Yep, i silly answer because you didn't give another basic piece of information. Power availalbe and power required. I'm asking power available, just to make sure you understand that you can't make more power than you have.
You need to say: " I have 400 VDC @ ___ amps available and I need to convert this to 12 V @ ____" How can I do that?
Sorry for too less info . well I have boosted 220V 50Hz AC to 400V DC @ 80Watts and now I want to step it Down to 12V 30W. I know I can use a Buck or a Boost but wont it be unsuitable to just use these converter . I have a suggestion of using forward converter. In forward converter I want to know about transformer design and how to do switching ???
Most switch mode power supplies that are rated to 240 V AC work by rectifying the voltage to start with. They will be quite happy to run on DC. 400 V is a bit high as they will normally run at about 350 V after the rectifier.