I am designing a board that needs power rails of +3.3V, +5V, and +15V. I am wondering if I should get 3 on-board psu's with 24V inputs and power all of them from my external supply, or is it better to connect the power modules in series? For example the 24V powers the 15V which powers the 5V and finally 3.3V...
Does anyone know of any logic supplies that have all three output voltages?

Thanks.
Omer

Thanks for the reply. ATX is way overkill for what I need. I need about 10 watts max to power the board. I also can't decide if i should go with switching dc-dc converters, or linear voltage regulators.

The linear voltage regulators are much cheaper and they don't produce switching noise.

This looks very attractive but it is $50!!! So expensive, and I'm not sure why
V-Infinity | DC-DC Converters | PTK15-Q24-T515 : 15W, 5/15/-15V, 1500mA, triple output, isolated DC-DC converter

I'm surprised I can't find something more standard and cheap since it seems that the voltages I need are pretty common as far as rails go.

Thanks,
Omer

Linear regulators, as said above are going to make a lot of heat from 24v if you are going to pull more than about 40-50mA from them. (A big series resistor can help take the load of a linear regulator though, by dropping off some voltage before the regulator, although a bit of a crude method)
Switchers are much more efficient. If you are like me, and shy away from making your own switchers, you could look at the Recom website. They make switchers that can be directly fitted in place of a Linear item. More expensive than what Mneary suggested, but good for an easy one-off solution..

Thanks for all the help.
It looks like I actually need +15V and -15V as well. I will use a switcher for that and a switcher for the 5V. I found a dual output linear LDO from TI (TPS70451) that will power +3.3V and +1.8V for my DSP.

I plan on using switchers from TDK Lamda
DC-DC Converter CC-E Series - TDK-Lambda

Anyone have experience with these?
Again thanks, you guys are very helpful.
Omer