24VAC Power Supply options

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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.

Re: 24VAC Power Supply options
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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

snipped-for-privacy@XYarius.com
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Re: 24VAC Power Supply options
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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

Re: 24VAC Power Supply options
snipped-for-privacy@aol.com (Dave) wrote in message
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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

Re: 24VAC Power Supply options

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look at power trends. They have pincompatible switcher replacement for the
old 78xx and 79xx linear regulators.

cheers



Re: 24VAC Power Supply options
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That's the name, Power Trends.  Aren't they the company that TI bought?

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: 24VAC Power Supply options

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the


Yes TI was hungry, PowerTrends was one of thouse that got eaten......
On the TI homepage I din't found those nice things again.




Re: 24VAC Power Supply options

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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.

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OK, so that's 20 Watts DC output.

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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 ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: 24VAC Power Supply options
snipped-for-privacy@aol.com (Dave) writes:
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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.

Re: 24VAC Power Supply options
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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

snipped-for-privacy@XYarius.com
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Re: 24VAC Power Supply options
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

Re: 24VAC Power Supply options
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There are two things I don't understand.  If you checked out the Power
Trends stuff, how is that not the simplest solution?  They are modules
that need no external components, AFAIK.  They are not chip level
devices that need coils and caps.  

The other is how are the higher freq parts more complicated?  Are you
talking about the compensation components?  Counting components may have
a value, but resistors and small caps don't cost much at all.  A reel of
5000 resistors costs maybe $15.  Caps are the same until you get above
0.1 uF or so.  The input and output caps need to be larger and start
costing some money (still under a buck), but the higher freqs allow
these to be smaller and cheaper.  Normally the chip is the most
expensive part regardless.

Am I missing something?  Are the slow, older converters just a lot
cheaper than the new high freq parts?  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: 24VAC Power Supply options
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Sorry - The power trends modules are also completely expensive (~30-60
bucks.)  The entire board costs about 60 bucks before this part.  A
non-starter...

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The National semi - low freq parts solution (~150kHz) was something
like 4-5 components on the order of about 5 bucks total.  Picking the
same input/output parameters from the TI site put together about 25
components (my "complexity" argument.)  I didn't even bother pricing
them out.

I can handle the big inductor on the low freq part.  It just looked
weird on the board.  Simple/cheap/but big.  The board's all through
hole anyhow (old equipment) so no biggie.

For other product, I'll probably use the TI solution (but will also
look into National because someone mentioned some higher frequency
parts that WebBench didn't know about.)  These other products will
need the smaller footprints because of more dimensional constraints.

I guess there is also the EMI problem you can create for yourself
using the low frequency parts.  Something else for me to think about.

Thanks again.

-- Dave

Re: 24VAC Power Supply options
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Can you give me the part no.  I thought that there were simple higher
Vin regulators that would do what you want with the standard arrangement
of components.  25 sounds very excessive unless this is a controller and
you need external FETs and diodes, etc.  


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I am not a power supply engineer, but I have had to do a couple for
myself.  I don't really know much about EMI, but I would expect the
higher freqs and smaller inductor would generate less EMI.  But then
again, the higher freq will be harder to deal with.  So it may be a
trade off.  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
We've slightly trimmed the long signature. Click to see the full one.
Re: 24VAC Power Supply options
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I didn't save the design.  I just downloaded the TI tool, input my
parameters, and out popped a schematic and BOM.  It did have external
fets, diodes, etc.  I'd do it again and report the part to you, but
it's on my machine @ work.

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Dunno.  Guess I'll find out during UL and FCC testing.  Gulp.

-- Dave

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