Characteristics and Standardized Design of Planar Transformers

0 Introduction

The design of magnetic components is an important part of the switching power supply. Since the planar transformer has great advantages in improving the characteristics of the switching power supply, it has been widely used in recent years. For an ideal transformer, the magnetic flux generated by the primary coil passes through the secondary coil, that is, there is no leakage flux. 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 electromagnetic coupling requirement cannot be met. The planar transformer has only one turn of mesh secondary winding. This turn of winding is also different from the traditional enameled wire. It is a piece of copper sheet, which is attached to the surface of multiple stamped ferrite cores of the same size. Therefore, the output voltage of the planar transformer depends on the number of cores, and the output current of the planar transformer can be expanded by parallel connection to meet the design requirements. Therefore, the characteristics of the planar transformer are obvious: the close coupling of the planar winding greatly reduces the leakage inductance; the special structure of the planar transformer makes its height very low, which makes the idea of ​​making the converter on a board come true. However, the planar structure has problems such as high capacitive effect, which greatly limits its large-scale use. However, these disadvantages may also be converted into an advantage in some applications. In addition, the planar core structure increases the heat dissipation area, which is beneficial to the heat dissipation of the transformer.

1 Insertion Technology

Insertion technology refers to arranging the primary and secondary windings of the transformer in such a way that the primary winding and the secondary winding are placed alternately, increasing the coupling between the primary and secondary windings to reduce leakage inductance, while making the current evenly distributed to reduce transformer losses.

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

1.1 Insertion Technology for Planar Transformers

The main advantages of insertion technology applied in transformers are as follows:

1) Reduce the space for storing magnetic energy in the transformer, resulting in a reduction in leakage inductance;

2) Make the current ideally distributed on the conductor during transmission, resulting in a reduction in AC impedance;

3) Better coupling between windings, resulting in lower leakage inductance.

1.2 The role of planar transformers in different topologies

The role of magnetic components is different in different topologies. In the transformer of the forward converter, 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 transfers energy to the secondary winding when it is turned off. Therefore, the advantages of this insertion technology are different from the above. The characteristics of the insertion technology applied to this transformer are as follows:

1) The energy stored in the core does not decrease, because the current can only flow in one winding at a time and there is no current compensation;

2) The current distribution is not ideal, for the same reason as above, so the AC impedance is not reduced;

3) Insertion results in better coupling between windings, resulting in a smaller leakage inductance value.

1.3 Advantages of planar structure in multi-winding transformers

Another important advantage of the planar transformer is its low height, which allows more turns to be set on the magnetic core. A high-power density converter requires a relatively small magnetic component, and the planar transformer meets this requirement well. For example, a multi-winding transformer requires a very large number of turns. If it is an ordinary transformer, it will cause the volume and height to be too large, affecting the overall design of the power supply, but 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 value will increase, which will increase the error of the secondary voltage. The planar transformer has good coupling, making it the best choice.

2 Study on the Characteristics of Planar Transformers

As mentioned above, the advantages of planar transformers are mainly concentrated in lower leakage inductance and AC impedance. The larger the gap between the windings, the greater the leakage inductance, which will produce higher energy losses. Planar transformers use the close combination of copper foil and circuit board to make the gap between adjacent turns very small, so the energy loss is also very small.

In a planar transformer, the "winding" is a flat conductive wire made on a printed circuit board or directly made of copper foil. The flat geometry reduces the skin effect loss, that is, eddy current loss, at high switching frequencies. Therefore, the surface conductivity of the copper conductor can be most effectively utilized, and the efficiency is much higher than that of a traditional transformer. Figure 1 shows a cross-sectional view of a planar transformer, and uses the different distances between the two layers of windings to obtain the leakage inductance and AC impedance values ​​at different gaps.

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

To illustrate the characteristics of the insertion technology, Figure 4 shows the structures using three different insertion technologies, where 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 the three structures at 500 kHz. By comparison, it can be easily found that the AC impedance and leakage inductance values ​​of the transformer using the insertion technology have been greatly reduced.

However, the capacitive effect is very important in planar transformers. The tightly wound wires on the printed circuit board make the capacitive effect very obvious. In addition, the selection of insulating materials has a great influence on the capacitance value. The higher the dielectric constant of the insulating material, the higher the capacitance value of the transformer. The capacitive effect will cause EMI because only the winding of the capacitive loop from the primary to the secondary winding propagates this interference. To verify, the author conducted an experiment. When the gap of the copper wire increased by 0.2mm, the capacitance value decreased by 20%. Therefore, if a lower capacitance value is required, a compromise must be made between leakage inductance and capacitance value.

3 Standardized design of planar transformers

The advantages of planar transformers are as mentioned above, but they also have disadvantages. The main disadvantage is that the design process is very complicated and the design cost is very high.

The following is a standard procedure for designing a planar transformer [3]; it provides a standard turns model design so that it can be used in different planar transformers, thereby greatly simplifying the design process and reducing costs.

Each layer of a double-sided PCB is composed of one or more turns of winding, and all layers maintain the same physical properties: the same shape and the same external connection point. In some multi-turn layers, this external connection point is the electrical connection point between different turns. If some layers have only one turn, they can also be printed on both sides of the PCB to reduce the AC impedance. Using copper foil directly printed on the PCB to replace traditional wires, even in many switching power supplies that require many turns, the transformer can still maintain a very small size, which greatly reduces the size of the entire machine. For specific design steps and precautions, please refer to reference [3].

Figure 6 shows an example of a top-layer standard turns design using a pot-shaped (RM) core.

The copper foil height is selected according to the skin depth corresponding to the maximum switching frequency, so that all parts of the copper foil can become current paths, greatly reducing the impact of the skin effect. Therefore, each switching frequency should correspond to a different copper foil height.

4 Experimental demonstration

In order to compare the planar transformer and the traditional transformer, two transformer models were made, one using a planar structure and using the insertion technique, and the other using copper wire wound on the primary and secondary. Both transformers are used in a complementary controlled half-bridge converter. The parameters of the two transformers are as follows:

Primary 12 turns:

The secondary has two l-turn windings (1:1 center tap).

Conventional transformers use enameled wire as windings. Although the current density in these coils varies, the current density is selected to be less than 7.5A/mm.

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

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

As shown in Table 1, the leakage inductance of the planar transformer is only 1/5 of that of the traditional transformer, and the AC impedance is only 1/3, which can greatly improve the working characteristics of the converter. In addition, due to the more compact structure, a smaller RM8 core can be used.

5 Conclusion

Planar transformers have great advantages in reducing leakage inductance and AC impedance, and their small size makes them a very good magnetic component. A standard method for designing planar transformers is given, which makes it easier to design planar transformers and greatly reduces costs. It can be foreseen that planar transformers will have a very good application prospect.