RE: [TCML] DRSSTC weird waveform behavior

[Justin B <boshjack@xxxxxxxxxxx>]

Steve, When I do my scope work, I have the probe connected to the OUT (pin 3) from the CM300.  The black clip on the probe goes to the other primary connection. I use an isolation transformer so I can do this without any issues of that black clip, blowing up my bridge.  So that being said, I have no real mains ground being connected to the bridge, to ground any of the 0V lines if thats what you ment by "Wheres your ground clip connector"  If you ment the probe, other connection of the primary, which if im not mistaken is the middle of the voltage doubler.  So, as your schematic shows this as a neutral, well, I have an isolation transformer, so theres no real ground.

I did find I can adjust the voltage waveform left/right by having the resistor and inductor in series. and have the two items, in parallel to the CT.  I was able to shift it, and knock down ringing.  But, I figured i couldn't get it fully down due to lack of variable inductor.  So, before I call anything else an issue, I have one on order that will do the job.  Hopefully things will quiet down, and fix my cross sectional issues too.  The real issue comes down to the fact that im using the old slow CM300s, so I need to play with delay time, which I can only do by slowing the gate by adding more resistance.  Obviously too much is bad too.

So, right now im awaiting to test that, see if the variable inductance can do anything different, since the pot was able to shift the waveform.  But if im doing isolation, DOES my bridge need an actual mains ground at some point on the 0V connections?

> Date: Sun, 3 Jul 2011 14:36:10 -0500
> Subject: Re: RE: [TCML] DRSSTC weird waveform behavior
> From: steve.ward@xxxxxxxxx
> To: tesla@xxxxxxxxxx
> 
> 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
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