John Larkin wrote:
On Sun, 15 Aug 2010 00:24:10 -0700, Robert Baer
wrote:

The diode across the E-B junction of Q1 was added, mostly to prevent
zenering and then beta reduction.
D1 is removed for the waveforms seen; it clamps the negative swing
seen at V3 (and the +20V supply current increases with reduced HV output).
The question is, if the voltage on C5 is changed from about +10V to
+50V, that negative swing DOES NOT change, but the positive swing of Vc
does.
Can anyone explain how and why?

This looks like it's operating as a forward converter, and it looks to
me like the transformer is saturating. If that's true, it would work
better if the ON time of Q1 were reduced.

If it's saturating, that would explain why the flyback voltage
(negative spike at TP2) isn't changing as the C5 voltage changes.

John



Ahh...will check that out; thanks.