Re: RE: [TCML] DRSSTC weird waveform behavior

[Tuomas Koivurova <tkoivuro@xxxxxxxxx>]

I can confirm that even though this R/L network trick is far from
optimal way to get some phase lead it still works with your first
universal driver design. After i played around with some R/L
combinations my ringing mess turned out to something like this:

http://koti.mbnet.fi/kizmo/HV/DRSSTC/340VDC1200A.jpg

Still some random spikes but the difference is like night and day. Too
bad i dont have pics about how bad it was before.



2011/7/3 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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