Current step to LC circuit
A buck converter is basically a pulse width modulated (PWM) voltage in series with a low-pass filter. The filter's output is (ideally) the DC average value of the PWM voltage:







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The PWM voltage is generated by switching a DC input voltage. When switch 1 is on, switch 2 is off, and vice-versa:







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The switching is commonly done with a MOSFET and a diode. The MOSFET is controlled by a PWM signal, which can be produced by an analog circuit (using a sawtooth wave and a comparator) or a digital circuit (using a counter):







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The low-pass filter could be an R-C filter, but that would waste a lot of power. An L-C filter is used instead because LC filters are (ideally) lossless:







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Finally, a feedback control system is added to compensate for component variation and improve the transient performance of the converter:







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And that's a buck converter. A common variation is the synchronous buck converter, which replaces the diode with another MOSFET to reduce conduction losses. Practical converters may have other features such as overload protection or soft start-up.

In a buck converter, the inductor is part of an L-C filter. But it's sometimes more helpful to think of it as an energy relay, especially when looking at other converter topologies. The basic idea is that when the MOSFET is on, the input voltage source stores energy in the inductor. When the MOSFET is off, the inductor releases that energy into the load. This is easiest to see in a buck-boost converter, where the input source and the load are never directly connected:







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In all cases, the capacitor is there to smooth the output voltage. The inductor current is not constant, so without the capacitor the output voltage would vary during the switching cycle. (In buck and buck-boost converters, the inductor isn't even always connected to the load!) It's not a snubber since there's always a current path for the inductor.