[sdiy] Using switching power supplies in series to get bipolar outputs?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Sat Nov 18 21:08:05 CET 2023


You often see different current ratings for the different voltage 
outputs on SMPSUs.  This is because the feedback for the voltage 
regulation mechanism is typically only taken from a single output, 
(usually the one with the largest power rating.)  And the 
cross-regulation of the other outputs is improved by winding their 
"smoothing" chokes onto a common iron-powder core to couple them 
together.  This trick typically means that all outputs have reasonably 
decent regulation for evenly distributed loads.  However, if you draw a 
lot of current from the main regulated output and almost nothing from 
the secondary "cross regulated" outputs, the voltage at these lightly 
loaded outputs will usually be a bit on the high side.  Some power 
supplies that use this technique stipulate a minimum load to prevent the 
voltages on these outputs rising too high.

Not all SMPSUs are made equally though.  Some of the better designs make 
use of things like mag amps or linear post-regulation to improve the 
regulation of the secondary outputs that might otherwise be quite slack.

Finally, connecting together two or more separate SMPSUs can result in 
audible tones due to beating between their slightly different switching 
frequencies which would otherwise be inaudible above 20kHz.

-Richie,



On 2023-11-18 19:18, Shawn Rakestraw via Synth-diy wrote:
> I have 2 of the RS-75 12V supplies connected together with the center
> grounded and it works fine. My only concern with these power supplies
> is that they may be a little noisy. If you really crank the gain up (I
> mean a lot!) you will hear the whiney switching sounds that they make.
> It's not a complete deal breaker because I have not replaced them, but
> something to consider. Also, so few modules use the 5V, it's easy to
> just plug a 5V converter into a busboard if you have a module that
> needs that.
> 
> Shawn
> 
> On Sat, Nov 18, 2023 at 12:37 PM Oren Leavitt via Synth-diy
> <synth-diy at synth-diy.org> wrote:
> 
>> That will work as long as the + and - outputs are floating, isolated
>> from ground. The center tap can be grounded in your system.
>> On 11/18/23 8:08 AM, Scott Bernardi via Synth-diy wrote:
>> 
>>> I've noticed that most switching power supplies that offer bipolar
>>> outputs(i.e., to get +/-12v) have less current capability on the
>>> negative supply than the positive.  For example, the Meanwell 65B
>>> which has +/-12v and +5v, great for Eurorack, has
>>> +5v max 8 amps
>>> 
>>> +12v max 3.5 amps
>>> 
>>> -12v max 1 amp
>>> What about buying two +12v supplies, or maybe one that has +12v
>>> and +5v with another with just +12v, and hooking them in series
>>> with a center tap to get +/- 12v?
>>> 
>>> This article
>>> 
>> 
> https://www.mouser.com/pdfDocs/blog_how-to-operate-parallel-and-series-connection.pdf
>>> looks like it's possible, although they only talk about hooking
>>> them up in series to increase the output voltage rather than
>>> bipolar.
>>> Has anybody done this?
>>> 
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