Transformer dual secondaries for independent power supply channels?

[ManuelMcLure]

Hi, all, first time poster here.

I'm working on designing an independent dual output lab power supply - 2 floating channels with 0-20V per channel/0-1A per channel based on Dave's PSU design, with I2C control through opto-isolators to an MCU to ensure no connection between the two channels (unless I tie them together myself). It will be fully linear so I need a decent transformer to run it. So I went to DigiKey and looked for a mains transformer with dual 18-20V secondaries and found the Triad Magnetics VPP36-1560 (http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=230900071&uq=636350446503474458) which seems like it would meet the requirements. But when I looked at the datasheet (http://catalog.triadmagnetics.com/Asset/VPP36-1560.pdf) I saw the following warning:

"Primary and secondary windings are designed to be connected in series or parallel. Winding are not intended to be used independently."

Upon looking at other power transformers, I see similar warnings for many of them.

My question is - how much of an issue is this? Can I assume that if a transformer does not have this warning it's OK to use for my purpose? Should I just go for either a -20/0/+20 non-isolated power supply, or use two independent transformers?

Of course, I could just go buy a DP832, but what's the fun in that?

Thanks!

[Zero999]

The warnings are there because the insulation between the windings doesn't provide adequate protection against electric shock. You can make two separate power supplies with the two windings but the insulation will only be good enough for 100VDC or so and if one power supply is floating at a hazardous voltage, then the outer power supply should be presumed to be a shock hazard.

[ManuelMcLure]

I think I understand. So as long as I don't have either of the power supplies floating at a hazardous voltage I'd be OK, right? Is that why supplies such as the DP832 have the "+/-240VDC Max to Ground" warning? From Dave's tear down there only seems to be a single transformer so I would expect the same sort of issues would apply...

[Zero999]

I think I understand. So as long as I don't have either of the power supplies floating at a hazardous voltage I'd be OK, right? Is that why supplies such as the DP832 have the "+/-240VDC Max to Ground" warning? From Dave's tear down there only seems to be a single transformer so I would expect the same sort of issues would apply...

Yes that's it.

One potential problem could be using the power supply to power a non-isolated, high voltage boost converter. If the high voltage output becomes earthed, or connected to the other power supply, then it could damage the transformer. Connecting the output of the power supply to earth would help to prevent this but it's not something you'd want permanently wired in. The usual solution is to provide a separate earth terminal, connected to the chassis and mains earth/ground conductor.

[endevor100]

You will could also run into weird stability and heat issues if you run the coils independently if you have noticeably different currents running through the individual coils. Depending on how you wire the primary it might work for a prototype or proof of concept, but you want better results from your lab bench supply.

[Zero999]

You will could also run into weird stability and heat issues if you run the coils independently if you have noticeably different currents running through the individual coils. Depending on how you wire the primary it might work for a prototype or proof of concept, but you want better results from your lab bench supply.

I don't see who this can happen. Please give more detail.

[anishkgt]

I initially had that question as well when I had designed my second pcb for a Spot welder. My first basic version has the link below.

I initially had them connected only in series and then latter followed the advise from bill at triad magnetics as he said connecting them parallel improves efficiency of the transformer and using to its full potential. That was for TRIAD and I used the same idea for the BLOCK brand of pcb transformers.

www.georgehobby.wordpress.com


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