What's the 'standard' way to generate +/-15v from +15v?

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FETlife

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Jun 5, 2014
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48
Hi all,

I've always just used mains and a linear power supply but now need to design something to use a DC wall wart. I assumed there would be a selection of DC-DC converters that could invert +15v to give me -15v at half an amp or so, but it seems I was wrong, unless I'm looking for the wrong thing? If anyone has done this, part numbers would be much appreciated.

How do the manufacturers do it? I'm guessing they're not doubling the +15v to 30v then splitting the rails? Are they using standard switchers in a non standard way somehow?

I realise Meanwell and others do prepackaged modules that would work but at $20+ each is too much.

Thanks.
 
The standard way for wall-wart'ing was to supply AC to the unit, then split this to + and - inside the unit. attached.

recently the switch-mode takeover has made AC wallwarts relatively expensive, yes

the 20$ meanwell seems to be a good offer, unless you design for big amounts and thus can afford to put real money and time into designing DC/DC conversion

/Jakob E.
 

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The typical way these days is to have a wall-wart that is actually an AC/DC SMPS (although "wall warts" used to be transformers and sometimes still are) and then there's a barrel jack that delivers that single voltage of 12V or 24V or whatever and then inside the unit there's a DC/DC converter to make whatever voltages the circuit needs.

How much power do you need? It is important to match the SMPS power to the required load. That is one of the gotcha's of SMPS. There are smaller DC/DC modules that make +-15V that are cheaper. But once you get to 500mA or so it's going to be ~$15 at least which is really a bargain IMO. But at full load the voltage of those modules will actually dip below 15V by quite a bit. It might only be 13 volts or so.

An alternative is to buy two modules of say 24V, "stack" them in series and use the negative output of the upper connected to teh positive output of the lower to make ground and then regulate the 24 down to 15.

If you're serious about manufacturing something in quantity, you need to design a DC/DC converter to make the exact voltages you need. The key to that is the right IC, a special SMPS transformer and a very very careful PCB layout. Then you get probably make it cheap. But this last option requires a lot of testing and probably 2-3 iterations of PCBs to get it just right.
 
FETlife said:
Hi all,

I've always just used mains and a linear power supply but now need to design something to use a DC wall wart. I assumed there would be a selection of DC-DC converters that could invert +15v to give me -15v at half an amp or so, but it seems I was wrong, unless I'm looking for the wrong thing? If anyone has done this, part numbers would be much appreciated.
There are two ways, a DC to DC switcher, or a charge pump while the charge pumps are generally for lower current applications.
How do the manufacturers do it? I'm guessing they're not doubling the +15v to 30v then splitting the rails? Are they using standard switchers in a non standard way somehow?
Manufacturers are not generally constrained to operate from +15V DC power source. While automotive is close.
I realise Meanwell and others do prepackaged modules that would work but at $20+ each is too much.

Thanks.
Many ways to skin that cat... are you stuck with only using +15v DC input?

JR
 
FETlife said:
How do the manufacturers do it? I'm guessing they're not doubling the +15v to 30v then splitting the rails?
Actually, that's often how it's done. There are multirail wall-warts, but they are custom-built and tend to be expensive when available.
Splitting the main rails is not a problem, but adding 48V is an issue.
For multirail use, a DC/DC converter is the best solution, however the standard ones do only +/- rails, and tend to be quite expensive.
 
abbey road d enfer said:
Actually, that's often how it's done. There are multirail wall-warts, but they are custom-built and tend to be expensive when available.
Splitting the main rails is not a problem, but adding 48V is an issue.
For multirail use, a DC/DC converter is the best solution, however the standard ones do only +/- rails, and tend to be quite expensive.

Something along the lines of fig.2 at this link? https://sound-au.com/project192.htm
 
JohnRoberts said:
There are two ways, a DC to DC switcher, or a charge pump while the charge pumps are generally for lower current applications. Manufacturers are not generally constrained to operate from +15V DC power source. While automotive is close. Many ways to skin that cat... are you stuck with only using +15v DC input?

JR

Many manufacturers do choose to though. It's ultimately my choice but with the space constaints and with AC wall warts seemingly going obsolete, I think it's probably my best option.

Having looked around a bit more I've found these, maybe I ought to find out the IC and copy it.
https://www.ebay.com/itm/5-30V-to-5V-6V-9V-10V-12V-15V-24V-DC-DC-Boost-Buck-Converter/352817645972?hash=item5225920994:g:wrgAAOSwg8xb4Hp4
 
FETlife said:
How do the manufacturers do it? I'm guessing they're not doubling the +15v to 30v then splitting the rails? Are they using standard switchers in a non standard way somehow?
They don't rely on ready-to-use modules, but design themselves using the proper ICs.
Or you look for a module with isolated output and connect the positive output to your ground and use the common terminal as negative output.
 
volker said:
Or you look for a module with isolated output and connect the positive output to your ground and use the common terminal as negative output.
It seems the ones that are commonly available are either very expensive or have not enough power, typically 1 or 2W.
 
FETlife said:
Many manufacturers do choose to though. It's ultimately my choice but with the space constaints and with AC wall warts seemingly going obsolete, I think it's probably my best option.

You might be better off by implementing your own 'wall wart' = or as I prefer an 'inline' AC supply. It's basically simply a Mains in transformer with relevant protection etc. Not particularly convenient to implement - but if you're DIYing then not really a big deal ?

I've used stacked isolated DC/DC modules for similar - generating +/-15V from a single 5V rail using Newport/C&D/XP etc type 'black blocks' but at the power levels you are talking about it probably still comes to over £15 (I'm in UK) or so ?.Plus maybe some cost (+ pcb estate) for filter components...
 
Thanks all, I tried the search on Digikey as suggested by volker and found more or less what I was looking for in the first place and was left wondering why I didn't find these parts on Mouser.

Well, using the LT1931 as an example, on digikey the output voltage is specified as a negative but on Mouser as a positive, so searching for inverters with a positive output voltage gives plenty of results. This is probably obvious to most but you don't know what you don't know, hopefully this will help someone in the future.
 
These things can be wired in all sorts of ways to make positive or output. The output is isolated so you can make the positive ground to make a negative voltage or make the negative ground and get a positive voltage. But data on supplier websites is also sometimes just wrong. LT1931 looks like it's specifically designed to make inverting.

However, if you're going to make your own circuit, the 'standard' way to do that would be to have one IC make both. If you're going to design something, you might as well make three voltages: +15, -15 and +48 for phantom. Feedback to the IC only comes from one voltage but if you care you can go higher and regulate down.

Also, LT1931 is very low power. You might get only 100mA out of it. If that's all you need, then why not use one of the dual output MeanWell supplies?
 

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