What purpose do these components serve in this circuit?

What is the purpose of the capacitor and the inductor in this basic PWM circuit? Why does the capacitor need be there for the Va to equal Vin when the switch pole is in the up position? Same with the inductor, what does it do?

May be there to debounce the switch, or protect from damage (snub).

In general I would think the cap would prohibit rapid changes in voltage and the inductor would prohibit rapid changes in current when the switch poisiton changes (duh). So you are also correct that the cap would hold the voltage at the input level, maybe making the pulses a little more square.

As you can see, I'm sort of guessing too. I'd be interestied to hear the true answer. I'm sure somebody on here will know.

Edit: Since I can't see the input or the output, this may also form a low-pass filter when the switch is in the up position. Don't know if that matters, depends on the input. That could flatten your pulses.

Your circuit looks like a buck dc-dc converter.

There are several reasons capacitors are used. The electrolytic cap is there to provide a stable DC voltage to the switch. Output pulse width is calculated based on input DC voltage. If it changes during the pulse, an output error will result. Amount of capacitance required is determined by the output load and type of DC supply. For instance, single phase, 6, 12, 18 pulse rectifier, active front end, battery. The active front end would require the least amount of capacitance where the single phase rectifier would require the most.

Also, there will ALWAYS be impedance in the DC link to the switch. A snubber capacitor would be used to absorb the energy in the inductance and prevent large voltage spikes across the switch. Remember, V=L*di/dt. The di/dt of that switch is very large resulting in a very large voltage spike across your switch. If your switch is a FET, the large Vds caused by this inductive spike can destroy the FET. Snubber caps are usually ceramic or film type caps.

The inductor is there to allow the control circuit to either control the current or regulate the voltage. The inductor does the same thing in both cases, limits di/dt.

Forget about capacitor. That is not a real circuit, and Vin is a voltage source, and voltage source always have zero resistance - will you add to it capacitor (by the way, real capacitor will not help ideal voltage source - it has his own ESR) or not.

What about inductor - I am agree with 2 previous posts, but in my opinion that piece of circuit can be used for step-down, and also step-up, step-up-down and inverting SMPS.