# SEPIC converter : why average secondary inductor is the output current?

I can't get the reason the why the average secondary inductor current is the output current. Simulation confirms it's true, as well as documentation. But I haven't found any clear explanation. Sorry if it seems obvious to you. Thank you.

A SEPIC converter can be simplified like this: -

Picture from here.

why average secondary inductor is the output current?

Average current is basically a DC current and we know that DC cannot pass through C1 or C2. C1 and C2 are functionally needed of course but they do not provide DC currents. Hence, the only source has to be L2 in the picture above: -

• Thank you for the answer. It makes things clear to me.I think I should also simulate a boost converter to see how true is the common approximation that the diode average current equals to the output current. For a SEPIC it seems not far, but inductor average current is a better approximation. Commented Jun 3, 2020 at 16:20
• .... if you've got such a clear explanation for the reason why using a coupled inductor divide the inductor ripple current by 2, I would be happy. But I think I should go through more mathematics. Commented Jun 3, 2020 at 16:24
• Sorry @inico I'm not sure what you are asking me? What do you mean by "divide the inductor ripple current by 2, I would be happy"? Commented Jun 3, 2020 at 18:04
• Thank you gain for your reply Andy. Don't bother, that was just a comment out of topic. About the fact that when using a coupled inductor rather than seperate inductors for a SEPIC converter divide by 2 the inductor ripple current. Getting the reason why is another thing I'd like to investigate. Commented Jun 4, 2020 at 17:16
• @inico is there any question about the validity of my answer or something about my answer that you don't understand? Commented Jun 4, 2020 at 17:31