This article presents a transformerless high step-down dc–dc converter based on a stacked active bridge (SAB) configuration. The SAB converter consists of series-stacked capacitively coupled inverter modules and parallel-connected rectifier modules. The nominal step-down conversion ratio is determined by the number of the inverter modules, while the output current capability scales with the number of the paralleled rectifier modules. SAB operating principles, including phase-shift control and soft switching, achieved through series inductors, are similar to transformer-isolated dual active bridge converters. It is found that the best size versus loss tradeoff is achieved by integration of the series inductors on a custom core. Furthermore, it is shown that how soft switching contributes to natural voltage sharing among the series-stacked inverter modules. The approach is verified by experimental results on a 400–48 V, 3-kW SAB prototype using GaN devices and featuring 400- power density. A flat efficiency curve is obtained with 99% peak efficiency, 97.5% full-load efficiency, and 98% efficiency at 20% load.