# 15 VDC to +/-12 VDC - What's to most simple solution?

#### galingong

Joined Mar 13, 2016
6
Greetings,

I'd like to create a PSU, input voltage is 15 VDC, and I need to have regulated +/-12 VDC on the outputs. I'd like to oversize it a bit, so the unit should be able to handle 2A output current on both + and - rails. If that's too high and makes things more complicated, then at least 1-1.5A per rail.

What's the most simple solution to design such a power supply? I have experience designing and building simple units (using rectifiers and linear regulators), but I guess I'll need a more complex circuit for this.

#### Marcus2012

Joined Feb 22, 2015
425
Hey welcome to AAC! I haven't had a need to use a -12V rail for more than 1A so I use my ATX -12V output. I'd suggest thinking about using one of those if you don't require more than that. If you want to go ahead and try and make one I would suggest something simple as it's relatively low current and a low voltage drop. LM317 and LM337 can be used in conjunction to provide complementary outputs but unfortunately require a negative input. This application would be much simpler with a rectified AC retaining it's original polarity in respect to 0V.

As for creating a negative voltage from a positive input TI have published this application note on the subject which is probably more useful than me in explaining it

Creating an Inverting Power Supply Using a Synchronous Step-Down Regulator

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#### ian field

Joined Oct 27, 2012
6,536
Greetings,

I'd like to create a PSU, input voltage is 15 VDC, and I need to have regulated +/-12 VDC on the outputs. I'd like to oversize it a bit, so the unit should be able to handle 2A output current on both + and - rails. If that's too high and makes things more complicated, then at least 1-1.5A per rail.

What's the most simple solution to design such a power supply? I have experience designing and building simple units (using rectifiers and linear regulators), but I guess I'll need a more complex circuit for this.

For the sort of current and split supplies, you probably need a push pull. The simplest is 2 transistors like an astable driving a centre tapped primary. Centre tapped secondary makes the split outputs easy. Keep the frequency low and you can use standard recovery diodes, but you'll need bigger smoothing capacitors.

There's schematics online - the Royer inverter is one variant.

#### Marcus2012

Joined Feb 22, 2015
425
Can you get a 240/12V (or 120/12V) centre tapped secondary transformer to supply the initial input? It would be much simpler to gain the output you desire.

Basically how this guy has done it

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#### galingong

Joined Mar 13, 2016
6
Thank you for your suggestions, I'm going to do my research based on them. At first glance the TI doc describes pretty much what I need.

#### galingong

Joined Mar 13, 2016
6
Can you get a 240/12V (or 120/12V) centre tapped secondary transformer to supply the initial input? It would be much simpler to gain the output you desire.

Basically how this guy has done it

This is how I do it usually, I have built several devices using traditional center-tapped transformer and linear regulator, but for this one unfortunately I'll have to stick to 15VDC input voltage.

#### AnalogKid

Joined Aug 1, 2013
10,556
The TI app note is an excellent introduction to a common technique. Keep in mind that the total voltage across the circuit is 27 V, which limits your choice of controllers. Linear Technology makes the LT1070 and LT1270 series of single-chip boost devices. Their buck-boost circuit is weird-looking, but works.

ak

#### galingong

Joined Mar 13, 2016
6
How about the LM2596 (-12 or ADJ version)? According to the datasheet it is applicable and it should be fine with ~1.2A load if I read Fig.38 right.

#### Sensacell

Joined Jun 19, 2012
3,257
Simplest- buy a multiple output DC-DC converter puck.

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#### galingong

Joined Mar 13, 2016
6
Simplest- buy a multiple output DC-DC converter puck.
This is brilliant (just as expensive though),but - correct me if I'm wrong - I'll need +18VDC input voltage for this to work.

#### AnalogKid

Joined Aug 1, 2013
10,556
RP40-2412DGW - this is shown as 9V-36V input, so it should work. All high density DC/DC converters are expensive compared to discrete designs, and multi-output parts are even worse because they are not nearly as popular. If the +15 source is stable then a LM317 or LM350 will give you the +12 output with no magnetics, and probably *better* efficiency than a low-cost switcher. A compromise is to buy a single-output isolated converter for the -12 rather than build a non-isolated one.

Back to your question, National's Simple Switcher line is very well regarded.
IF you can get by with only 1.2 A
AND IF you want to build rather than buy
THEN it is a good option. The LM2678 has a 5 A switch, so it probably will run cooler and be able to give you more output current.

ak

#### galingong

Joined Mar 13, 2016
6
Thank you for your help, much appreciated. 1.2A should be fine, but I'll do the math again to see.

The 2678 has no inverting application example in the datasheet, but I assume if I connect it's Output Pin to GND, and place some additional diodes (as with the 2596), I will get -12V on it's GND Pin right?

#### Sensacell

Joined Jun 19, 2012
3,257
This is brilliant (just as expensive though),but - correct me if I'm wrong - I'll need +18VDC input voltage for this to work.
Another idea is to get TWO single output ISOLATED converters and connect the outputs to produce the + and - voltages.