Why does the Cuk converter need an input inductor?

[Chacabucogod]

Hi,

I'm currently studying the topology of the cuk converter and I'm wondering why do you hhave to add that first inductor to the topology? Can't you just charge the capacitor straight through the voltage source?

Thank you.

 

[analogdesign]

If you didn't have the input inductor and connected the voltage source directly to the capacitor you would have a very large current spike. The first inductor acts like a current source so it kind of smooths the current going into the capacitor. This reduces loss.

So, to summarize, if you connected the voltage source straight to the capacitor it would still work, but it wouldn't work very well.

A useful exercise would be to simulate it in LT Spice or something similar. It's a simple circuit. Make sure you put a small output resistance for the voltage source (some fraction of an ohm). You'll see there is much more variation in the current without the inductor than with.

 

[Chacabucogod]

Mhh... I get it! So the large current spike would dissipate in the resistance and you would lose a lot of energy right? Cool.

 

[zoki85]

Just imagine what would happen to the transistor if it is turned on and put directly across the voltage source...

Edit: circuit drawing :)