Efficiency of Transformer, increasing output current

In a standard (low voltage eg 40V)transformer - same number of input
turns as output turns, should the effiency go up with increasing the
output current or down?

 

[PeteS]

The efficiency will eventually go down for a number of reasons.

1. Copper losses will increase due to P = I^2R. Increase the current
and you increase the losses in the windings (notably by a square
factor).

2. The input and output windings will heat up, thereby increasing their
resistance (copper has a positive temperature coefficient). With a
higher resistance, you exacerbate the problem in (1)

3. Core and eddy current losses in the magnetic core will increase.
Increased output current (which implies increased input current) will
increase the magnetic flux density. As you increase it, the losses in
the core will increase, up to magnetic saturation, where you can
effectively get no more current (the maximum energy through a
transformer is limited by the magnetics as well as the winding limits).

That's a simple overview - there's a lot more to it, but those are the
highlights.

Cheers

PeteS