Here’s an application note (PDF!) from Texas Instruments on MOSFET power losses and how they affect power-supply efficiency:
Power-supply efficiency is a critical criterion for today’s cloud-infrastructure hardware. The efficiency of the chosen power solutions relates to system power loss and the thermal performance of integrated circuits (ICs), printed circuit boards (PCBs), and other components,which determines the power-usage effectiveness of a data center.
This article revisits some of the basic principles of power supplies and then addresses how MOSFETs—the power stage of any switching-voltage regulator—affect efficiency.