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Characterization of Low Temperature Sintered Ferrite Laminates for High Frequency Point-of-Load Converters
Keywords: Ferrite laminates, Low temperature sintering, Point-of-Load converter
The high power density Point-of-Load (POL) converter with high operating frequency has gained increasing interest in the growing market for portable communications and handheld electronics. Low profile magnetic components and associated integration techniques are desired for design and fabrication of highly integrated POL converters. Low fired ferrite tapes for multilayer chip inductors and high frequency applications have been developed and applied in hybrid integration of power devices. The inductor made of multilayer ferrites can be fabricated as the magnetic substrate in an integrated POL converter with active components on top. The developed 3-D integration approach is widely used to reduce the footprint and increase the power density for the high frequency POL converter. The current work reports the characterization of magnetic properties, microstructures, and chemical compositions of commercially available low-fire Ni-Cu-Zn ferrite materials (ESL 40010, 40011, and 40012) and their heterogeneous laminates. Permeability and core loss density are measured on toroidal shaped samples sintered at 885oC for 3.5 h. Because the high DC current is always applied to bias the core for the inductors used in high frequency POL converters, the influence of superimposed DC-bias on the magnetic properties of ferrite laminates has also been evaluated. The microstructures and chemical compositions of low temperature sintered ferrite laminates are observed and analyzed in order to explain the difference of magnetic properties among samples. The laminate with alternating layers of ESL 40010 and ESL 40012 in 1:1 ratio has been successfully co-fired and presents the highest permeability and the lowest core loss density in all examined samples when the DC-premagnetization is above 1000 A/m. Finally, a 12 V to 1.2 V, 15 A, high frequency (1-5 MHz) integrated POL converter with ferrite laminated inductor has been fabricated and demonstrated working at high efficiency with power density as high as 1000 W/in3.
Wenli Zhang,
Center for Power Electronics Systems
Blacksburg, Virginia

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