Two power supplies in series; voltage changes on one...

I am putting two power supplies in series with each other in order to
obtain a higher voltage than either provides. If I measure the
potential across one of the supplies to be 1.15 V and the connect it in
series with the other supply, the voltage across the supply drops to
..75V. I am not measuring any current flowing between the supplies in
the mA range and have checked the potential right at the supply outputs
itself ruling out drops in the wires.

Any ideas what might be causing this drop? I can connect loads in the
2-5A range to the supply and the voltage fluctuates less than 10mV.

Thanks for any feedback,

David.

 

Operating two power supplies in series is a bad idea.
You can often get away with it if the power supplies are of robust
design and the designers considered operating them in series.

Things can look rosey until you have a current limit or a turn-on
transient. With foldback limiting, things start to go to hell rapidly.
I've seen limit cycle oscillations that exceed the total programmed
voltage. I once saw two power supplies in series to charge a battery.
A transient crowbared one.
That overvoltaged the second one causing it to crowbar.
Smoke ensued from both supplies as they shorted the battery.
No, it wasn't me...that's my story and I'm stickin' to it.

Number 1, don't do it.
#2, make sure your supply outputs are floating.
#3, make sure your outputs are reverse polarity protected.
A reverse diode helps, but if the supply design is crummy, even
a volt backwards can cause problems. The even slightly reversed input
can exceed the common mode range on the sense circuit and latch it up.
#4 make sure any overvoltage limiting settings are appropriate.
#5 make sure you've got fuses everywhere.
#6 see #1.
mike
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