[sdiy] switched mode PSU design

[Mark Smith]

>Try the "Topswitch" family of ICs (available from Farnell - just search for 
>"topswitch" on their website). They contain pretty much an entire off-line 
>switching PSU on one chip, in DIL8/TO220 etc. depending on power rating.

Power integrations do a full dev kit and supply all the software for doing the basic transformer design. It's easy to wind your own transformer and the devices are pretty forgiving when you get it wrong, but laying your hands on the cores may not be as easy.

If you can blag samples from TDK, etc then you have to wind them, do the drawings, order samples, test them, and then order ready made transformers in reasonable quantity.

but there is an easier and safer way...

If you are looking at switch mode for efficiency and you do need dual rails, then it's much easier and safer to buy an off the shelf type approved supply. Then make a small DC/DC switching supply that is a low voltage input buck/boost using a single chip device from TI/Linear tech/Maxim, etc. These are far easier to build as they use standard radial inductors, diodes, etc and there is no mains !

This ends up being cheaper for small runs and much easier to work on, achieving dual or triple regulation cheaply from a mains switcher can be tricky - you also need to understand the magnetics quite well and then approvals and emc, etc.

The simple DC/DC circuits can all be adjusted very simply for output voltage/current, etc, and they are generally fairly quiet as far as radiation goes - unless you opt for a super efficiency high speed switcher (1MHz), but these are only really necessary when there is not enough space for a large inductor.

So doing the conversion twice is more expensive, but safer,easier and still quite efficient.

mark