Isolated power supply

[eenaoo]

Hi,

The simple way to build an isolated power supply, the most common way, is to use a transformer with a number of secondary windings, i.e. one winding for each power outpout. A diode bridge followed by a voltage regulator is then used for each output and the ground is kept uniqe for each output.

I recently saw the Strymon Ojai, and got impressed. Here they seems to be using a switch mode power supply down to 24V (not sure if it is AC or DC, assume AC), then creating isolated outputs from this source. Are there any othe way to do this isolation without a transformer? Or is a method to use a number of small SMD 1:3 transfomers to create this isolation?

Regards Arne

[antonis]

Welcome Arne..!! 

Is there a chance to post a schematic/diagram of what you refer on..??
(your query about "isolation" is, more or less, confusing - at least, as far as it concerns separate circuits grounding..)

[eenaoo]

Hi,

With isolation i mean that each output are not connected together through a common ground, i.e they are completely independent to prevent any ground loops.

Arne

[amz-fx]

Murata, CUI, Recom and other manufacturers make small modules that isolate a DC source from the DC output. There is typically a driver circuit and a tiny transformer inside the module that provides the isolation. The small size of the module (and transformer) will limit the amount of current that they can output.  I have tested modules that provide 75 to 150 ma but higher current devices are probably available.

https://www.mouser.com/Power/DC-DC-Converters/Isolated-DC-DC-Converters/_/N-brwkv

regards, Jack

[anotherjim]

You can do it with many little transformers. One for each output.
This is essentially the same as the conventional multiple secondary power transformer. Once you raise the supply frequency with the switcher to ultrasonic ranges, you need much smaller transformer construction for the power.
If you look at the pcb's of most switchers, you can tell by the ground pours that it's really separated between HV input and the output with a small transformer in between providing the galvanic "safety" isolation. Extra outputs can have their own transformer and ground and probably aren't even regulated directly, but trusted to be close enough according to the transformer characteristics.

[EBK]

Once you start going down the many-transformers path, the just-plug-a-bunch-of-wall-warts-into-a-power-strip option might start to look elegant again. 

[anotherjim]

I mean really small transformers. They can look similar to ferrite core inductors, but often like audio/modem transformers.
.

[Rob Strand]

Are there any othe way to do this isolation without a transformer?

It's not really possible to get isolation in the true sense of the word.

Isolated supplies also lets you connect positive ground effects and negative ground effects.

If only have negative ground effects and you only want to isolate the ground then there are different schemes which will reduce ground loops without using full-blown isolation.

Or is a method to use a number of small SMD 1:3 transfomers to create this isolation?

Yes as anotherjim mentioned.

[eenaoo]

I found what I was looking for here:

https://www.strymon.net/effects-pedal-power-supplies-white-paper/

My PSU design so far has used a typical PCB mounted Toroidal transformer with 4 secondary windings this one except for 9VAC outputs

https://www.elfa.se/en/ring-core-transformer-15-va-5vac-noratel-aa-021506/p/15620067?q=*&filter_Category3=Toroidal+Power+Transformers&filter_Buyable=1&filter_Secondary+Voltage=4+x+7.5VAC&page=1&origPos=124&origPageSize=25&simi=99.9

Not sure if they are made anymore. Anyway I will instead look into this flyback type. Here is a pic of my PSU