In fly back converter the primary inductance should be less to store more energy in core. This can be achieved either by increasing the air gap length and keeping the number of turns as constant or decreasing the no of primary turns without air gap in core. Decreasing the no of primary turns has advantage of decreasing the copper and transformer size, but why air gap is introduced in most of the fly back converter designs. So Where I am missing the concept..?

 

In fly back converter the primary inductance should be less to store more energy in core.

That is not a goal.

The purpose of a gap in the core is to increase the core reluctance and thus increase the point at which the inductor material saturates. This incidentally will reduce the inductance of the core for a given number of turns but that is not the reason it is done. The air gap helps because the inductance goes up as the square of the turns but the flux only goes up in direct proportion to the number of turns.

For example, suppose the air gap increases the core reluctance by a factor of two, so for the same mmf at a given current you can increase the number of turns also by a factor of two to get the same flux density. But this means the inductance goes up by a factor of four and so the energy stored at that current also goes up by that same factor.

The disadvantage is that more turns means more winding resistance, especially if you have to go to smaller wire to fit the extra turns into the available core winding space.

 

Most of the energy on a flyback transformer stays on the air gap. As you decrease the length of a gap, the energy tends to be located inside the core. To transmit the same energy to secondary inductance you have to increase the volume of the core.
MOR_AL

 

...................... To transmit the same energy to secondary inductance you have to increase the volume of the core.

I don't understand that statement. I don't see that the volume of the core has anything to do with the transfer of energy from the primary to the secondary (aside from any room needed for the secondary windings).