Back to the OP questions.
Questions :
1. What is the best topology . Is push/pull the best for this
Since you want to isolate, which requires using a transformer, an buck-derived, boost-derived, or flyback is possible. So, at 50 watts you could use any topology - flyback, forward, push/pull, full bridge. Choice becomes one of overall efficiency, complexity, cost, EMI, and such.
2. Has anyone got a design that could used or simply modified for this application
First – it’s best to check the design examples and application notes from the manufacturers out there. Texas Instruments (who also has the formerly National semiconductor and Unitrode parts and design examples/tools), Linear Technology, Micrel, etc.
3. How can it be done to minimise the cost ?
Minimization of cost is a very iterative process of paper design-analyze cost, paper redesgn-reanalyze cost, analyze-tradeoffs, finally working up to a working design. For minimum cost you will probably find a flyback topology is best, but a flyback will have the worst efficiency and EMI.
For the topologies I listed in question 1, they go from (generally) lowest efficiency to highest efficiency, lowest complexity to highest complexity (which translates into cost), and highest EMI to lowest EMI.
4. Is there an off the shelf transformer i could use or modify ?
There are winding companies out there that make transformers for the design examples that the IC companies publish. There are transformers made by companies like Pulse that IC companies will publish an example built-around. Otherwise, almost all DC/DC SMPS when you begin to build more than few bench items get into designing a custom transformer, which means conquering the skills of magnetics design. Only when doing AC/DC designs at 50Hz or 60Hz do you find a large supply of ready-made transformers. For switch-mode, there are only a few pre-designed transformers available, and some of the ones that vendors advertise are really non-available 'examples' that they publish as a starting-point for discussions on doing a custom. However, 50 watts is a common power level, and you Vin/Vout is fairly standard so there might be some off-the-shelf available.
Don't overlook the online tools and downloadable software that is out there that is available at Linear or TI (National). You may be able to plug-in your numbers and have it pop-out a design complete with BOM.
Of course, I will point out page one of Linear Technology application note AN73. Do your sanity check and proceed if you must. Doing SMPS design is not just another project to get out-of-the-way, it is, in itself, a complete and all-encompassing career-path if you want to do it well.
I just love pointing people to AN73 page 1 every time this type of question comes up on Eng-Tips.
