What role do inductors and capacitors play in buck converters?
I know it acts as an LC low-pass filter.
What other role does it have? When the switch opens, does the capacitor discharge in the load? If it discharges then due to an increase in current, doesn't the voltage increase in load due to V=R*I?
The output also depends only on the duty cycle controlled by switching. So how do we know what value of inductor and capacitor to use?
The main role that inductors and capacitors play in dc/dc converters is energy storage. Otherwise you'd have to use a resistive device to regulate voltage or current (like an ldo) and that burns up power.
If you can store energy in an inductor or capacitor for short period of time it can be switched with a transistor. In basic terms this allows you to 'chop' the voltage and lower the voltage is kind of like turning an inductor capacitor into a variable resistor over time. The last problem is you must do this at the right frequency for a variable load to maintain the voltage so you need something to measure the voltage and then provide negative feedback to regulate the voltage and change the switching action. The last thing is you need to be able to ensure that the inductor capacitor does not get saturated. In addition you may also need to put an output filter on the buck converter to reduce the ripple created by the switching
