This　article　proposes　a　variable　transformer　equivalent　turns　ratio　dual-active-bridge　(VTDAB)　dc-dc　converter,　which　aims　to　increase　the　efficiency　of　the　conventional　dual-active-bridge　(CDAB)　dc-dc　converter　over　a　wide　voltage　range.　By　changing　the　driving　logic　of　the　secondary-side　three　bridge　legs,　the　VTDAB　controls　the　voltages　across　the　dual　transformers　with　two　different　turns　ratios,　resulting　in　three　operating　modes　with　different　equivalent　turns　ratios　that　do　not　require　any　extra　components.　First,　the　principle,　mathematical　model,　and　zero-voltage　switching　(ZVS)　conditions　of　VTDAB　are　introduced.　Second,　the　turns　ratios　of　the　dual　transformers　are　designed,　the　boundaries　between　the　different　operating　modes　are　determined,　and　a　complete　control　strategy　is　proposed　with　the　aim　of　lowering　the　inductor　root　mean　square　(rms)　current.　Finally,　a　1-kW　prototype　with　a　voltage　gain　range　of　0.4-2.5　is　built.　Comparisons　are　made　between　the　maximum　attainable　transmitted　power,　the　inductor　rms　current,　and　the　ZVS　range　of　VTDAB　and　CDAB.　According　to　the　experimental　results,　the　VTDAB　maintains　an　efficiency　of　92%-98.3%　across　the　whole　operating　range　while　having　a　lower　inductor　rms　current　and　higher　efficiency　over　a　wide　voltage　range.　©　1986-2012　IEEE.