Smart Transformer (ST) is a promising technology, which usually requires a high frequency high voltage dc-dc converter. This paper investigates the application of a Quad-Active-Bridges (QAB) converter using multilevel bridges as a solution for the dc-dc stage of Smart Transformer. The focus of this work is to compare in terms of efficiency 2-level and 3-levels bridges in a QAB converter for ST applications. The considered power cell topologies are: H-bridge, Neutral-Point-Clamped (NPC), Flying-Capacitor (FC) and T-type (TT). The NPC and FC show as main advantage reduced voltage over their semiconductors, which allow the utilization of IGBT and diodes with lower forward drop voltage. Meanwhile, the HB topology shows as advantage reduced current through its device. Thus, a comparative study is performed in order to find out the most suitable topology for this application. Two scenarios are defined and the comparison and final results are presented for both scenarios.