1 Introduction
In recent years, with the development of power electronic technology and mature,people gradually realize the magnetic element is not only functional elements in a power supply, and its volume, weight loss, also accounted for a considerable proportion in whole. According to statistics, the magnetic element weight is generally converter total weight of 30% ~ 40%, the volume of the total volume of 20% ~ 30%, for the high-frequency power supply modular design, volume, weight of magnetic components proportion will be higher. In addition, the magnetic element is one of the most important factors affecting power output dynamic performance and output ripple. Therefore, to increase the switching power supplypower density, efficiency and output quality, the key is to improve the power density of the magnetic components, reduce the volume and weight of magnetic components. Planar transformer for tightly coupled to the special plane structure and winding, the high frequency parasitic parameters has been greatly reduced,greatly improved the performance of switching power supply. Therefore, in recent years in the switching power supply has been widely used. This paper reviews the planar transformer technology developed in recent years, compared with the traditional transformer, analyzes the advantage and Prospect of planar transformer in the switching power supply development and application.
The structure of 2 planar transformer
Transformer is one of the key elements in a power supply. Traditional windingtransformer is composed of ferrite core and coil of copper wire, size and it is easy to generate electromagnetic interference. But the biggest difference betweenplanar transformer winding transformer and traditional is core and coil winding.Planar transformer with E type, small size RM or ring type ferrite core, usually made of high frequency power ferrite material, has lower core loss at high frequency; the winding adopts multilayer printed circuit board and wound into,and then put a magnetic loop transformer in the plane of the high-frequencymagnetic core. Figure 1 shows a typical multilayer planar transformer structure.The design of DC copper resistance, low low leakage inductance and distributed capacitance, which can meet the design requirements of the resonant circuit. And because the magnetic shielding core good, can suppress the RF interference.And the number of turns of the primary winding of the planar transformer is usually only a few turns, not only to reduce the copper loss and distributed capacitance, inductance, and brought a lot of convenience for winding. Because the core is stamping parts simple combination and become, consistency of performance is greatly improved, but also for the mass production and reduce the cost. To effectively solve the volume and high frequency problems, provides the possibility for the lightweight, compact power supply converter.
Planar transformer iron core on the market with a clamping groove and noclamping slot two. Clamp plate with a clamping groove core provided by the manufacturer to fixed; no between clamping groove core by resin bonded fixed.The transformer core clamping groove is suitable for high temperatureapplications, and more firmly, as shown in Figure 2A; transformer iron core made of no clamping groove height ratio transformer with a clamping groove is lower.Designers can choose according to the actual situation of iron core. If you choose no clamping groove core, attention can not be stuck in resin with twopieces of iron surface, there will be air gap between this core, the resin bonded in the iron core outside, as shown in Figure 2b.
Combination of Figure 2 two planar transformer core
The advantages and development status of 3 planar transformer
The planar transformer and traditional variable winding transformer has many advantages:
1) has the advantages of small volume, low profile. The planar transformer withferrite core, reducing the volume of the transformer, the general height is less than 20mm.
2) high power density. Because of the advantage of structure of planar transformer, improves its electrical characteristics, which is 3 times higher thanthe power density of traditional transformer.
3) high voltage, high current. In the planar transformer, wire is actually someplanar conductor, so the current density, the winding of each layer maximum current can reach 200A, the secondary voltage is more than 20kV.
4) high power. Power single device can reach 0.5kW ~ 25kW.
5) efficiency. Can reach 98% ~ 99%.
6) low leakage inductance. About the primary inductance of about 0.2%.
7) wide operating frequency range. The frequency range is from 50kHz to 2MHz.
8) wide operating temperature range. -40 ℃ ~ 130 ℃.
9) for a fixed and pre processing winding structure is good, stable and repeatablecharacteristics parameters.
10) good heat dissipation. Hot channel of short distance, low temperature rise,large radiating area, good heat conduction effect.
11) low EMI radiation. Because of high efficiency magnetic shielding, which can suppress the RF interference.
12) high insulation. Between the windings, initial secondary and secondary -secondary with high insulation, initial secondary insulation isolation of up to 4kV.
13) assembly performance.
Transformers three existing types of (conventional transformer, piezoelectric ceramic transformer and planar transformer) to compare the performance, see table 1:
In view of the superiority of the planar transformer various technical parameters,it can be widely used in notebook computer, digital camera, digital TV,communication power supply, automotive electronics and other fields. Low height of a 200W planar transformer is only 1 4mm higher than the traditional high-frequency products, small size, light weight a lot. Because of its small parasiticreactance. The interwinding capacitance and leakage inductance. So the efficiency is as high as 97%, the highest frequency of 2MHz. leakage resistanceis less than 0.2%. The present miniature electronic transformer foreigndevelopment faster, has produced a miniature transformer and thickness of 5mm× 5mm × 5mm only for planar transformer 0.2mm, Chinese minority foreign enterprises