Easy step down converter

I am looking to build a step down voltage converter.it should first rectify a 230V AC supply to DC and step it down so that the max current u get is 350mA.This will be used to power up a series of POWER LEDs and will be kept constant with a CCS(constant current source).We could use a transformer but we want the power efficiency to be 90% or above.So does anyone have an idea about it?

So use an efficient transformer... efficiencies of 90+% are VERY difficult to obtain.

We actually want to use an SMPS,but we dont have an idea about the design considerations of it..and also,if u say so,how exactly will i use,or for that matter,design an efficient transformer?

https://www.electro-tech-online.com/custompdfs/2010/01/di131.pdf
https://www.electro-tech-online.com/custompdfs/2010/01/di74.pdf
https://www.electro-tech-online.com/custompdfs/2010/01/di173.pdf
I think this is above your level. Search around powerint.com to find more of this.