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Technical advantages of planar transformer in switching power supply

Abstract: high power density is the main trend of the development of switching power supply today. In order to achieve this, we must improve the power density of magnetic components. Because of the special planar structure and the close coupling of windings, planar transformer greatly reduces the high-frequency parasitic parameters and greatly improves the working state of switching power supply, Therefore, it has been widely used in recent years. Several different planar structures and winding manufacturing methods are studied, and a standard method for designing planar transformers is introduced, which makes the design process simpler and greatly reduces the design cost. Finally, some parameters of planar transformer and traditional transformer are compared, and the design guidelines are given

key words: planar transformer; Leakage inductance; Insertion technology

0 introduction

the design of magnetic components is an important part of switching power supply. Because planar transformers have great advantages in improving the characteristics of switching power supply, they have been widely used in recent years. For an ideal transformer with many years of practical experience, the magnetic flux generated by the primary coil passes through the secondary coil, that is, there is no magnetic flux leakage. For ordinary transformers, the magnetic flux generated by the primary coil does not pass through the secondary coil, so leakage inductance is generated, and the tight requirements of electromagnetic coupling cannot be met. The planar transformer has only one turn of secondary winding, which is also different from the traditional enameled wire, but a piece of copper skin, which is attached to the surface of multiple stamped ferrite cores of the same size. Therefore, the output voltage of planar transformer depends on the number of magnetic cores, and the output current of planar transformer can be expanded in parallel to meet the design requirements. Therefore, the characteristics of planar transformer are obvious: the close coupling of planar windings greatly reduces the leakage inductance; The special structure of the planar transformer makes its height very low, which makes the idea of the converter on a board come true. However, the high capacitive effect of planar structure greatly limits its large-scale use. However, these shortcomings may also be converted into advantages in some applications. In addition, the planar magnetic core structure increases the heat dissipation area, which is conducive to the heat dissipation of the transformer

1 study on the characteristics of planar transformer

as mentioned above, the advantages of planar transformer mainly focus on low leakage inductance and AC impedance. The greater the gap between windings, the greater the leakage inductance, and the higher the energy loss. The planar transformer uses the close combination between copper foil and circuit board, so that the gap between adjacent turns is very small, so the energy loss is very small

in a planar transformer, its "winding" is a flat conductive conductor made on a printed circuit board or directly made of copper. The flat geometry reduces the loss of skin effect, that is, eddy current loss, when the switching frequency is high. Therefore, the surface conductivity of copper conductor can be used most effectively, and the efficiency is much higher than that of traditional transformer. Figure 1 shows the sectional view of a planar transformer, and the leakage inductance and AC impedance values under different clearances are obtained by using the different distances between the two layers of windings. High speed sampling

Figure 2 and figure 3 show the changes of leakage inductance and AC impedance under different gaps. It can be clearly seen that the larger the gap is, the greater the leakage inductance is, and the smaller the AC impedance is. When the gap is increased by 1mm, the leakage inductance increases by as much as 5 times. Therefore, under the condition of meeting the electrical insulation, the thinnest insulator should be selected to obtain the minimum leakage inductance

however, the capacitive effect is very important in planar transformers. The conductor tightly wound on the printed circuit board makes the capacitive effect very obvious. Moreover, the selection of insulating materials also has a great impact on the capacitive value. The higher the dielectric constant of insulating materials, the higher the capacitive value of transformers. Capacitive effect will cause EMI, because only the windings of capacitive circuit in the primary to secondary windings transmit this interference. In order to verify, the author did an experiment. When the gap of copper wire was increased by 0.2mm, the capacitance value was reduced by 20%. Therefore, if a relatively low capacitance value is required, a compromise must be made between leakage inductance and capacitance value

2 insertion technology

insertion technology refers to that when the primary and secondary windings of the transformer are arranged, the primary and secondary windings are placed alternately, and the coupling of the primary and secondary windings is increased to reduce the leakage inductance. At the same time, the current is evenly distributed and the transformer loss is reduced

now the research of insertion technology is divided into two aspects, that is, the insertion applied to transformer (forward circuit) and the insertion applied to connecting inductors (flyback circuit). Therefore, insertion technology has now been studied in different topologies as different magnetic components

2.1 insertion technology applied to planar transformers

the main advantages of the insertion technology applied to transformers are as follows:

1) reduce the storage space of magnetic energy in transformers, resulting in the reduction of leakage inductance

2) make the ideal distribution on the conductor during current transmission, resulting in the reduction of AC impedance

3) better coupling between windings leads to lower leakage inductance

in order to illustrate the characteristics of the insertion technology, figure 4 shows the structure of applying three different insertion technologies. P represents the primary winding and s represents the secondary winding. The test shows that the SPSP structure is the best, because the primary and secondary windings are inserted at intervals. Figure 5 shows the AC impedance and leakage inductance values of three structures at 500 kHz. Through comparison, it can be easily found that the AC impedance and leakage inductance values of transformers with insertion technology have been greatly reduced

2.2 advantages of planar structure in multi winding transformer

another important advantage of planar transformer is its low height, which makes it possible to set more turns on the magnetic core. A high power density converter needs a relatively small magnetic element, and the planar transformer meets this requirement well. For example, a transformer with multiple windings needs a lot of turns. If it is an ordinary transformer, it will cause excessive volume and height, which will affect the overall design of the power supply, while the planar transformer does not have this problem

in addition, for multi winding transformers, it is very important to maintain good coupling between windings. If the coupling is not ideal, the leakage inductance increases, which will increase the error of the secondary voltage. The planar transformer is the best choice because of its good coupling

2.3 the role of planar transformer in different topologies

the role of magnetic components is also different in different topologies. In the transformer in the forward converter, the magnetic energy is transferred from the primary winding to the secondary winding when the main switch is turned on. However, the "transformer" in the flyback converter is not exactly a transformer, but two connected inductors. The "transformer" in the flyback topology stores energy in the primary winding when the main switch is turned on, and transmits energy to the secondary winding when it is turned off. Therefore, the advantages of this insertion technology are the same as above, so as to avoid the damage surface of the tested product caused by uncontrolled environmental conditions. The characteristics of the insertion technology applied to this transformer are as follows:

1) the energy stored in the magnetic core is not reduced, because the current can only flow in one winding at a certain time, and there is no current compensation

2) the current distribution is not ideal for the same reason as above, so the AC impedance does not decrease

3) insertion leads to better coupling between windings, so there is a relatively small leakage inductance value

3 standardized design of planar transformer

the advantages of planar transformer are described above, but it also has disadvantages. Its main disadvantage is that the design process is very complex, and the design cost is also very high

the following describes a standard procedure for designing planar transformers [3]; By providing a standard turn model design, it can be used in different planar transformers, so that the design process is greatly simplified and the cost is greatly reduced

each layer of double-sided PCB is composed of one to more turns of windings, and all layers maintain the same physical characteristics: the same shape and the same external connection points. In some multi turn layers, this external connection point is the electrical connection point between different turns. If some layers have only one turn, it can also be printed on both sides of PCB to reduce AC impedance. Copper foil directly printed on PCB has been used in ordinary cars to replace traditional wires. Even in many switching power supplies that require many turns, the transformer can still maintain a very small volume, which greatly reduces the volume of the whole machine. Please refer to literature [3] for specific design steps and precautions. Figure 6 shows an example of a top-level standard turn design using a can shaped (RM) core

the height of copper foil is selected according to the skin depth corresponding to the maximum switching frequency, which can make all parts of copper foil become current paths and greatly reduce the influence of skin effect. Therefore, each switching frequency should correspond to different copper foil height

4 experimental demonstration

in order to compare the planar transformer and the traditional transformer, two kinds of transformer models are made respectively. One uses the planar structure and the insertion technology, and the other uses the copper wire wound in the primary and secondary respectively. Both transformers are used in a half bridge converter with complementary control. The parameters of the two transformers are as follows:

primary 12 turns:

secondary two 1-turn windings (1:1 center tap)

traditional transformers use enameled wires as windings. Although the current density in these coils is different, the current density is less than 7.5A/mm

the primary winding of plane transformer is made into 4 layers, with 4 parallel secondary windings. The final structure of this transformer is shown in Figure 7

both transformers use the same magnetic core RM10. The leakage inductance, AC impedance and occupied area of the two transformers are compared. The results are listed in Table 1

according to table 1, the leakage inductance of planar transformer is only 1/5 of that of traditional transformer, and the AC impedance is only 1/3. It can be seen that this will greatly improve the working characteristics of the converter. Moreover, due to the more compact structure, smaller RM8 cores can be used

5 conclusion

planar transformer has great advantages in reducing leakage inductance and AC impedance, and it has become a very good magnetic component because of its small size. A standard method of designing planar transformer is given, which makes the design of planar transformer easier and the cost will be greatly reduced. It can be predicted that the planar transformer will have a very good application prospect. (end)

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