transformer works on principle of faraday's law of electromagnetic induction. So if D.C supply use, no indction are produced and due to this E.M.F not induced in winding and transfer of electrical energy is not possible.
In step up votlage, Output votlage > Input votlage Output current < Input current
In step down transformer Output votlage < Input votlage Output current > Input current. Power and frequency remains almost constant in stepup or stepdown transformer.
EXAMIANS