We've got a 800V dc supply in the lab. We needed to test something at
900V so we series connected the 800v supply on top of a 100v supply. The 100v supply was on the bottom and the neg lead for that was referenced to earth. The 800v supply connected neg to pos of the 100v so that the pos output of the 800v supply was at 900v relative to earth. Things seemed fine. At least in the sense we checked out what we wanted to see with 900v applied without any troubles. We took out the 100v supply and hooked things up "normally" again. Later in the day, we had some problems with noise in the prototype unit we are working on that was not there before. This morning I started looking into it and the problem is coming from the 800V supply. Its common mode noise and is everwhere in our board. If I use a battery powered scope, I do not see it, but a regular earth grounded one I do. I then put the regular scope on a isolation transformer to isolate it from earth and the problem is not as bad. So, now I finally check the manuals for the 800v supply to confirm it was OK to stack up supplies to get more voltage since I was told by the mfg I could do it. I find out the way I was supposed to do it was to connect the supplies up differently than a simple stack so that the output terminals of the supply never get >800v from earth. Not what I did since I had 900v the way I did it. So now I am thinking maybe I damaged some filter capacitors or the like in my 800v supply. Other than this problem, the supply seems to be working OK.Thoughts or comments? I am not real familiar which what may be in the supply. Its a MagnaPower Electronics switching supply and runs off 208
3phase. Rack mount unit PQD series.Thanks, Jeff