configuring logic power rails....

[opeleg]

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

 

[Sceadwian]

You could do it either way, it would influence the exact regulator you get as if you daisy chain them all the 5V and 3.3V current will be going through the 15V regulator first, however the 5V and 3.3V regulatars will dissipate less power because the 15V regulator is doing the brunt of it, it should increase ripple rejection as well, especially on the 3.3V line. Have you thought about using an ATX or micro ATX PC power supply? They're very standardized and do all the work for you. You can even get some pretty small form factors.

 

[opeleg]

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
**broken link removed**

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

 

[mneary]

To get 10 watts at 5V (2A) from 24V your linear regulator needs to dissipate 38 watts. Probably a big heat sink and forced air ventilation. Your 24V supply had to provide 48 watts (not counting the fan), which probably isn't free either. That's definitely not cheaper than a decent switching design.

An 85% efficient 10W switching regulator dissipates less than 2W, with minimal (if any) heat sink. The total 24V consumption is less than 12 watts, probably half the cost. You can build a simple switching supply for a few dollars - See National, TI, or Linear Tech. They all have design aids.

One of my old favorites for simplicity is the LM2575 series.

 

[jimmythefool]

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..

 

[Sceadwian]

How about switch mode for the +15V and linear for the other two?

 

[opeleg]

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