1 Introduction In order to reduce the size and weight of power electronic equipment, its operating frequency has been raised to the MHz level. It is well known that the main problem affecting the overall size and weight of a power converter is the size of the magnetic element. In the development of high-efficiency magnetic elements in this frequency range, it is important to focus on the skin effect and eddy current losses in the design of windings and cores and the various restrictions imposed on the choice of core material and conductor material specifications. This article discusses the design of an electronic transformer with an operating frequency of 2MHz, a power density greater than 400W/cm3, a power range of 50 to 100W, an output voltage of 1.5V, and an efficiency higher than 99%. In order to achieve the above-mentioned index, it is necessary to increase the density of the copper foil of the winding first to exceed the density of the conventional multi-strand strand, and to solve the process problem of manufacturing a multi-layer winding by using the copper foil winding technique. This winding eliminates the need for external connections between the winding layers, thus forming a tightly overlapping zig-zag winding structure, thereby achieving a high copper foil winding density, low copper loss, and a very low leakage inductance.
2 Miniaturization of Power Electronic Equipment Smaller, thinner, and lighter electronic components are gradually advancing with the demand for miniaturization and multi-functional development of electronic devices such as desktop personal computers, laptop computers, and mobile communications. Since the proportion of the magnetic element in the electronic device is 25% or more by volume, electronic devices have for a long time been able to deliver small-sized electrons with low voltage, large current, and low loss, occupying a small mounting area, and having good heat dissipation properties. Transformers pay special attention. Under these requirements, distributed power systems emerged in the 1980s, allowing the use of small power components for individual circuit board installations. For example, a switching power supply for a desktop personal computer has a power of 200 W, an output voltage of 5 V and 12 V, an efficiency of 80%, and a package power density of 1 W/in3. Since then, with the adoption of new high-efficiency power topology and resonance, quasi-resonant switching method to reduce the switching losses, and the operating frequency increased to MHz, not only the size of the magnetic components in the circuit is greatly reduced, but also developed a 100W And above output power, compact electronic transformer with output voltage as low as 1.5V and efficiency as 99%. The first problem encountered in the design of such a compact transformer is the trade-off between high power density and high efficiency. The main technology developed is the use of a planar winding structure with overlapping copper foils to increase the copper foil density. The method reduces the skin effect and eddy current loss in the high frequency (MHz level) range. It has been reported in the literature that, according to calculations, a copper foil with a thickness of less than 4.3 mils (mils) with twice the skin depth of copper at a frequency of 2 MHz is made into a flat ring-shaped single turn to obtain 75 μΩ. The single-layer resistance value for the 51-mil inner layer single-turn, if the outer layer adopts two parallel layers, the outer-layer single-turn radius needs 9.79 inches, and if it is four-layer parallel, it is 7 inches. Therefore, in order to ensure a sufficiently small installation area, many single-layers must be connected in parallel to form an approximately cubic geometry.
The small and thin process of magnetic electronic components (mainly electronic transformers and inductors), based on the conventional three-dimensional transformer inductors, has gone through three stages of development and have the following types: The first stage of development is "Plane type" (thin type) electronic transformer, see Figure 1, Figure 2 shows the type. Is now serialized mass production. The second stage of development is a "chip" transformer and inductor, as shown in Figure 3 and Figure 4. They are gradually improved with the development of SMT process technology, and now have formed series and batch. The third stage of development is the "thin film type" transformers and inductors, shown in Figure 5, which have been produced in small quantities. The various types of transformer inductors developed in these four stages each have their application fields, and they have not been completely replaced by "generation" in accordance with the development stage. Small, thin, lightweight transformer inductors have considerable advantages compared to conventional stereo transformer inductors, as shown in Table 1.
The manufacturing process of the various miniaturized magnetic components mentioned above can be said to have been relatively mature, but planar wire wound coil type, "PCB" type, and hybrid magnetic elements thereof are not applicable to very low resistance magnetic elements. Moreover, the magnetic elements of the PCB manufacturing method use an external pin to achieve interlayer interconnection, which significantly increases the interconnect resistance, and thus limits the use of more layers of coils. In the manufacture of hybrid components, the resistivity of the thick film adhesive and the thickness of the printed conductor are easily limited. Many flat copper foil windings with overlapping layers can obtain extremely low resistance, which is mainly limited by the low current monolayer design and the barrel winding structure.
In the transformer design and manufacturing discussed in this paper, a variety of methods for flexing the windings were investigated. The PCB copper foil windings included the fabrication of inter-layer interconnections, resulting in a zigzag folded multilayer primary winding; similar The method is zigzag-folded, and a center tap is drawn on the copper-clad foil. The copper foil is welded on each layer and the output ends of the copper foils are connected, and the output ends of the layers are connected in parallel to form a secondary winding. In the Z-stack fabrication process, the proper overlap sequence between the primary and secondary laminations should be used at the same time to ensure close overlap between the primary and secondary layers.
