i have researched how to design and build a flyback converter, most websites use ac as input to the flyback converter. they specify a minimum input voltage and maximum input voltage. what should i use as an ac source?
You are asking about off-line SMPS, which are comprised of a rectifier, a filter capacitor and a flyback converter. Unless you are designing a PFC flyback, you can power it with a DC source as well.
To test an off-line SMPS with varying input voltage, you can use a variac (variable transformer).