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Features and advantages of planar transformer _ planar transformer structure diagram
1. Introduction to planar transformer
The successful development of planar magnetic core can realize planar transformer design. Since the planar transformer requires the magnetic core and windings to be planar structure, multi-layer PCB windings should be adopted.Now, many companies have developed planar transformer, Pulse, the company developed a planar magnetic components, Israel Payton Payton company has developed a Planetics planar transformer, power from 5 w to 20 kw, since 20 KHZ and 2 MHZ frequency, efficiency can reach 98%, usually is the telecommunications, electric welding machine, computers and peripherals, network, medical electronics, industrial control, security system and the choice of electronic equipment.
The biggest difference between planar transformer and traditional transformer is iron core and coil winding.Planar transformer adopts small-sized e-type, RM type or ring-type ferrite core, which is usually made of ferrite material with high frequency power and has low core loss at high frequency. The windings are made up of multilayer printed circuit boards, and the windings or copper pieces are superposed on the plane high-frequency iron core to form the magnetic circuit of the transformer. This design has low dc copper resistance, low leakage inductance and distributed capacitance, which can meet the design requirements of resonant circuit. And because of the good magnetic shielding of magnetic core, rf interference can be suppressed.
2. Main features of planar transformer
(1) High current density. The conductors of a planar transformer are actually some planar conductors, so the current density is high.
(2) High efficiency. The efficiency can reach 98% ~ 99%.
(3) Low leakage. About 0.2% of the primary inductance.
(4) Good heat conduction. Short distance of hot channel and low temperature rise.
(5) Low EMI radiation. Good core shielding can reduce the radiation to very low.
(6) Small volume. The volume can be reduced by using small magnetic cores.
(7) Good parameter repeatability. Because the winding structure is fixed and easy to be processed in advance, the parameters are stable.
(8) Wide operating frequency range. The frequency ranges from 50kHz to 2MHz.
(9) Wide operating temperature range. Working temperature is to 40 ℃ ~ 130 ℃.
(10) Good insulation. The planar transformer is composed of conductive circuit and insulating sheet overlapping each other, so as to ensure 4kV insulation isolation between windings, primary and secondary and secondary.
3. Performance of planar transformers
The transformers of this structure are small in size and have heights of 8mm and 12mm.The number of winding turns is much less than the traditional transformer, the structure is more compact, the magnetic coupling is much better than the traditional transformer, the leakage resistance is less than 0.2%, so it can work at a higher frequency, which is conducive to the improvement of power conversion efficiency. The compact core geometry limits hot spots and reduces heat consumption, thus allowing for higher energy density. At the same time, the thermal condition of itself is much better than the traditional transformer. Therefore, the volume and weight of planar transformer are greatly reduced and the efficiency is higher. More importantly, it provides a universal choice for switching transformer in the switching power supply, and saves the complicated calculation, material selection and transformer winding process. It simplifies and optimizes design while reducing volume and costs. Therefore, planar transformer is very suitable for the design of switching power supply or inverter power supply with low voltage (1 ~ 60V, high current (30A/per core), and there is no limitation on the topological structure of transformer.
4. Structure principle of planar transformer
Planar transformers usually have two or more cylindrical cores of the same size (FIG. A).The structure of planar transformer with two magnetic cores is introduced as an example. The two diagonal angles of each core column are connected by copper sheets that cling to the inner wall of the core as they pass through the core column (FIG. B).Two magnetic cores are placed side by side, the two adjacent angles are welded together with copper sheet. The copper sheet on two corners of an outer surface of a magnetic core is welded together with a piece of copper sheet. This is the center of the plane transformer secondary coil. The copper sheet at two angles on one of the outer surfaces of the other core is the two ends of the planar transformer secondary coil (FIG. C).
This basically constitutes the main part of a planar transformer.Its secondary has only one turn and can have a central tap.A full of energy storage inductor planar transformer and a preset (1.4 mH @ 500 KHZ, DC20A), it often meet in the center tap, at the end of the upper and lower each have a fixed copper, they will be core and filter inductance caught in the middle, at the same time as the poles (d) of rectifying power supply and cooling plate (used to add cooling plate according to the size of the power).
5. Classification of planar transformers
(1) PCB type transformer, printed circuit PCB type transformer can eliminate the winding skeleton, increase the heat dissipation area, reduce the eddy current loss caused by skin effect and proximity effect at high frequency, and increase the current density, the current density can be up to 20A/mm, with large power and simple process.But with PCB, the window utilization rate is low, only 0.25~0.3, the window utilization rate of traditional transformer is 0.4, and its volume is also large. The power of PCB transformer can be up to 20kW and the frequency can be up to the order of megahertz. Pulse planar technology is used. The multilayer PCB is sandwiched between magnetic cores. The thin and efficient ferrite planar transformer has a small base area and a height of only 7.4mm.
(2) Thin-film transformer. Thin-film transformer is a laminated miniature transformer developed with magnetic thin film. Most of them use metallic magnetic materials, such as permalloy, ferrosilicon-aluminium and amorphous alloys. Mainly because they have high BS and high permeability. Tsuijimotl et al. used strip transformers (copper 35 m thick, 34mm long, 3mm wide) with insulating film (100 m thick), amorphous CoNbZr film (1.8 m) to form a film transformer with controllable output voltage at high frequency -- pinhole type transformer, and also made a chip transformer with a thickness of 210 m.It is made of two layers of 10 m thick CoZr amorphous film, for 5V, 0.3a, 1MHz switching power supply, 77.5% ferrite material (mainly MnZn series) can also be made into A thin-film transformer, but it is difficult to make A suitable micro magnetic film using conventional methods, so A new film-forming technology needs to be developed. At present, PVD, CVD and other deposition technologies are mainly used in foreign countries, such as chemical etching, laser ablation, low temperature coating by light irradiation and other film-forming technologies. The miniature transformer designed and manufactured by Yamaguchi K et al has an area of only 2.4mm * 3.1mm, and its efficiency can reach 67% at 10MHz.
(3) Thick film transformer, thick film transformer is to overcome the film transformer conductor resistance in the defect. Primary and secondary windings are printed on the upper and lower surfaces of alumina based on thick film technology. The planar transformer made of ferrite has an efficiency of 85% when the output power is 75W at 2MHz.The efficiency of planar transformer produced by the thick film process is generally low, so it is the key to realize the high frequency integration of planar transformer to seek further technology to perfect the thick film process.
6. Advantages of planar transformers
Compared with the conventional transformer, the size of the magnetic core is greatly reduced, especially the height is greatly reduced. This feature has considerable attraction to power equipment in the case of space constraints, which can become the preferred magnetic element in many power equipment. The advantages of planar transformer structure also bring many advantages to its electrical characteristics: high power density, high efficiency, low leakage, good heat dissipation, low cost, etc.
7. Application of planar transformer in power supply
(1) In addition to reasonable layout and control circuit, surface sticker technology is adopted to save space, more effective measures are taken to avoid the limited space occupied by the traditional large-volume high-frequency power transformer.
(2) High working environment temperature. Relative to other rectifier module - 25 ℃ ~ + 50 ℃ work environment,
The module can work in 25 ℃ ~ + 70 ℃ environment, to meet the needs of some bad conditions. Therefore, when working normally, the temperature rise inside the module will be higher, which requires the transformer to withstand high temperature.
(3) The module requires high EMI, noise and other indicators. Practical measures are required to improve these aspects.
(4) Small size and high efficiency of the module indirectly require small heat loss of the module.
In view of the above requirements, combined with the advantages of planar transformer, planar transformer is preferred in transformer design. The planar transformer has high current density and low leakage resistance under the condition of small structure volume, which is very suitable for switching power supply with low voltage and large current. It should be noted that since the conventional transformers are cylindrical wire wound around the ferrite core, the high-frequency current is concentrated near the surface of the wire (skin effect), which will reduce the effective conductivity. In planar transformers, the "windings" are flat conducting wires on copper-coated circuit boards. The flat geometry reduces the loss of skin effect at higher switching frequencies. Therefore, the surface conductivity of the copper conductor can be used most effectively, and the efficiency is higher than the traditional one.