Re: RE: [TCML] DRSSTC weird waveform behavior

[Steve Ward <steve.ward@xxxxxxxxx>]

With regards to the LR network on the feedback CT output: if you are using
clipping diodes, you will get non-linear behavior of the circuit, and it
will no longer provide a constant phase lead as it is intended to because
the current will start flowing through the diodes rather than the linear L
and R components, and this will cause the behavior to change.  The series
R/L network across the CT output causes the voltage developed across the CT
output to lead the current seen at the input of the CT.  This is quite
different than how i used CTs for my controller which your coil is based.
 In that case the voltage was simply in phase with the current, and produced
whatever voltage necessary to allow that current to flow in the clipping
diodes.  There is no way to add Ls and Rs to this circuit and have it work
properly.

Also, given the waveforms, it does look like an awful big voltage drop on
the bridge output, i suspect either a lot of stray inductance with the
measurement reference (where is your ground clip connected??), or a lot of
stray inductance from the IGBTs to the main bus storage capacitance, in
which case the bus inductance is really *part of* your primary coil
inductance, and thus shows some voltage drop.

Finally, and getting back to the original issue at hand.  Im not sure if i
ever posted the final conclusion on these voltage spikes due to late IGBT
switching.  Yes, i did experiment with more gate turn on resistance to slow
down the transition, and this was helpful.  But in the end it seemed that
these voltage spikes, caused by free-wheeling diode recovery, were so low in
energy that they posed no real threat to the IGBT.  So from then on, i used
a moderately low 5 ohm gate resistance and kept the switching speed up.

After seeing Finn's "Prediktor" controller use the lead compensator so
nicely, i developed my own, though have yet to publish it in any formal way:

http://www.stevehv.4hv.org/leadcomp/UD2_1revB/

And now the switch timing can be controlled much better, allowing the IGBT
turn on to happen really at zero current, which keeps the wheeling diode
recovery from causing big voltage transients.

Steve




> >
> > > Fra: Justin B <boshjack@xxxxxxxxxxx>
> > > Til: TCML <tesla@xxxxxxxxxx>
> > > Dato: Lør, 02. jul 2011 22:49
> > > Emne: RE: [TCML] DRSSTC weird waveform behavior
> > >
> > >
> > > Just giving an update to progress.
> > >
> > > currently, I have an inductor and resistor (1ohm - 100K) in parallel
> > > with my CT. Turning the potentiometer, does nothing at all. However,
> > > the inductor has helped. If any one can give feedback as to what im
> > > doing wrong with this POT, please let me know. because it does
> > > NOTHING. The pot is in working condition.
>
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