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The temperature rise data of the receiving end magnetic disk usually appears as a single number in the specification sheet or test report: temperature rise is 7-8 ℃ lower, or surface temperature does not exceed 40 ° C. This number comes from a measuring point of the thermocouple or thermal imaging, reflecting the average state or highest point of the magnetic disk.
But the difference between the "highest point" and the "average value" is itself an overlooked risk information. The magnetic flux density within the plane of the magnetic sheet is not uniform, and the loss density is also uneven. The average value meets the standard, but it does not mean that there is no local overheating; The fact that the highest surface temperature has not exceeded the threshold does not mean that there are no thermal spots inside the magnetic sheet.
Where do hot spots come from: Uneven magnetic flux is the first entry point
The distribution of magnetic flux density in the plane of a magnetic sheet is influenced by the shape of the coil, the offset state, and the structure of the magnetic sheet itself. When the geometric shapes of the transmitting coil and the receiving coil do not match, the projection of magnetic field lines on the magnetic sheet is no longer uniform; The lateral displacement of the user when placing the phone significantly increases the magnetic flux density on the offset side; The gaps between fragments and interlayer interfaces of fragmented magnetic separators will introduce magnetic resistance changes locally, causing magnetic flux to concentrate near the edges or interfaces of the fragments.
Local heating in magnetic flux concentration areas
In areas with higher magnetic flux density, the bulk density of hysteresis loss and eddy current loss is also higher. If these heat cannot diffuse laterally in a timely manner, they will accumulate locally and form hot spots with temperatures higher than the surrounding area. The patent literature has a clear description of this: uneven flux density distribution leads to higher total losses and causes local heating in areas where magnetic flux is concentrated.
Why are hot spots dangerous: positive feedback of loss increasing with temperature
The danger of hot spots is not that the temperature at a certain point is slightly higher, but that it may trigger a positive feedback loop.
The loss of magnetic materials is not a constant independent of temperature. In certain operating ranges, hysteresis losses and eddy current losses increase with temperature rise. When a hot spot forms in a local area, the temperature in that area increases, the loss further increases, more heat is generated, and the temperature continues to rise. If the heat dissipation path is not sufficient to take away the heat, this cycle will accelerate on its own.
The warning given in patent literature is that in some cases, temperatures can reach 100 ° C or more and may degrade the characteristics of magnetic materials and the performance of wireless power transfer; In more severe cases, magnetic materials may be damaged, or surrounding electronic components, packaging, and casings may be subjected to additional thermal damage. This means that the risk of hot spots cannot be evaluated solely based on "average temperature rise". A magnetic disk with an average temperature rise of only 20 ℃ may have approached or exceeded 100 ℃ in a certain local area. The average data conceals the starting point of local failure.
Adhesive layer thermal resistance: why hot spots cannot dissipate
The formation of hot spots is partly due to the concentration of local heat sources, and partly due to the obstruction of transverse heat dissipation paths.
The receiving end magnetic sheet is usually composed of multiple layers of magnetic layers stacked through adhesive film. The adhesive film is a polymer material with a much lower thermal conductivity than the metal magnetic layer. Each layer of adhesive film in a multi-layer structure is a series thermal resistance on the thermal conduction path. The patent literature points out that the thermal conductivity of double-sided tape is poor, and the adhesive layer is distributed between the nanocrystals, which greatly increases the thermal resistance of the entire magnetic isolation sheet, reduces the heat dissipation capacity of the system, and leads to excessive temperature rise in the wireless charging system. The impact of this structural problem on the heat spot is that the conduction of heat in the thickness direction is hindered by the adhesive layer, and the diffusion in the plane direction is limited by the thermal conductivity of the magnetic layer itself. If the thermal conductivity in the plane direction of the magnetic sheet is not sufficient to spread out the heat in the hot spot area, the heat will accumulate locally. For magnetic sheets with anisotropic thermal conductivity, the ratio of thermal conductivity in the plane direction to thickness direction is in the range of 5-15:1, which means that the plane diffusion ability is relatively strong, but the heat dissipation in the thickness direction still depends on the adhesive layer and interface. If the thermal resistance of the adhesive layer is too high, even if a portion of the heat is spread out by planar diffusion, it is still difficult for the heat to be effectively conducted to the metal shell below.
The "seam hot spot" of ferrite and the "interface hot spot" of nanocrystals
The formation location of thermal spots varies among magnetic sheets of different material systems.
Ferrite: Local overheating at joints
Ferrite magnets are usually assembled from multiple tiles at the receiving end. The seams and air gaps between tiles are areas of sudden magnetic resistance changes. The magnetic flux is redistributed near the seam, which may result in local magnetic flux concentration and loss concentration at the edge of the seam. IEEE literature clearly states that local overheating at the connection of ferrite blocks often occurs, posing potential risks to system reliability; One approach is to fill the air gap with magnetic powder to form a "magnetic flux bridge" to alleviate heat concentration at the joint.
Nanocrystals: interlayer interfaces and fragment boundaries
The thermal spot risk of nanocrystalline magnetic sheets is more concentrated at interlayer interfaces and fragment boundaries. The fragmented gaps after magnetic fragmentation cut off the eddy current path, but also introduced magnetic resistance non-uniformity. In multi-layer composite structures, the interface between the adhesive layer and the magnetic layer is both a thermal resistance and an impedance transition point on the magnetic flux path. If the size distribution of fragments is uneven or the interlayer alignment deviation is large, the concentration of magnetic flux in local areas will increase, and the risk of hot spots will also increase.
Selection judgment from "average temperature rise" to "hot spot risk"
Firstly, suppliers are required to provide thermal imaging or local temperature distribution data, rather than just providing average temperature rise or single point maximum temperature
Meeting the average temperature rise standard does not mean there are no hot spots. Thermal imaging can display the temperature distribution on the surface of magnetic sheets, helping to identify whether local hotspots appear near seams, edges, or interlayer interfaces. If the supplier can only provide a temperature rise number, the basis for judging the risk of hot spots is insufficient.
Secondly, confirm the position and amplitude of the hot spot under offset conditions, rather than measuring the average temperature rise only under center alignment conditions
Offset is the main source of magnetic flux non-uniformity. When selecting, it should be confirmed whether the hot spot appears at the edge or seam area of the magnetic sheet under the maximum offset condition, and whether the temperature in this area is close to the sharp increase range of material loss or the temperature resistant boundary of adjacent structures.
Thirdly, inquire about the thermal resistance and planar thermal conductivity data of the multi-layer structure adhesive layer
The severity of hot spots depends on whether heat can diffuse out from local heat sources. The thermal resistance of the adhesive layer, the planar thermal conductivity of the magnetic layer, and the anisotropy ratio of thermal conductivity jointly determine the "heat dissipation radius" of the thermal spot. When selecting, it should be confirmed whether the supplier can provide these thermal parameters, rather than just providing magnetic performance and loss data.
Fourthly, in the selection of material systems, separate the assessment of "hot spot risk" from "average temperature rise"
The thermal spots at the joints of ferrites and at the interfaces of nanocrystals have different causes and mitigation methods. A plan with a lower average temperature rise may not necessarily have a lower risk of hot spots; If the average temperature rise is slightly higher, and the thermal diffusion path is more uniform, the peak temperature of the hot spot may actually be lower. The criterion for selection should be whether the peak temperature of the hot spot falls within the safe range under the target operating conditions, rather than simply the average temperature rise.
Advanced Institute (Shenzhen) Technology Co., LtdThe receiving end magnetic sheet product line covers ferrite composite sheets and nanocrystalline composite sheets. The saturated magnetic induction of the nanocrystalline series is ≥ 1.25 T. At the same efficiency, the typical thickness is 0.11 mm, which is 7-8 ℃ lower than the temperature rise of the ferrite scheme.
The product can provide multi-layer structure adhesive layer thermal resistance and planar thermal conductivity data, and support thermal imaging or local temperature distribution evaluation under offset conditions. For more technical information, please visithttp://www.avanzado.cn/.