A power conversion device in the form of a compressor drive constitutes a three channel power sharing transmission which allows power input and/or output from shafts on two of the channels along with hydraulic, electric or potentially pneumatic power input and/or output from the third channel. Varying the input and/or output of hydraulic, electric or pneumatic flow provides a continuously variable transmission function. Several embodiments of the power conversion device are described to drive a supercharger for an internal combustion engine providing a variable ratio coupling allowing effective use of a centrifugal type compressor across a broad range of operational engine speeds.