I think maybe what your question is not correct.
We want the power supply has a possible high efficiency,But if the power supply have a high efficiency at light load,but the transformer get saturated during full load,that is not acceptable.
I think :
1:it is necessary that the transformer will never be saturated under any load and input condition(that include overload,unless your power supply has a very sensitive current detection function;)
2:At full load,the unit have possible high efficiency.
