Transformers are powerful electrical components which enable the conversion of a sinusoidal voltage into another sinusoidal voltage of the same frequency, but with a different amplitude.

The physical principle of induction, which is the basis of the transformer, does not work with direct current (DC).

A chopper or Buck converter is analogous to a transformer, but it does work with direct current. A voltage, V, is reduced (stepped down) or increased (stepped up) to a different level of direct current.

We call this a DC-DC converter, as opposed to the transformer, which converts from alternating current (AC) to a different alternating current (AC-AC).

The chopper  (Buck converter) underlies  switching power supplies  and the speed controllers for direct current machines.