A non-isolated DC-DC converter topology which outputs voltage less than or equal to the input voltage

A buck converter is a non-isolated DC-DC converter topology used to step down voltage. It is one of the most fundamental converter topologies in existence.

The basic buck converter power train is comprised of a switch (usually a MOSFET or transistor), an inductor, a diode, and a capacitor.

Public-domain buck converter image from Wikipedia

The duty cycle of the switch determines the ratio of load voltage to supply voltage:

$ \frac{V_{out}}{V_{in}} \mbox = D $

An improvement on the buck converter is the synchronous buck converter, where the diode is replaced with another switch. This increases the efficiency of the converter but requires more complicated control.

Wikipedia: Buck converter