High efficiency and high power density are critical in megawatt-class mechanical-to-electrical energy conversion systems that operate within a limited speed range, as in wind- and gas-turbine-driven generators. Generator output is connected to an ac-to-dc conversion system for processing and controlling power flow to an electric grid. Conventional high-power ac-to-dc conversion architectures rely heavily on active rectifiers, which consist of fully-controlled power-electronic switches. These make the system bulky, lossy, and less reliable. This paper presents an alternative approach: integrating a multi-port permanent-magnet synchronous generator (PMSG) with series-stacked power converters. An active rectifier processes only a fraction of the total converted power while regulating the dc bus. The remaining power is processed by diode bridges, which allows a substantial increase in overall efficiency, power density, and reliability. Theoretical analysis shows that for wind-power generation applications, the active rectifier processes a maximum of 25% of the rated power while the PMSG operates in a speed range similar to the conventional doubly-fed induction machine. The conversion loss is reduced by 66%. Results from a laboratory-scale experimental setup corroborate the proposed architecture. This approach potentially increases integration of medium-voltage dc distribution to the megawatt-class mechanical-to-electrical energy conversion systems to achieve higher efficiency, power density and lower cost.