A transformer cannot convert AC to DC or DC to AC. The transformer has the ability to step up or decrease current. A step-up transformer is a transformer that raises the voltage from the primary to the secondary

Transformers only work on alternating current (a.c.). The current in the primary coil causes it to become an electromagnet. The continually changing current produces a continually changing magnetic field in an iron core.

Transformers only work for alternating current, because you need a changing magnetic field. The size of the induced voltage depends on the number of turns on each coil – if there are less in the secondary coil, the voltage falls, and if there are more in the secondary coil, the voltage rises.