What's the 'standard' way to generate +/-15v from +15v?
« on: January 14, 2021, 05:04:55 AM »
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.
« Last Edit: January 14, 2021, 09:14:07 AM by FETlife »


gyraf

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #1 on: January 14, 2021, 05:46:55 AM »
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.
..note to self: don't let Harman run your company..

squarewave

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #2 on: January 14, 2021, 09:21:42 AM »
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.

JohnRoberts

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #3 on: January 14, 2021, 10:02:57 AM »
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.
Quote
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.
Quote
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
It's nice to be nice....

abbey road d enfer

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #4 on: January 14, 2021, 10:05:32 AM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #5 on: January 14, 2021, 10:52:24 AM »
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

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #6 on: January 14, 2021, 10:56:55 AM »
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

abbey road d enfer

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #7 on: January 14, 2021, 11:53:22 AM »
Something along the lines of fig.2 at this link? https://sound-au.com/project192.htm
That is a possibility.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

volker

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #8 on: January 14, 2021, 12:00:06 PM »
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.

abbey road d enfer

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #9 on: January 14, 2021, 12:24:32 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Newmarket

Re: What's the 'standard' way to generate +/-15v from +15v?
« Reply #10 on: January 14, 2021, 06:52:24 PM »
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...


 

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