Buck Converter Analysis w/ Assumptions

Thread Starter


Joined Aug 13, 2009

I think I get the basic idea of how a buck converter works, but I'm having a little trouble. With the following assumptions:

1. ideal components
2. constant output voltage V_0
3. steady state operation
4. large enough capacitor to ensure constant output voltage

I am completely comfortable with generating the equations for the current in the inductor when you assume constant output voltage, so in CCM, the inductor current rises and falls linearly.

Most texts state that it is "obvious" then, that the average current in the inductor is equal to the constant current in the load. I'm not sure why this is so obvious. If we're assuming that the capacitor is large enough to maintain constant voltage across the load, then the current through the load (assuming it's a resistor) will be constant. So there must be current through the capacitor, but why is this current only the AC component of the inductor current? Why can't it have both a DC and an AC component? I thought capacitors only block DC when they're fully charged. I guess the main question is: why does the capacitor in this case block all DC current?

Thanks for your help.