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Design of integrated inductances based on ferromagnetic LTCC layers

: Hahn, R.; Sommer, G.; Dörr, I.; Schwerzel, S.; Reichl, H.; Muller, E.

Advancing microelectronics 33 (2006), Nr.6, S.8-16
ISSN: 2222-8748
Fraunhofer IZM ()

A compliant LTCC material system was developed which allows the integration of soft magnetic layers with a relative permeability ranging from 200 to 300. On this basis, ultra low profile inductances can be fabricated for miniaturized dc - dc converters. Inductances can be buried into the LTCC substrate to give more room for the semiconductor chips on top, or they can be fabricated as very small size planar inductances or arrays of inductances as surface mountable devices. In both manners, significant size and cost reductions are achievable. A number of toroidal and planar spiral coil designs were characterized and analyzed. At a coil diameter below 2.8 mm and a total thickness of 300 m, inductance values of up to 4.7 H were achieved at frequencies of up to 3 MHz and maximum currents of 800 mA. Q - factors were measured in the range between 100 KHz and 100 MHz with help of an impedance analyzer. The coil design was optimized in terms of via hole placement, inner and oute r coil diameter, and the number of ferromagnetic layers. All devices show a pronounced inductance maximum as a function of current which is typical for the ferrite material used. since all inductances use the same magnetic substrate and there is no physical separation, the effect of magnetic flux crowding and coupling between adjacent coils was additionally investigated. Based on the measured parameters, the saturation induction and magnetic losses as a function of frequency and magnetic flux of the ferrite tapes were calculated.