This paper proposes an air-core slab inductor structure to achieve ultra-high Q for on-chip power conversion circuits with over 90% efficiency. The structure uses a wide, thick metal slab with specially designed current return paths on either side to minimize DC resistance. Experiments demonstrate Q values as high as 25-35 at 200-300MHz, achieving 96.6% estimated inductor efficiency. Simulations of buck converters using the proposed inductor design show over 90% conversion efficiency for 2:1 voltage ratios. The structure can be implemented in a standard CMOS backend process without thin-film magnetics.