Abstract: High power density is the main trend in the development of switching power supplies today. To achieve this, the power density of magnetic components must be increased. Planar transformers, due to their special planar structure and tight coupling of windings, significantly reduce high-frequency parasitic parameters, greatly improving the operating state of switching power supplies. Therefore, they have been widely used in recent years. This paper studies several different planar structures and winding fabrication methods, and introduces a standard method for designing planar transformers, which simplifies the design process and greatly reduces design costs. Finally, some parameters of planar transformers and traditional transformers are compared, and design guidelines are given. Keywords: Planar transformer; Leakage inductance; Insertion technology 0 Introduction The design of magnetic components is an important part of switching power supplies. Planar transformers have significant advantages in improving the characteristics of switching power supplies and have therefore been widely used in recent years. For an ideal transformer, the magnetic flux generated by the primary coil passes through the secondary coil, i.e., there is no leakage flux. However, for ordinary transformers, the magnetic flux generated by the primary coil does not all pass through the secondary coil, resulting in leakage inductance, and the requirement for tight electromagnetic coupling cannot be met. Planar transformers have only one turn of mesh secondary winding. This single turn differs from traditional enameled wire; instead, it's a copper foil wrapped around the surface of multiple equally sized stamped ferrite cores. Therefore, the output voltage of a planar transformer depends on the number of cores, and the output current can be expanded through parallel connection to meet design requirements. Thus, the characteristics of planar transformers are obvious: the tight coupling of the planar windings significantly reduces leakage inductance; the special structure of a planar transformer results in a very low height, enabling the conversion to be integrated onto a single board. However, the planar structure suffers from high capacitive effects, greatly limiting its large-scale application. However, these disadvantages can be transformed into advantages in certain applications. Additionally, the planar core structure increases the heat dissipation area, which is beneficial for transformer heat dissipation. 1. Study on the Characteristics of Planar Transformers As mentioned earlier, the advantages of planar transformers mainly lie in their lower leakage inductance and AC impedance. Larger gaps between windings mean greater leakage inductance and higher energy loss. Planar transformers utilize the tight bonding between copper foil and the circuit board, resulting in very small gaps between adjacent layers of turns, thus minimizing energy loss. In planar transformers, the "windings" are either flat conductive wires on a printed circuit board or directly made of copper foil. The flat geometry reduces skin effect losses, also known as eddy current losses, at higher switching frequencies. Therefore, it most effectively utilizes the surface conductivity of copper conductors, resulting in significantly higher efficiency than traditional transformers. Figure 1 shows a cross-sectional view of a planar transformer and illustrates the leakage inductance and AC impedance values ​​at different gaps by varying the distance between the two winding layers. Figures 2 and 3 show the changes in leakage inductance and AC impedance at different gaps, clearly demonstrating that a larger gap results in a larger leakage inductance and a smaller AC impedance. Increasing the gap by 1 mm increases the leakage inductance by more than five times. Therefore, while meeting electrical insulation requirements, the thinnest insulator should be selected to obtain the minimum leakage inductance. However, capacitive effects are very important in planar transformers, and the tightly wound wires on the printed circuit board make the capacitive effect very pronounced. Furthermore, the choice of insulation material has a significant impact on the capacitive value; the higher the dielectric constant of the insulation material, the higher the capacitive value of the transformer. Capacitive effects can cause EMI because this interference propagates only through the capacitive circuit from the primary to the secondary winding. To verify this, an experiment was conducted where increasing the gap in the copper conductors by 0.2 mm resulted in a 20% reduction in capacitance. Therefore, a trade-off must be made between leakage inductance and capacitance if a lower capacitance value is required. 2. Insertion Techniques Insertion techniques refer to alternating the primary and secondary windings in the transformer's primary and secondary winding arrangement. This increases the coupling between the primary and secondary windings, reducing leakage inductance and simultaneously ensuring a more even current distribution, thus reducing transformer losses. Current research on insertion techniques is divided into two aspects: insertion applied to transformers (forward circuits) and insertion applied to connecting inductors (flyback circuits). Therefore, insertion techniques are now being studied as different magnetic components in different topologies. 2.1 Insertion Techniques Applied to Planar Transformers The main advantages of insertion techniques applied to transformers are as follows: 1) They reduce the space for storing magnetic energy in the transformer, leading to a reduction in leakage inductance; 2) They allow for ideal current distribution on the conductors during transmission, resulting in a reduction in AC impedance; 3) Better coupling between windings leads to even lower leakage inductance. To illustrate the characteristics of insertion techniques, Figure 4 shows structures using three different insertion techniques, where P represents the primary winding and s represents the secondary winding. Experiments show that the SPSP structure is the best because the primary and secondary windings are inserted alternately. Figure 5 shows the AC impedance and leakage inductance values ​​of the three structures at 500 kHz. By comparison, it is easy to see that transformers using insertion techniques show a significant reduction in both AC impedance and leakage inductance. 2.2 Advantages of Planar Structures in Multi-Winding Transformers Another important advantage of planar transformers is their low height, which allows for a greater number of turns on the core. A high-power-density converter requires a small magnetic component, a requirement well met by planar transformers. For example, multi-winding transformers require a large number of turns. Ordinary transformers would result in excessive size and height, affecting the overall power supply design. Planar transformers do not have this problem. Furthermore, maintaining good coupling between windings is crucial for multi-winding transformers. Poor coupling increases leakage inductance, leading to greater errors in the secondary voltage. Planar transformers, with their excellent coupling, are the optimal choice. 2.3 The Role of Planar Transformers in Different Topologies The role of magnetic components differs in different topologies. In a forward converter, magnetic energy is transferred from the primary winding to the secondary winding when the main switch is turned on. However, in a flyback converter, the "transformer" is not a single transformer but rather two connected inductors. In a flyback topology, the "transformer" stores energy in the primary winding when the main switch is turned on and transfers it to the secondary winding when the main switch is turned off. Therefore, the advantages of this insertion technique differ from the above. The characteristics of the insertion technology applied to this transformer are as follows: 1) The energy stored in the core is not reduced 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 makes the windings have better coupling, so there is a relatively small leakage inductance value. 3 Standardized design of planar transformers As mentioned above, planar transformers also have disadvantages. The main disadvantage is that the design process is very complicated and the design cost is very high. The following introduces a standard design procedure for planar transformers [3]; it provides a standard turns model design, which can be used in different planar transformers, thereby greatly simplifying the design process and reducing the cost. Each layer of the double-sided PCB consists of one or more turns of winding, and all layers maintain the same physical characteristics: 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, it can also be printed on both sides of the PCB to reduce AC impedance. Using copper foil directly printed on the PCB board to replace traditional wires, the transformer can still maintain a small size even in many switching power supplies that require a large number of turns, which greatly reduces the overall size of the machine. For specific design steps and precautions, please refer to reference [3]. Figure 6 shows an example of a standard number of turns design for the top layer, which uses 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 influence of the skin effect. Therefore, each switching frequency should correspond to a different copper foil height. 4 Experimental demonstration To compare planar transformers and traditional transformers, two transformer models were made, one using a planar structure and using insertion technology, and the other using copper wire wound on the primary and secondary sides respectively. Both transformers were used in a complementary control half-bridge converter. The parameters of the two transformers are as follows: 12 turns for the primary winding and two 1-turn windings for the secondary winding (1:1 center tap). Traditional transformers use enameled wire as windings, although the current density in these coils is not the same, the current density is selected to be less than 7.5A/mm. The primary winding of the planar transformer is made into four layers, with four parallel secondary windings. The final structure of this transformer is shown in Figure 7. Both transformers use the same RM10 magnetic core. The leakage inductance, AC impedance, and area occupied by the two transformers are compared, and 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 greatly improves the operating characteristics of the converter. Moreover, due to the more compact structure, a smaller RM8 magnetic core can be used. 5 Conclusion Planar transformers have significant advantages in reducing leakage inductance and AC impedance, and their small size makes them excellent magnetic components. A standard method for designing planar transformers has been presented, making the design easier and significantly reducing costs. It is foreseeable that planar transformers will have a very promising application prospect.