Forums

24VAC Power Supply options

Started by Dave February 20, 2004
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.
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
daveoman@aol.com (Dave) wrote in message news:<26a6bac8.0402192010.18abcdcd@posting.google.com>...
> 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.
Yikes! Typical linear regulators a terrible for big step downs in voltage, especially at any significant current. The problem is basic physics, you are stepping 36VDC down to 12V. That means the regulator is dropping 24volts at 1.5 amps, or about 36 Watts! Now go look at the watt rating on your soldering iron! :) In a situation like this, you definitly need a switching regulator. I have seen some switcher supply bricks with most of the passive components built in. I think Lucent was one of the companies making these. Try ST and Maxim (amongst many others), they have all kinds switching supply ICs. I have used the L5973D from ST with good results, it has settable output from 5 to 35 Volts and comes in an SO-8 package. The physical inductor size that we use is much smaller that what you are describing as well. Good luck -J
"Dave" <daveoman@aol.com> wrote in message
news:26a6bac8.0402192010.18abcdcd@posting.google.com...
> 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.
look at power trends. They have pincompatible switcher replacement for the old 78xx and 79xx linear regulators. cheers
Dave <daveoman@aol.com> wrote:

> 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.
You didn't say explicitly: does the new one *have* to be a 24V one? Because those 24 Volts, or rather the rather large distance between that and the actual output voltages you need, are really the root of your problem.
> 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.
OK, so that's 20 Watts DC output.
> 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").
If you really think 0.85 cubic inches for a part that has to handle 20 Watts of electrical power qualifies as a "beast", maybe you'll have to re-check some physics. So let's conduct a little survey: go around your house and look --- is there *any* AC->DC power supply (wall wart or laptop power converter, e.g.) that comes anywhere near as small as 1 cubic inch in size? I doubt you'll find any. Your best bet by far may be to get away from those 24V input voltage of that power converter. You'ld be a whole lot better off with 15VAC for input, e.g. Even investing in a small-ish transformer to make 12 VAC from your 24 would already simplify things a lot, and allow you to stick with your simplistic design using linear regs. -- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
rickman <spamgoeshere4@yahoo.com> wrote in message news:<403590AE.3F28E1E2@yahoo.com>...
> 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... ;) > > -- >
Thanks Rick. Could the TI site by better for a firmware guy or what? Much nicer than National's. I did find another solution there that kept the size down, but added a bunch more components (guess that's probably the tradeoff.) But, their software gives you a complete BOM with part numbers and everything! Too bad they don't have a firmware site like that. Thanks for the pointer. -- Dave
Felix Bonjour wrote:
> > "Dave" <daveoman@aol.com> wrote in message > news:26a6bac8.0402192010.18abcdcd@posting.google.com... > > 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. > > look at power trends. They have pincompatible switcher replacement for the > old 78xx and 79xx linear regulators.
That's the name, Power Trends. Aren't they the company that TI bought? -- 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
daveoman@aol.com (Dave) writes:
> 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").
To use smaller inductors, you need a higher switching frequency. National has a newer line of their Simple Switcher chips, but when I last checked Webbench didn't support them. That was quite a while back, so it seems likely that they would support them now. Lately I've been using Linear Tech switchers. They make a wide variety of high frequency ones, and they have free downloadable design software that's fairly good. The name is LTCad if I recall correctly. It will come up with designs for you, and includes a specially customized Spice simulator that you can use to tweak the designs if they aren't exactly what you want. I was particularly impressed that they actually officially support running their software on Linux using Wine.
Eric Smith wrote:
> > daveoman@aol.com (Dave) writes: > > 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"). > > To use smaller inductors, you need a higher switching frequency. National > has a newer line of their Simple Switcher chips, but when I last checked > Webbench didn't support them. That was quite a while back, so it seems > likely that they would support them now. > > Lately I've been using Linear Tech switchers. They make a wide variety > of high frequency ones, and they have free downloadable design software > that's fairly good. The name is LTCad if I recall correctly. It will > come up with designs for you, and includes a specially customized Spice > simulator that you can use to tweak the designs if they aren't exactly > what you want. > > I was particularly impressed that they actually officially support running > their software on Linux using Wine.
Most of the LT parts are pretty good, but most of their switchers are not synchronous and require the use of a fat Schottky diode. Many also require a diode for the bias voltage boost circuit. Adding a couple of diodes starts to eat up real estate that other chips don't need. Otherwise, LT makes very good opamps. :) -- 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
Thanks to everyone that responded.  To summarize:

- Yup, I need 24VAC.  That's what is present on the incoming power
supplies I need to interface with.
- TI did buy power trends, and the software that TI puts up for power
supply design is nice.  Going with a high switching frequency makes
the inductor smaller, but also puts many more parts on the board. 
Price-wise, it's actually better to go with the national 2576 switcher
because it can actually support > 48VDC (which is a bonus for me).
- I checked on the LT parts also, but again, National's solution had
fewer parts.  Cost-wise, it works better.
- I still need to check through for those other higher frequency parts
that aren't supported in WebBench.

You guys rock.  I have the utmost respect for hardware folks.

-- Dave