Ferrite sheets and absorbing materials are both magnetic media with magnetic absorption and dielectric with electrical absorption, and are a class of absorbing materials with excellent performance.
In the low frequency band, the loss of ferrite sheet to electromagnetic waves is mainly caused by the loss of hysteresis effect, eddy current effect and magnetic after-effect; in the high frequency band, the loss of ferrite sheet to electromagnetic wave mainly comes from natural resonance loss, domain wall resonance loss and dielectric loss.
Dielectric loss is the main cause of electrical loss in microwave ferrite sheets. Charges cannot pass through dielectrics in an electric field like conductors, but under the action of the electric field, the charge particles will be displaced from each other, causing the positive and negative charge centers to separate, forming many electric dipoles. In the process of polarization, part of the charge that is lost in the form of heat produces electrical losses.
It is generally believed that the polarization of polycrystalline electromagnetic media mainly comes from four mechanisms: electron polarization, ion polarization, intrinsic electric dipole orientation polarization and interface polarization.
The existence of lattice vacancies, dielectric inhomogeneity and high electrical conductivity such as galvanic polarization are the main reasons for the dielectric loss caused by the intrinsic galvanic orientation polarization; The main reason for the dielectric loss caused by the interface polarization is the zero-phase dispersion distribution of high conductivity. The dielectric loss of ferrite sheet is basically due to the existence of two valence states of iron and the excess of electrons, and electrons will run from one iron ion to another. Some conduction and dielectric losses are incurred during this process.
Magnetic loss is the energy loss generated by magnetic materials in the alternating magnetic field, mainly caused by hysteresis loss, eddy current loss and residual loss. Hysteresis loss refers to the loss of part of the energy supplied by the external magnetic field in the process of irreversible jump dynamic magnetization, overcoming various damping effects.
The area of the hysteresis loop is numerically equal to the value of the hysteresis loss per magnetization cycle, that is, the method to reduce the hysteresis loss is to reduce the coercive force of the ferromagnetic material, and the reduction of the coercivity makes the hysteresis loop narrower , its so-called area is reduced, thereby reducing hysteresis losses. When the conductor is placed in a changing magnetic field, an induced current, that is, an eddy current, will be generated inside the conductor. The eddy current cannot be transported out like the current in the wire, but causes the magnetic core to heat up and cause energy loss, that is, eddy current loss.
In addition, frequency has little effect on ferrite sheet eddy current losses. Residual losses refer to all losses other than eddy current losses and hysteresis losses, resulting from the magnetization relaxation process. Different materials have different mechanisms of residual loss in different frequency ranges due to the different mechanisms of their magnetization relaxation processes.
In the low frequency weak field, the residual loss is mainly the magnetic after-effect loss. In the case of high frequency, size resonance loss, domain wall resonance loss and natural resonance loss belong to the category of residual loss.
In summary, to obtain high-loss ferrite sheet absorbent, the ways are: increase the saturation magnetization of the ferromagnet; increase the impedance coefficient; reduce the magnetocrystalline anisotropy field; Since the resonance frequency is proportional to the magnetocrystalline anisotropy field, it is possible to control the absorption band of the material by changing the magnetocrystalline anisotropy field of the ferromagnet. In the actual preparation process, the composition and preparation process of the material can be changed by changing the be controlled.
Ferrite sheets play an important role in today’s electronic digital products! It solves the problems of magnetic isolation and anti-interference such as RFID, NFC, wireless charging, and notebook computers.
Application of magnetic isolation ferrite sheet in wireless charging
At present, most wireless charging products on the market are based on the QI standard, and the principle of the QI standard is that electromagnetic induction generates current, so in all wireless charging products, an indispensable accessory is the magnetic separator. The processing and assembly of the magnetic isolation sheet is very important. It is directly related to the performance and user experience of the entire wireless charging product, so both manufacturers and consumers should pay great attention to this.
In general, the role of the magnetic isolation ferrite sheet is no more than three points: magnetic conduction, magnetic blocking, and heat conduction.
As we all know, the principle of QI wireless charging standard is electromagnetic induction. When the primary coil (wireless charging transmitter) works, it will generate an interactive magnetic field (the direction of strength and weakness is constantly changing). In order to make the magnetic field energy emitted by the primary coil act on the secondary coil (wireless charging receiver) as much as possible, it is necessary to the magnetism of the coil conducts guidance.
The magnetic isolation ferrite sheet should not only be able to effectively conduct magnetism, but also play a role in blocking magnetism. Why block magnets? We know that when a changing magnetic field encounters a conductor such as a metal, a current will be generated if the metal is a closed wire, and if the metal is a non-closed wire, especially a solid piece of metal, an eddy current effect will occur.
However, in wireless charging products, we should try to avoid the heating of the device as much as possible, because the heating not only consumes too much power, but also causes damage to the electronic equipment. In order to avoid heat generation, it is necessary to constrain the magnetic field generated at the transmitting end so that the magnetic field can only act on the coil at the receiving end, and cannot act on other devices outside the coil.
The magnetic field acts on the coil to generate high-frequency current. During this process, the coil itself also generates heat. If this heat is not effectively dissipated, it will accumulate. Sometimes we feel that the heat is very hot when wireless charging is carried out. Generally, it is caused by the heating of the coil or the heating of the circuit board.
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