High winding ratio, high frequency, ultracompact step-up transformers with laminated metallic magnetic cores

Abstract

The development and characterization of step-up transformers with high winding ratios and laminated metallic magnetic cores for MHz-range step-up transformers used in DC-DC power converters is presented. The use of metallic magnetic cores with higher saturation flux density than traditional ferrites offers the possibility of reducing the physical size of the transformers. Core eddy currents are suppressed by forming the magnetic cores in a sequential electrodeposition process through toroidal lithographic molds, followed by self-assembly. This approach results in relatively thick magnetic cores (in the hundreds of microns range), comprised of many individual laminations with micron-scale thicknesses. Transformers were formed by manual winding of primary and secondary coils on these cores. The resultant transformers, with typical dimensions in the cubic millimeter range, exhibited primary and secondary inductances in the microhenry and hundreds of microhenry ranges, respectively. Typical transformer coupling coefficients were in the range 0.85–0.95. The transformers exhibited expected voltage step-up ratios as high as 10:1 at typical operation frequencies of 1–2 MHz. These ultracompact devices exhibit up to seven times higher inductance per unit volume and five times higher inductance per weight than typical commercial coupled inductors, and show promise for use in space-constrained electronic systems.