Re: 24VAC Power Supply options

From: rickman (spamgoeshere4_at_yahoo.com)
Date: 02/20/04 

Date: Thu, 19 Feb 2004 23:44:30 -0500

Dave wrote:
>
> Hi all.
>
> I'm looking to replace a 24VAC power supply. Currently, the board
> takes 24VAC with a center tap. Piece of cake taking the + and -
> sides, rectifying them, sending them to 7812s, 7805s and 7912s, and
> away we go. I need to convert to one that does not use a center tap.
> I've eliminated the negative rail needs of the board, so now just need
> +12/+5 VDC outputs. The current requirements are ~ 1.5A on the 12V
> line and 500mA on the +5.
>
> I've used the national semi webbench stuff and it puts a nice little
> switching supply together for me. The problem is, it sticks a big ol'
> inductor on there. This thing is a beast (like 1.5" by 0.75" by
> 0.75"). I can find a horizontal mount one, but it takes up a ton of
> board real estate.
>
> The linear regulator design gets to the Vin limits of a typical 7812.
> Max input voltage is 35VDC, and the full wave rectified voltage is
> about 36VDC (the actual transformer voltage varies between 19 and
> 28VAC.) Needless to say, that regulator gets too darn hot and
> eventually blows.
>
> I'm just a lowly firmware guy, so any hardware types have any answers
> to a smaller component version of that power supply? I'd really like
> something that didn't take up so much room and have such a high
> profile.

I don't have any specific recommendations, but I think an LDO (the 7812)
would be a problem. Dropping 20 volts at 1.5 amps gives you 30 Watts!
That is what an older Intel CPU dissipates and they use a fan on the
heat sink!

A switcher is what you want and they should not be hard to come by.
There are lots of newer inductors around that are smaller and better.
But the key to getting the inductor and capacitors smaller is to up the
switching frequency. I bet the designs you came across are running
below 200 kHz. Look for one that switches closer to 500 kHz or even 1
MHz. Try the TI site, I know they have some nice ones, but all the ones
I have looked at only work up to 6 volts in. TI also bought PowerOne or
someone similar who makes PS modules! No real design work, just pick
the input and output voltages and current and you are done! *That*
sounds like the way a firmware guy should go... ;) 

-- 
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design      URL http://www.arius.com
4 King Ave                               301-682-7772 Voice
Frederick, MD 21701-3110                 301-682-7666 FAX

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