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The selection discussion of wireless charging magnetic separators often starts from the material: how high the magnetic permeability is, how low the loss is, and how much saturation magnetic inductance is. This perspective generally holds true at the transmitting end, as there is relatively ample axial space at the transmitting end, and the thickness of the magnetic separator can be adjusted upwards according to the power level, providing ample room for optimizing material parameters.
The design logic of the receiving end is different. The receiving module of flagship phones is sandwiched between the coil and the battery/frame, with a total thickness often limited to within 0.3 mm, and the budget for the magnetic separator itself is usually only 0.1-0.15 mm. At the same time, Qi2.2 requires the device surface temperature to not exceed 40 ° C at 25W output. The thickness should be increased while the temperature should be decreased, and the efficiency must find a position between these two boundaries. Magnetic barriers are not about "designing materials", but about "managing a narrow corridor".
The triple role of the receiving end: flux guidance, eddy current suppression, and a part of the thermal path
At the transmitting end, the main task of the magnetic separator is to guide the magnetic flux and shield the metal behind it. The conditions faced by the magnetic isolation plate at the receiving end are more complex: it is closely attached to the back of the coil, with metal structures such as batteries, middle frames, or camera modules behind it. It is not only the component of the magnetic flux path, but also the location of eddy current losses. At the same time, it is also a link in the thermal conduction path from the coil to the metal shell.
Three constraints are simultaneously established
Magnetic flux guidance requires sufficient effective magnetic permeability and magnetic flux capacity; Eddy current suppression requires low loss and fragmented/insulated structures; The thermal pathway requires the material's own thermal diffusion ability, as well as controllable interface thermal resistance between the backing adhesive and the heat equalizing layer. Within a thickness of 0.1 mm, none of these three constraints can be independently satisfied and must hold simultaneously.
That's why 'magnetic permeability standard' is not enough to predict the actual performance of the receiving end. If a material with a high μ 'loss is too high, eddy current heat will accumulate in the thin layer, and the surface temperature will first exceed 40 ° C. The system will be forced to land, and the efficiency target will be missed; A material with extremely low loss, if the magnetic flux capacity is insufficient, magnetic field lines will penetrate into the metal backplate after saturation, and eddy current losses will occur outside the magnetic separator, also manifested as uncontrolled temperature rise. If any one of the three constraints is violated, the optimization of the other two will be offset by system level consequences.
Geometric Matching: Why Df/Dr and Coverage are the 'First Sieve'
Before determining the material parameters, the receiving end of the magnetic separator first needs to solve a geometric problem: whether the relative size between it and the coil matches. When there is a geometric mismatch, no matter how optimized the material parameters are, they cannot compensate for the losses caused by magnetic leakage and detuning.
The priority range given by Advanced Institute Technology based on coil matching experience is: circular magnetic separator/coil outer diameter ratio Df/Sc=1.08-1.15, coverage coefficient ≥ 90%, edge margin 0.5-1.0 mm. When Df/Sc is below 1.05, the edge of the magnetic separator fails to fully "take over" the overflow magnetic flux at the edge of the coil, resulting in increased magnetic leakage and a possible efficiency decrease of 0.3-0.8 percentage points; After Df/Sc exceeds 1.20, the additional inductance and area cost introduced by the magnetic separator in the near-field begin to increase, and the benefits decrease. When the coverage rate is below 90%, the uncovered antenna projection area still allows magnetic lines to enter the metal backplate and excite eddy currents, weakening the suppression effect of the magnetic barrier on temperature rise.
The reason why this set of geometric boundaries is the "first sieve" is that it determines whether the subsequent material selection is based on an effective foundation. Comparing the temperature rise differences between nanocrystals and ferrites on schemes with insufficient coverage or Df/Sc deviation from the window, the results of both being contaminated by magnetic leakage and detuning have been compared, and the difference between the data has lost its significance at the material level.
Thickness and Material: 0.11 mm vs. 0.27 mm is not about "who is thinner", but about "whether three lines can be held simultaneously within a thin layer"
After geometric matching is determined, thickness becomes the most rigid constraint at the receiving end. The typical thickness window of the magnetic separator on the mobile phone receiver is 0.1-0.3 mm, and models that pursue thin and light designs further compress it to around 0.1 mm.
Within this thickness range, the differences in material systems are amplified. The saturation magnetization of ferrite is usually between 0.4-0.5 T, and that of nanocrystals can reach 1.25 T. 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. The engineering significance of this set of data is not "nanocrystals are better overall", but rather: in the corridor where the receiving end is compressed to the order of 0.1 mm, the residual magnetic flux capacity of the ferrite in the thickness direction is first exhausted, and the cost of further thinning is first exposed in the form of saturation and temperature rise. But this does not mean that nanocrystals are the default answer at the receiving end. The cost of ferrite is about half that of nanocrystals, and in the 5W level wearable or cost sensitive low to medium power receiving end, 0.1-0.2 mm ferrite flakes can still fall within the available window. The boundary of material selection is not about "who is more advanced", but whether the combination of target power and thickness budget falls within the saturation margin and loss boundary of the material system.
Thermal pathway: Magnetic separators cannot solve temperature rise alone, but their choice determines how thick "patches" are needed for thermal management
The control of temperature rise at the receiving end has never been a task that can be independently completed by a single component of the magnetic separator. The 40 ° C surface temperature constraint of Qi2.2 requires the joint satisfaction of magnetic loss, coil copper loss, IC loss, and structural heat dissipation path.
The contribution of the magnetic separator is twofold: its own loss level determines the strength of the heat source; Its thickness and thermal diffusion ability determine the length of the path through which heat is conducted from the coil side to the metal shell. In a thin layer on the order of 0.1 mm, the heat capacity and lateral diffusion ability of the magnetic separator are limited, and relying solely on the intrinsic thermal conductivity of the material is not sufficient to spread out the heat. A common cooperation scheme in engineering is to introduce thermal insulation or soaking layer between the magnetic separator and the battery/shell: aerogel film (thermal conductivity 0.018 – 0.025 W/(m · K)) blocks the heat conduction to the battery and shell, and graphite film or soaking layer is used to spread local hot spots.
Here is a causal relationship that is easy to be ignored: the role of aerogel insulation film is to "block the residual heat", rather than "reduce vortex heating". If the eddy current loss of the magnetic separator itself is not suppressed and the heat source strength is too high, the insulation layer can only delay the time for heat to reach the shell and cannot reduce the total heat generation of the system. The relationship between the selection of magnetic separators and thermal management design is not a "binary choice", but rather "the magnetic separator first presses the heat source to a manageable level, and the thermal management layer then processes the remaining conduction paths". In the narrow corridor of the receiving end, the order of these two steps is reversed, often resulting in the result of "adding insulation film but still exceeding the surface temperature limit".
Selection and judgment of receiving end based on the three-dimensional boundary of "space magnetic flux heat"
Firstly, use geometric matching to screen out invalid solutions, and then proceed to material comparison
Confirm the outer diameter and available area of the coil, and determine the plane size of the magnetic separator according to Df/Sc=1.08-1.15, coverage factor ≥ 90%, and edge margin 0.5-1.0 mm. If the geometric window itself cannot meet the requirements, the optimization of material parameters cannot compensate for the system level losses caused by leakage and detuning. After determining the geometry, compare the materials in the schemes that meet the dimensions.
Secondly, cross confirm the saturation margin based on the target power and thickness budget, rather than confirming it based on "room temperature μ '"
The thickness window of the receiving end is rigid. Within the budget of 0.1-0.15 mm, suppliers are required to provide the proportion of operating magnetic flux density relative to Bs at the target power, rather than just providing static Bs or initial μ '. If the working point is close to 70% -80% of Bs, the saturation margin at this thickness is not sufficient to support the target power, and the material system needs to be adjusted or a thicker solution needs to be accepted, rather than looking for a "higher μ '" number on the specification sheet.
Thirdly, incorporate the thermal role of the magnetic separator into the system's thermal resistance calculation, rather than treating it as an independent "low loss device"
Confirm the total loss of the magnetic separator under the target operating conditions (rather than just one of hysteresis loss or eddy current loss), as well as the contribution of the backing and interface materials to the thermal resistance. If aerogel thermal insulation film or graphite soaking layer is required behind the magnetic separator, confirm whether the thickness of these layers has been included in the total thickness budget of the receiving module, to avoid "the space saved by thinning the magnetic separator is re occupied by the thermal management layer".
Fourthly, when comparing materials of equal thickness, put the "temperature rise difference" and "efficiency difference" in the same table for reference
The efficiency of nanocrystals at 0.11 mm is 2.5 percentage points higher than that of ferrite at 0.27 mm, and the temperature rise is 7-8 ℃ lower. This set of data was measured under the premise of "equivalent efficiency". If the project chooses ferrite for cost, it is necessary to confirm whether the efficiency and temperature rise still fall within the acceptable range of the system at the target thickness (rather than the "comfortable thickness" of ferrite). The cost of material replacement is ultimately determined by both temperature rise and efficiency, rather than a single parameter.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging receiver's magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets. The nanocrystalline series has a typical thickness of 0.11 mm and a saturation magnetic induction of ≥ 1.25 T under the same efficiency, which is 7-8 ℃ lower than the ferrite scheme in temperature rise and about 2.5 percentage points higher in efficiency. It supports thickness specifications such as 0.08/0.11/0.15 mm and ± 0.05 mm die-cutting accuracy.
The product can be precision die cut according to the coil matching rule with Df/Sc=1.08-1.15 and coverage coefficient ≥ 90%, and can be synergistically evaluated with the stacking design of thermal conductive adhesive and thermal management layer. For more technical information, please visithttps://www.avanzado.cn/products/gold-plated-conductive-cloth.html.