Transform is used to step up or step down the AC BUT it is not Possible to convert AC to DC.
however by using a center tapped step down transformer is used to convert AC TO pulsating DC.
by connecting filters to it u can get almost DC.
IN half wave and full wave rectifier transformer used for stepping down the AC not to convert AC TO DC
No transformer cannot convert AC into DC cause we giving an alternating supply to transformer and alternating flux are generate at primary side and this fluxes are traveling through the core comes in secondary side and alternating emf is induced so we can get only AC supply at secondary side.
If we want a DC supply we can use a rectifier converts AC into DC supply at the output of transformer