Buck Converter - Output Current Ripple

Question

I have a follow up question to this question posted here Why dc component I must flow entirely through the load resistance R in buck converter?

I know now that the DC component of the inductor current flows to the resistor, creating a DC voltage V.

The AC component of the inductor current flows to the capacitor, creating a voltage ripple on the capacito voltage which centered around the DC voltage

However, the voltage across the capacitor (with ripple) IS THE voltage across the resistor. So how can we claim that the resistor has only DC current if it has the same voltage across it as the cap which has a ripple??

Answer 1

So how can we claim that the resistor has only DC current if it has the same voltage across it as the cap which has a ripple??

Technically we can't but, given that the ripple voltage might be 50 mV p-p atop an average DC voltage of (say) 5 volts, it's significantly less than an error of 1%. In fact this error might be significantly less than the DC error in regulation due to other components so, it's not really a big deal to assume this.

And, none of the DC current flows into the capacitor; only the ripple voltage atop the DC voltage causes ripple current in the capacitor (which is of course the same ripple current as the inductor (within reason).

You can easily prove this with a simulator; use a 5 volt DC source in series with a ramping up and down 100 kHz voltage of 50 mV and just look at where the currents flow.