DC/DC converter solar without current variation?
The voltage output of a buck converter will be constant, as long as there is enough input power to supply your load.  If there isn't, the output will either sag, or the buck will go into undervoltage lock-out and shut down.  With a switching regulator Power in = power out with some losses from conduction and switching.  

The current drawn will obviously depend on your load.  If the load is constant and the buck is in regulation, the current will be constant.  Not sure what you mean by "a large current interval".

The better way to use all the available power from a solar panel is to use an MPPT controller (Maximum Power Point Tracking).  This part optimizes the current draw from the panel over operating conditions to extract the maximum possible power from the panel.

If you Google MPPT you should be able to find lots of information on the subject.