Improvement of the film transformer characteristics

Abstract

Previously, we have proposed a thin film transformer. The operating principle of this transformer is based on the skin effect. Thereby, its weight and size are extremely small but its operating frequency having high efficiency is limited only to high frequency region. In order to remove this fault, we employ the thin ferrite tiles which have been sandwiched between the film conductors. As a result, we succeeded in widening the operating frequency range about 100 times compared with those of original one. I .INTRODUCTIONIn order to develop a small size and light weight high frequency transformer for DC to DC converter use, we have previously proposed a coreless transformer utilizing the skin effect of the adjacent current carrying conductors [l]. 15W output power was obtained by the coreless transformer having only 7g weight [2]. Satisfactory result of the coreless transformer spurred to exploit a film transformer, which was fabricated by the photo etching technique. When we installed our film transformer to a push-pull type DC to DC converter, l l w output power was obtained with 72% overall efficiency [3]. This record is fairly good comparing with those of the other similar type of converters [4,5]. However, we have found that magnetizing inductance of the film transformer is too small to use the DC to DC converters instead of the conventional core type transformers. In the other words, the coreless film transformer is light and small so that it is possible to realize a light weight DC to DC power supply. However, it is difficult to replace the conventional core type transformers because of the film transformer's peak frequency characteristics, i.e., useful operating frequency range of the film transformer is extremely narrow for practical use.In the present paper, it is revealed that the f i l m transformer sandwiched by the ferrite tiles yields about 70 times larger self inductance and gives about 100 times wider usefbl operating frequency range comparing with those of the original coreless film transformer. Thus, we have succeeded in improving the characteristics of film transformer by means of a simple sandwich structure. II .EXPERIMENTAL A. Basic principle and structureFigure l(a) shows a typical core type conventional transformer which utilizes the magnetic flux linking the conductors wound around the magnetic core. The proposal coreless transformer utilizes the magnetic flux enclosing the current carrying conductors as shown in Fig. 1 (b) [ 1,2]. Because of this, the internal inductances of the primary and secondary coils should be small in order to obtain higher coupling. This can be accomplished by the skin effect at high frequency. Thus, the coreless transformer should be operated only in the higher frequency range. To utilize the same operating principle in a flat surface, the primal and secondary conductors are arranged alternatively on a film base surface. Figure 2(a) shows a coaxial circular shape film transformer. The connection of the coils are shown in Fig. 2(b). Further details of the film tra...

A Survey on Microtransformers

“…This area has been mentioned in almost 40% of the papers analyzed in this work [15], [22], [24], [31], [39], [40], [42]- [47], [50], [52], [54], [58], [59], [62], [66], [67], [70]- [73], [79]- [81], [84]- [91], [93]- [99], [101], [105], [116], [120]- [122], [131]- [134]. Some of the examples of specific applications in this area are DC-DC converters, energy harvesting, and power systems on chip.…”

“…3). An advantage of multilayer over coplanar devices is that they can reach higher L-values, up to 500 µH [31]. The performances of multilayer and coplanar microtransformers are comparable, with coupling coefficients higher than 70%, according to the articles that reported this parameter for multilayer devices [24], [26], [28], [32]- [38].…”

“…These inductors are usually designed according to the classic modeling by Greenhouse [11], [85]- [87]. The spirals can be circular, square, elliptical or n-sided polygons, and the coils can be coplanar (interleaved or side by side) or placed in different layers (with or without magnetic layers between the coils) [6], [17]- [19], [22]- [25], [27], [31], [33]- [37], [39]. Due to their compact spiral coil assembly architecture, microtransformers having very small area and volume can be fabricated.…”

“…Midorikawa et al demonstrated that the performance of microtransformers could be improved, even with a spiral assembly, by employing a ferrite film between the coils [31]. The improvement in efficiency was larger in the frequency range from 3 to 100 kHz, increasing from 0-50% to 65-85%.…”

Design of UHF Thin Film Transformer and Research of its S-Parameter Performance

“…Main research fields involve model simulation of thin film transformer, preparation, testing of thin film material and geometric structure of thin film transformer. In 1995, Y.Midorikawa from Japan fabricated a thin film transformer with two inductors in the same plane [9]. But its performance is greatly limited.…”