High-Inductance-Density, Air-Core, Power Inductors, and Transformers Designed for Operation at 100–500 MHz

This paper presents the microfabrication and measurement of high-inductance-density, moderate-Q, air-core inductors, and transformers intended for switch-mode power supplies operating in the 100-500 MHz frequency range. The inductors and transformers were fabricated on Pyrex substrates with four layers of electrodeposited copper with each layer up to 10 ¿m thick. Stacked winding layers allowed for mutual coupling between layers to increase areal inductance density. Inductors of various designs exhibited inductance densities of up to 100 nH/mm ² and quality factors approaching 20 in the frequency range of interest. Transformers were formed by interleaving primary and secondary coils and were designed with increased inductance in the secondary coil for voltage gain. A fabricated transformer, 1.5 mm × 1.5 mm in area, yielded 46 nH primary inductance and 500 nH secondary inductance with a coupling coefficient of 0.63. Measurements indicated that a maximum transformer efficiency of 78% at 125 MHz would be possible.