Power-Conversion Efficiency: Loss Dominance, Optimization, & Design Insight

Power-conversion efficiency is critical in power supplies. Switched inductors are popular in this space because they can deliver a large fraction of the power they draw. This fraction hinges on the power that switches, diodes, resistances, and capacitances need to conduct and transfer power to the output. So, understanding how these loss mechanisms set and dictate efficiency across power levels is important, especially when designing and targeting particular load levels. This article details how these losses scale, when they dominate, and how and when they balance. Gate drive and controller losses are the ones that become a smaller fraction of input power as output power increases, leading to the increase of the power efficiency at the low-end of discontinuous conduction scale, while only gate charge loss plays this role at the low-end of the continuous conduction scale. Ohmic loss is the one that reduces power efficiency at the high-end of discontinuous and continuous conduction scale as ohmic loss becomes a larger fraction of input power as output power increases. Power efficiency peaks in continuous conduction when ohmic loss and gate charge losses balance. Overlap and dead time losses, although still important, do not shape the power efficiency in continuous conduction mode. In discontinuous conduction mode, all losses play a role and efficiency peaks when they all trickily balance. With this insight, predicting and controlling when efficiency rises, peaks, and falls across loads are possible. The fractional loss analysis and the design insight that make this possible are new contributions to the state of the art.