There are many magnetic devices used in switching power supplies. The commonly used soft magnetic devices include the main transformer (high-frequency power transformer), common-mode choke coil, high-frequency magnetic amplifier, filter choke coil, and spike that are the core components of the switching power supply. Signal suppressor and so on. Different devices have different performance requirements for materials, as shown in the table for the performance requirements of various devices on magnetic materials.
(A), high-frequency power transformer transformer core size depends on the output power and temperature rise. The design formula of the transformer is as follows:
P=KfNBSI×10-6T=hcPc+hWPW
Among them, P is the electric power; K is the coefficient related to the waveform; f is the frequency; N is the number of turns; S is the core area; B is the working magnetic sense; I is the current; T is the temperature rise; Pc is the iron loss; PW Copper loss; hc and hW are experimentally determined coefficients.
From the above formula, it can be seen that a high working magnetic induction B can obtain a large output power or reduce the volume weight. However, the increase in B value is limited by the Bs value of the material. While the frequency f can be increased by several orders of magnitude, it is possible to significantly reduce the volumetric weight. The low core loss can reduce the temperature rise, which in turn affects the choice of frequency of use and magnetic sense of work. In general, the main requirements of the switching power supply are: high frequency loss as low as possible, high saturation magnetic susceptibility, high magnetic permeability, high enough Curie temperature, and good temperature stability. Some applications require The high squareness ratio is insensitive to stress, stability, and low price. Single-ended transformers Because the iron core works in the first quadrant of the hysteresis loop, the magnetic properties of the material are different from those of the main transformer. It is actually a single-ended pulse transformer and thus requires a large B = Bm-Br, i.e., a large difference between the magnetic induction Bm and the residual magnetic Br; and at the same time, a high pulse permeability is required. Especially for the single-ended flyback switch main transformer, or energy storage transformer, to consider the energy storage requirements.
The amount of energy stored in the coil depends on two factors: one is the working magnetic induction Bm or inductance L of the material, the other is the working magnetic field Hm or operating current I, and the stored energy W = 1/2LI2. This requires the material to have a sufficiently high Bs value and a suitable magnetic permeability, often a wide constant magnetic material. For transformers operating between ±Bm, the area of ​​the hysteresis loop is required, especially the loop area at high frequencies should be small. At the same time, in order to reduce the no-load loss and reduce the excitation current, there should be high magnetic field. Conductivity, the most suitable for the closed-loop toroidal core, its hysteresis loop shown in the figure, this core is used for devices in double-end or full-bridge working state.
In general, it is not easy for metal-crystalline materials to reduce iron loss at high frequencies. For amorphous alloys, they do not have magnetocrystalline anisotropy, metal inclusions, and grain boundaries, and they do not have a long range. The ordered atomic arrangement has a resistivity 2-3 times higher than that of a normal crystalline alloy, and a rapid cooling method forms amorphous thin ribbons with a thickness of 15-30 micrometers at a time, and is particularly suitable for high-frequency power output transformers. Has been widely used in inverter arc welding power supply, single-ended pulse transformer, high frequency heating power supply, uninterruptible power supply, power transformer, communication power supply, switching power supply transformer and high-energy accelerator core, in the frequency of 20-50kHz, power below 50kW, The transformer is the best core material.
In recent years, a new type of single-ended pulse transformer for inverter arc welding power supply has the characteristics of high frequency and high power, so transformer core materials are required to have low high-frequency losses, high saturation magnetic susceptibility Bs, and low Br to obtain large The working magnetic induction B reduces the welder's volume and weight. The commonly used core material for high frequency arc welding power source is ferrite, although it has low high frequency loss due to its high resistivity, but its temperature stability is poor, the working magnetic induction is low, and the transformer volume and weight Larger, can no longer meet the requirements of the new arc welding machine. After using the nanocrystalline toroidal core, because of its high Bs value (Bs>1.2T), high ΔB value (ΔB>0.7T), high pulse permeability and low loss, the frequency can reach 100kHz. The size and weight of the iron core can be greatly reduced. In recent years, tens of thousands of nanocrystalline cores have been used in inverter welding machines. Users report that they are made of nanocrystalline transformer cores and are equipped with amorphous high-frequency inductors. This is not only small, light and portable, but also Arc stability, small spatter, good dynamic characteristics, high efficiency and high reliability. This annular nanocrystalline core can also be used in high-frequency heating power supply, pulse transformers, uninterruptible power supplies, power transformers, switching power transformers, and high-energy accelerators. The core material can be selected according to the frequency of the switching power supply.
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