Structural principles and applications of planar transformers

    Abstract: Most DC/DC converters require an isolation transformer, and planar transformer technology has achieved important breakthroughs in many aspects of isolation transformers. The structure, performance and usage of planar transformers are introduced.

    Keywords: isolation transformer, planar transformer, switching power supply

    In DC/DC conversion, the basic Buck, Boost, and Cuk converters do not require a switching isolation transformer. However, if the output and input are required to be isolated, or multiple sets of output voltages are required, a switching isolation transformer must be used between the switching element and the rectifying element, so most converters have isolation transformers. The current development trend of switching power supplies is higher efficiency, smaller size, and lighter weight. However, the efficiency, volume, and weight of traditional isolation transformers have seriously restricted the further development of switching power supplies. At the same time, since the main parameters involved in the transformer include voltage, current, frequency, transformation ratio, temperature, core u value, leakage reactance, loss, dimensions, etc., it has not been possible to have ready-made transformers for selection like other electronic components. , it is often necessary to go through tedious calculations to select the magnetic core and winding wires, and the winding winding also has a great impact on the performance of the transformer. In addition, many important parameters of the transformer are difficult to measure, which brings a certain degree of blindness to the use, and it is difficult to determine the frequency. Satisfactory results are achieved in terms of response, leakage resistance, volume and heat dissipation. Planar transformer technology has achieved important breakthroughs in many aspects of isolation transformers.

    At present, planar transformer technology has begun to be used in many foreign power products, such as battery charging power supplies, distributed power supplies for communication equipment, UPS, etc. However, there is a certain gap between domestic isolating switching transformers and foreign advanced countries in terms of materials, processes, etc., which hinders the improvement of high frequency and efficiency of switching power supplies, and makes switching power supply products stay at a low level. Planar transformer technology will provide useful help for the design and productization of high-frequency switching power supplies.

    The windings of traditional transformers are often wound on a magnetic core and have a large number of turns. The planar transformer (unit) has only one turn of meshed secondary winding. This turn is also different from the traditional enameled wire. It is a piece of copper wrapped around the surface of multiple stamped ferrite cores of the same size. Therefore, the output voltage of the planar transformer depends on the number of magnetic cores, and the output current of the planar transformer can be expanded through parallel connection to meet the design requirements. Moreover, the number of turns in the primary winding of a planar transformer is usually only a few turns, which not only effectively reduces copper loss, distributed capacitance, and reactance, but also brings a lot of convenience to winding. Since the magnetic core is assembled from simple stamping parts, the consistency of performance is greatly improved and the cost is reduced for mass production.

1 Structure and performance of planar transformer

1.1 Structure

    Planar transformers usually have two or more cylindrical cores of the same size (Figure 1a). Now let’s take a planar transformer with two magnetic cores as an example to introduce its structure. The two corners of each magnetic core column on the diagonal are connected with copper sheets, and the copper sheets are close to the inner wall of the magnetic core when passing through the magnetic core columns (Figure 1b). Two magnetic cores are placed side by side, and the two adjacent corners are welded with copper sheets. The copper sheets on the two corners of one outer surface of one magnetic core are welded together with a piece of copper sheet. This is the secondary coil of the planar transformer. If the tap is drawn here, it is the center tap of the secondary coil; the copper sheets on the two corners of one outer surface of the other core are the two ends of the secondary coil of the planar transformer (Figure 1c). This basically forms the main part of a planar transformer. Its secondary coil has only 1 turn and can have a center tap. A complete planar transformer also has a preset energy storage inductor (1.4μH@500kHz, DC20A). One end of it is often connected to the center tap, and there is a fixed copper plate on the top and bottom. They connect the magnetic core and filter inductor. Sandwiched in the middle, it serves as the two poles of the rectified power supply (Figure 1d) and the heat sink (in actual use, a heat sink needs to be installed according to the power).

1.2 Performance

    Transformers with this structure are small in size and available in heights of 8mm and 12mm. The number of winding turns is much less than that of traditional transformers, and the structure is more compact. The magnetic coupling is much better than that of traditional transformers, and the leakage reactance is less than 0.2%, so it can work at higher frequencies, which is beneficial to the improvement of power conversion efficiency. . The tight core geometry limits the creation of hot spots, reducing heat dissipation and therefore allowing for higher energy densities. At the same time, its heat dissipation conditions are much better than traditional transformers. Therefore, the size and weight of the planar transformer are greatly reduced, and the efficiency is higher. More importantly, it provides a universal choice for switching transformers in switching power supplies, eliminating the complex calculation, material selection and transformer winding processes. It simplifies and optimizes the design while also reducing the size and cost. Therefore, planar transformers are very suitable for use in the design of low-voltage (1~60V, high-current (30A/per core) switching power supplies or inverter power supplies. There is no restriction on the topology of the transformer. Table 1 lists the FTI-12 Main technical parameters of ×2A-XA series planar transformers.

Table 1 Main technical parameters of FTI-12×2A series flat transformers

1
5
15

2 Use of planar transformers

    There are three main principles for the use of planar transformers:

    (1) Select the corresponding model of planar transformer according to the output voltage;

    (2) Determine the number of parallel planar transformers according to the output current;

    (3) Determine the transformation ratio and the number of turns of the primary winding according to the input and output voltage.

    For example, the switching power supply output voltage is 5V and the output current is 150A. Choose the 5V series planar transformer FTI-12×2A-XX. If you use FTI-12×2A-1A, you need 5 in parallel; if you use FTI-12×2A-5A (actually composed of 5 FTI-12×2A-1A in parallel), you only need 1.

    In addition, the actual application also needs to know the changes of the planar transformer and the number of turns of the primary coil. The transformation ratio can be calculated using the following formula:

    Transformation ratio = K × N × P: 1

    In the formula, K is the coefficient. When the output of the planar transformer passes through the center tap, K=0.5; when the planar transformer has no center tap, K=1. N is the number of parallel planar transformer units. The number of primary turns of P-plane transformer.

    In the above example, if the output planar transformer uses a center tap, the input DC voltage is 150V, and the transformation ratio can be set to 10:1, then the number of primary turns P = 10/(0.5×5) = 4. If the input DC voltage is 300V, the transformation ratio can be set as 201, then the number of primary turns P=20/0.5×5=8. It can be seen that the number of turns of the primary winding of a planar transformer is usually very small.

3 Conclusion

    The author was commissioned by FTI Company to use a planar transformer in the battery charging power supply. According to my experience, after using a planar transformer, the switching frequency is 300kHz, the conversion efficiency is greater than 95%, and the volume and weight of the entire device are also greatly reduced. Because planar transformers have been standardized, serialized, and productized, they have good performance consistency and are very convenient to use.

    Planar transformers have changed the design ideas of isolation switching transformers in traditional switching converters, providing designers with a more convenient and better choice. In 1998, it was named Outstanding Product of the Year by Electronic Products magazine.