On Nov 7, 5:47 pm, Grumpy The Mule wrote:
I forgot to mention that I made the assumption it's a forward
converter that produces symmetrical output. I'm expecting it's
a half-bridge converter. Other topologies would require an
additional winding to the transformer.

73
Grumpy

Right, to pick up the negative supply you has to have a waveform that
has enough negative going cycle you might be able to pick up sum
ringing waveform and rectify it. Don't expect to gits -15 volts @ 25
amps.

If you're lucky you have a push pull front end. But I don't think
your -15 is going to be regulated.

73 OM

n8zu