A Transformerless Composite Step-Down DC-DC Converter with Wide Input Voltage Range

This paper is focused on a composite step-down dc-dc converter architecture capable of operating efficiently over wide input voltage range. The composite architecture consists of a transformerless fixed-ratio stage (DCX) and a non-inverting buck-boost stage, which provides regulation while processing a relatively small fraction of the load power. The DCX stage is based on a 4-to-1 transformerless stacked active bridge (TSAB) converter and a capacitively isolated 1-to-1 dual active bridge (DAB) converter. A high overall system efficiency is obtained over a wide input voltage range as the bulk of the indirect power is processed through the highly efficient DCX stage. A decoupled control scheme is presented for the composite system, which allows for fast output voltage regulation as well as efficient operation of the DCX stage close to its nominal conversion ratio. Experimental results are provided for a composite converter prototype delivering a well regulated 12V output voltage with a maximum output power of 120W, while the input voltage varies from 48V to 65V. Using 30V and 40V rated Silicon devices and small planar coupled inductors, the measured power stage efficiency peaks at 97.8% and is greater than 96% over the 48-65V input voltage range and the 30-100% output power range.