I was doing an experiment on buck converter for a particular resistive load. As calculating the value of gain and respective efficiency i saw that from duty cycle 0.1 - 0.9 converter gain change drastically as 0.109, 0.199, 0.3, to 0.9. Matching with the duty cycle. However in terms of efficiency it increases faster (66.94%, 79%, and 88%). After reaching 50% duty cycle efficiency does not increase that much like 0.3 or 0.4. Why is that??? and what is the relation between gain and efficiency of buck converter when duty cycle changed?

Kindly explain it elaborately so that i can understand the basic concept.

 

The efficiency is affected by the fixed power and variable power consumed by the converter.
Since the fixed power does not vary with duty-cycle/load, it will cause the efficiency to go down as the load/duty-cycle are reduced.
The variable part of the power consumed generally goes up with load (due mainly to the switching losses, and switch and inductor resistance), causing the efficiency to level off or even go down at high loads/duty-cycles.

So the relation between load/duty-cycle and efficiency depends upon the values of the fixed and variable power consumed by the particular converter you are using.