Robert Baer a écrit :
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.

It's more like Q1 is desaturating: look how vc gets back up to 20V at
t=200us while Q1 is still driven "hard" Ib = 140uA with a given 40-160
HFE range for the 2N3439... That makes for about 15mA max collector current.

Checking the +20V current will sort that out.


--
Thanks,
Fred.