ED-02 1 Comparative Study of Microfabricated Inductors / Transformers for High Frequency Power Applications Dragan Dinulovic 1 , Member, IEEE, Mahmoud Shousha 1,2 , Martin Haug 1 , Sebastian Beringer 3 , Marc C. Wurz 3 1 Würth Elektronik eiSos GmbH & Co. KG, 85748 Garching, Germany 2 Electrical Power & Machines Department, Faculty of Engineering, Cairo University, 12611 Giza, Egypt 3 Institute of Micro Production Technology, Leibniz Universität Hannover, 30823 Garbsen, Germany This paper presents a comparative study on different microfabricated inductors (microtransformers), which are suitable for high frequency power application. The main topic of this study is comparing and analyzing the effect of some factors like material type, number of turns, and coil form on microinductor performance considering a fixed chip size, fixed magnetic core size, and core form. Two different microtransformer designs with different magnetic core materials are considered in this work. The core materials used in this work are NiFe45/55 and CoFe45/55. Also, two types of device packages are developed, namely QFN and FR4 embedded packages. For device testing, different high frequency switching regulators available on the market are used. This study shows the influence of different parameters on the performance of a certain high frequency application. In this paper, we discuss the influence of the design, magnetic material, and package type on thermal performance and the efficiency of the power application. Index Terms— Electromagnetics, High Frequency, Microinductor, Microtransformer, Power Electronics I. INTRODUCTION ECENT TRENDS in power electronics are miniaturization and integration of power supplies, especially for mobile and portable devices. The final target is to develop new miniaturized products known as power supply in package (PwrSiP) and power supply on chip (PwrSoC) [1, 2]. One possible technique for miniaturization is to increase the switching frequency of an electronic power circuit. Increasing the switching frequency sets new requirements for inductive components regarding the size, inductance, and current capability. At high switching frequencies, inductors and transformers can have a smaller inductance and hence smaller size. Therefore, thin-film microfabricated inductors and transformers are ideal components to be used in high switching frequency power applications. Both, microinductors and microtransformers are, in many research works, fabricated and tested at higher switching frequencies. [3-6]. II. DESIGN Our work is focused on development of microtransformers devices suitable for power application for switching frequency of 20 MHz and above. Thin-film magnetic microdevices considered are flexible transformers from a configuration point of view. The design allows usage of the device as an inductor, a variable-turns-ratio transformer or as a common mode choke for filter applications. The microtransformer device is a solenoid transformer type consisting of a closed magnetic core and multilayer coils surrounding the magnetic core. Using the same configuration, two types of microdevices are developed. First microtransformer design (generation 1) has, in total, six coils with a magnetic core of NiFe45/55 [7] and of CoFe45/55 [8]. The second design (generation 2) is improved in terms of increasing current capability and efficiency. Generation 2 device consists of only four coils [9]-[11]. A CoFe magnetic core material with both 5μm and 10μm thickness are used in the generation 2 design. It is worth mentioning that the size of all the devices and the form and lateral dimensions of the magnetic core are the same. An EIA standard (Electronic Industries Alliance) size 1008 is chosen as the total size of a microtransformer chip. Therefore, the whole microtransformer device features dimensions of 2.5 mm x 2.0 mm x 0.6 mm [length x width x hight]. However, generation 1 microdevices are packaged in a QFN package and generation 2 devices are embedded in an FR4 substrate. The microtransformer samples are prepared, characterized, and used in high frequency buck DC-DC converters as shown in the next subsections. Figure 1 shows x-ray micrographs of both microtransformer devices. It should be noted that both design generations have similarities. The main design parameters are summarized in Table I. In both designs, the width of the magnetic core track is 200μm. The major difference between the two generations is the design of the coil, allowing the second generation to achieve significantly reduced electrical resistance and higher current capability. Hence, the cross-section of the windings is increased from 20 μm x 15 μm to 60 μm x 20 μm in the second generation design. On the other hand, the number of turns is reduced which results in a reduction of the total inductance of the second generation devices compared to the R Fig. 1. Two microtransformer designs: (a) generation_1 and (b) generation_2