We are designing SMPS
this is one of the easiest things to design and one of the most difficult things to make work.

Virtually every manufacturer (e.g. LT, Maxim) of chips for this purpose have appnotes galore and you will note that most (all?) such appnotes have the PCB layout as well. How may other appnotes have you seen with PCB layouts?. This illustrates the fact that for switchers design is easy, layout is a bitch. I was talking with a DC/DC converter company engineer the other day and a quote from him "after 5 years of doing this you will sometimes succeed".

If you "are designing SMPS" to become a power supply manufacturer be ready for an uphill battle where there may be great rewards at the end, if you "are designing SMPS" to save two bucks by not using a manufactured unit, give it up.

If AC powered have a look a Phihong (Mouser catalog), if DC/DC have a look at Power Trends (TI). I doubt you can save anything worth while by fighting your way through making your own.

Issues to be concerned with: stability, transient response, EMI, fail-safe and more.

Erik