A buck converter is a voltage step down and current step up converter. It is used in SMPS circuits where the DC output voltage needs to be lower than the DC input voltage. The DC input can be derived from rectified AC or from any DC supply. It is useful where electrical isolation is not needed between the switching circuit and the output, but where the input is from a rectified AC source, isolation between the AC source and the rectifier could be provided by a mains isolating transformer.
The basic operation of the buck converter has the current in an inductor controlled by two switches. All the components are considered to be perfect in the idealized converter. The switch and the diode have zero voltage drop when on and zero current flow when off and the inductor has zero series resistance. Further, it is assumed that the input and output voltages do not change over the course of a cycle.
The switching transistor between the input and output of the Buck Converter continually switches on and off at high frequency. The circuit uses the energy stored in the inductor to maintain a continuous output, during the on periods of the switching transistor, to continue supplying the load during the off periods. The circuit operation depends on what is sometimes also called a Flywheel Circuit. This is because the circuit acts rather like a mechanical flywheel that, given regularly spaced pulses of energy keeps spinning smoothly at a steady rate.
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