Español
Français
Português
In mobile phones that support magnetic alignment, a circle of permanent magnets is arranged around the receiving coil. The magnet provides alignment force, allowing users to "put it on and suck it in", which is the source of the advantage of the magnetic attraction solution experience. But in electromagnetic design reviews, this magnet is often only discussed as a "structural component" - whether the magnetic force is sufficient, whether it will interfere with the compass, and whether it will affect NFC.
For magnetic separators, permanent magnets are not just structural components. It is a continuous static magnetic field source that is directly superimposed on the magnetic circuit of the magnetic separator. The magnetic separator area near the magnet has already been pre biased before starting wireless charging.
The meaning of pre bias: the working point of the magnetic disk is "raised" by a section
The magnetic permeability of soft magnetic materials is not constant, it varies with the operating point. When there is no external static magnetic field, the working point of the magnetic separator is located near the origin of the hysteresis loop, and the small signal magnetic permeability is at a relatively high level. When a permanent magnet generates a static magnetic field nearby, the operating point of the local area of the magnetic plate is pushed to one side of the hysteresis loop, and the small signal magnetic permeability changes accordingly.
How static bias increases the risk of local saturation
The direction and magnitude of this change depend on the relationship between the direction of the static magnetic field and the direction of the alternating magnetic field. If the direction of the static magnetic field is the same or opposite to that of the alternating magnetic field, the magnetic flux density in a local area will vary within the range of "static bias dynamic oscillation". The larger the static bias, the smaller the margin that can be used for dynamic oscillation. When the dynamic swing superimposed bias touches Bs, the region enters the saturation zone.
In 25W wireless charging, the amplitude of the alternating magnetic field itself is close to the upper edge of the linear region of the magnetic sheet. The static bias brought by permanent magnets is equivalent to pushing the "starting line" of a local area towards the saturation boundary. The magnetic plate area directly below the magnet may saturate earlier than the central area, even if the working point of the central area is still within the linear region.
Three local effects near the magnet: saturation, magnetic permeability modulation, and loss concentration
The influence of permanent magnets on magnetic separators is concentrated in three local effects.
The first is the narrowing of local saturation margin
The magnetic plate area directly below the magnet has a static bias that results in a lower effective saturation margin than the central area. At the target power, if the operating point in the central region is close to 70% -80% of Bs, the local operating points near the magnet may have crossed the boundary of the linear region. After saturation, the magnetic permeability of the region sharply decreases, and the magnetic field lines are "squeezed out" of the region, transferring to the surrounding area and forming local magnetic flux concentration.
The second is magnetic permeability modulation
Even if saturation is not reached, static bias can alter the small signal magnetic permeability of a local region. In the area where magnetic permeability decreases, magnetic resistance increases, and the distribution of magnetic flux in this area is redistributed. This non-uniform magnetic permeability distribution causes the effective magnetic circuit of the magnetic separator in the plane to no longer be symmetrical, and the equivalent inductance of the coil also changes accordingly. The symmetry of magnet layout not only affects mechanical alignment, but also electromagnetic symmetry.
The third is the concentration of losses
The loss density in the saturation region and magnetic permeability modulation region is higher than that in the surrounding area, and heat accumulates locally. The magnet itself is a metal with high thermal conductivity, which can dissipate some of the heat; But the interface between the magnet and the magnetic sheet, as well as the adhesive layer around the magnet, are also thermal resistances. The effective diffusion of heat depends on the thermal design of the magnet sheet interface and the heat dissipation path around the magnet.
Collaboration between magnet layout and magnetic plate design: gap, opening and material selection
The relative position between the magnetic separator and the magnet in the magnetic suction machine has been determined in the structural design stage. The adjustable space of the magnetic plate design is mainly located in three places.
The first is the opening or thinning of the magnetic sheet near the magnet
If the magnetic plate area directly below the magnet has the highest risk of saturation, one approach is to open holes or thin the area to reduce the occupation of "ineffective magnetic flux capacity" and guide the magnetic flux to the gap area between the magnets. The cost is a decrease in the magnetic flux guidance ability of the perforated area, and it is necessary to confirm whether the coil inductance and efficiency still meet the standards after perforation.
The second is the spacer layer between the magnet and the magnetic sheet
Introducing a non-magnetic spacer layer between the magnet and the magnetic sheet can reduce the strength of static bias in the magnetic sheet, thereby retaining more dynamic magnetic flux capacity. The thickness and material of the spacer layer need to be balanced with the magnetic requirements and stacking space - if the spacer layer is too thick, the magnetic force will decrease; Too thin, insufficient pre bias effect.
The third is the selection of material system
High Bs materials (such as nanocrystals, Bs ≥ 1.25 T) retain a larger dynamic margin under the same pre bias, and the risk of local saturation near the magnet is relatively low. Ferrite has lower Bs and is more prone to premature saturation near the magnet. When selecting ferrite solutions in magnetic suction machines, the layout of the magnet and the design of the spacing layer need to be more cautious.
Verification of magnetic isolation plates for magnetic suction models: cannot only focus on center alignment
If the verification of the magnetic separator of the magnetic suction machine is only carried out under ideal conditions of center alignment and no magnetic interference, it will mask the local effects near the magnet. The feasible verification directions include: measuring local temperature rise directly below the magnet and in the gap area between the magnets, and comparing the temperature difference between the two areas; Observe whether there are local hotspots near the magnet using thermal imaging under target power and offset conditions; Compare the differences in coil inductance and efficiency between "with magnet" and "without magnet" conditions, and quantify the magnitude of the influence of static bias. The purpose of these verifications is not to 'prove that the magnet has an impact', but to confirm whether the impact falls within an acceptable range. The alignment accuracy improvement brought by magnetic attraction structure can itself reduce the saturation risk caused by offset; The narrowing of local margin caused by static bias is the cost of this benefit. The task of selection is to ensure that the cost does not exceed the limits that the system can bear.
Selection judgment based on the spatial relationship between magnets and magnetic plates
Firstly, in magnetic suction models, the magnet layout is used as a pre input for the selection of magnetic separators, rather than a later structural detail
The number, position, and magnetization direction of magnets determine the distribution of static bias in the plane of the magnetic plate. The material system, hole design, and thickness of the magnetic separator need to be selected under these constraints, rather than selecting materials first and then "stuffing" the magnet layout.
Secondly, calibrate the saturation margin based on the local working points near the magnet, rather than the average value in the central region
Request suppliers or internal simulations to provide the proportion of magnetic flux density in the area directly below the magnet relative to Bs. If the working point in the area has approached or exceeded 70% -80% of Bs, it is necessary to adjust the magnet magnet spacing, opening design, or material system.
Thirdly, in the selection of material systems, Bs should be given priority as a screening criterion for magnetic attraction models
High Bs materials retain a greater dynamic margin under pre bias and lower saturation risk near the magnet. The ferrite scheme is not unusable in magnetic suction machines, but it requires stricter spacing layers and opening designs to compensate for its lower Bs.
Fourth, take the local temperature rise near the magnet as an independent verification term
The temperature difference between the area directly below the magnet and the gap is a direct reflection of the static bias effect. During the sample stage, use thermal imaging to confirm whether local hotspots appear near the magnet and whether the temperature in that area is close to the upper limit of the magnet's temperature resistance or the temperature resistance boundary of adjacent structures.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of wireless charging magnetic isolation sheets for mobile phones covers ferrite composite sheets and nanocrystalline composite sheets. The nanocrystalline series has a saturated magnetic induction of ≥ 1.25 T and a typical thickness of 0.11 mm under the same efficiency. It supports customized die-cutting for opening, thinning, and spacing layer design according to the magnet layout.
The product can provide saturation margin evaluation data for local working points near the magnet, as well as comparative data on coil inductance and efficiency under "with magnet/without magnet" conditions. For more technical information, please visithttp://www.avanzado.cn/.