an engineering boundary based on ideal alignment; Under offset conditions, the actual effective coverage area will "move" towards the offset direction, making it easier for the edge magnetic flux on the opposite side to overflow. If the planar size of the magnetic separator is designed only at the lower edge of Df/Sc=1.08, the coverage on the opposite side after offset may drop below the critical value, and the risk of leakage loss and FOD false triggering may increase simultaneously. This is also why the edge margin of 0.5-1.0 mm is not an "optional" in offset scenes, but a component of tolerance.
Edge margin
The effect is amplified during offset. In ideal alignment, the edge margin only serves as a conduit for the "overflow magnetic flux"; When offset occurs, the side with smaller margin will lose its magnetic flux constraint ability first, while the side with larger margin will still be 'surplus'. The size design of the magnetic separator needs to leave enough margin in both directions to cover the offset, rather than only meeting the coverage requirements in an average sense.
The selection of material systems is reordered under offset conditions
Under ideal alignment conditions, the ranking of material comparisons is relatively clear: high Bs materials have a greater saturation margin within the thin layer, while low loss materials have a lower temperature rise. The offset condition disrupts this sorting.
The performance of ferrite under offset conditions
Bs is relatively low and may fall within the available window when ideally aligned; After the magnetic flux density on the offset side increases, the saturation risk is first exposed on the offset side. The hard and brittle nature of ferrite also means that under repeated displacement and assembly stress, microcracks may occur in the edge region, further weakening the magnetic flux guidance ability on that side.
Performance of nanocrystals under offset conditions
Bs is higher, and the saturation margin on the offset side is greater; The fragmented magnetic structure enables the magnetic separator to maintain the continuity of the magnetic flux path through the redistribution of fragment gaps even when offset. However, the effective μ 'of nanocrystals depends on the demagnetization process, and the increase in local magnetic flux density under offset conditions may cause certain fragment regions to enter nonlinearity. The design margin of demagnetization parameters is equally important.
In other words, the deviation condition does not change the general conclusion of "which material is better", but it changes the judgment basis of "under what conditions is better". In scenarios with frequent offsets and uncontrollable user placement habits, the weights of saturation margin and coverage margin increase, while the weight of nominal efficiency decreases.
Selection judgment based on "offset ± 5 mm"
Firstly, treat the offset condition as an independent verification condition, rather than an inference from ideal alignment data
Require suppliers or internal testing to provide efficiency, temperature rise, and FOD trigger rate data for coil lateral offset of ± 3 mm and ± 5 mm, rather than just providing nominal values for center alignment conditions. The difference between offset data and alignment data is itself a measure of the design margin of the magnetic separator.
Secondly, confirm the saturation margin under offset conditions, rather than just looking at the ideal working point for alignment
Re calculate the proportion of the working magnetic flux density on the offset side relative to Bs based on the target power and maximum offset. If the ideal alignment is already close to 70% -80% of Bs, the offset side is likely to cross the linear region. At this time, it is advisable to increase the thickness of the magnetic layer or choose materials with higher Bs, rather than searching for "higher μ '" numbers on the specification sheet.
Thirdly, calculate the coverage coefficient and edge margin based on the "offset" effective area
For boundaries with a coverage rate of ≥ 90% and an edge margin of 0.5-1.0 mm, the effective coverage on the opposite side should be reconfirmed with an offset of ± 5 mm. If the size of the magnetic separator is only designed at the lower edge of Df/Sc=1.08, the coverage on the opposite side after offset may be lower than the critical value, and the risk of leakage loss and FOD false triggering may increase accordingly.
Fourth, distinguish between "efficiency decline" and "usability decline" at the user experience level
The efficiency decrease caused by offset may be perceived by users as weak by a few percentage points; However, FOD false triggering, intermittent charging interruption, and local overheating caused by offset are perceived by users as "not user-friendly". The offset tolerance design of the magnetic separator aims not to make the efficiency during offset completely consistent with that during alignment, but to keep the system within the range of "stable charging" during offset.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging receiver magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets. The nanocrystalline series has a saturation magnetic induction of ≥ 1.25 T and a typical thickness of 0.11 mm under the same efficiency. It supports precision die-cutting according to the rules of Df/Sc=1.08-1.15 and coverage coefficient ≥ 90%, and can provide efficiency and temperature rise evaluation data under the conditions of coil lateral offset of ± 3 mm and ± 5 mm.
The product supports thickness specifications of 0.08/0.11/0.15 mm and a die-cutting accuracy of ± 0.05 mm, suitable for mobile phone and car receiver scenarios with requirements for offset tolerance. For more technical information, please visithttps://www.avanzado.cn/products/gold-plated-conductive-cloth.html.