Re: [TCML] BIG solid state

[Michael Twieg <mdt24@xxxxxxxx>]

Hey Steve,

Well, it started off as "what to do with this big PVC tube and spools of
> wire".
>
I'm also surprised by how high your fres is with such a huge secondary and
topload.  Did you wind the secondary with thick wire or something?  Looking
at it I expected it to be in the 500mH range.

>
> Eh, not really as hard as it might seem.  So yes, there is 1 master
> driver/controller that takes the *summed* currents from all 4 primary
> circuits.  The summing is done by using 4 small CTs (one on each primary
> conductor) whose outputs all run through a 5th CT, which steps down the
> current again, and feeds this into the controller board.  If the primaries
> dont have exactly matching L's and C's and loading, then there is simply a
> small phase error and slightly different primary current per coil.  I
> measured each bridge output individually and found the currents to match
> within 10% or better.  Also saw that the phase error was acceptably small,
> meaning some IGBTs might switch a bit earlier than others, but i seem to
> recall being able to achieve nice switching waveforms on all the bridges
> relatively easily.
>
Yeah this occurred to me too.  Probably the best tradeoff between simplicity
and accuracy.

>
> I did find that i needed to add bleeders on the bridge outputs to discharge
> the MMCs before each "bang".  Typically, the MMC can be left with any
> voltage that is less than Vbus.  I suspect due to variations in IGBT
> leakage
> currents, i was getting different voltages left on each MMC, and so when
> the
> system would start up, there would be different starting conditions on each
> primary.  This did seem to cause some strange behavior, but adding 1600 ohm
> power resistors across the output of each bridge discharges the MMC between
> firings, and suddenly everything worked wonderfully.  I suspect the problem
> should be even less noticable when making big sparks, as this should help
> discharge the system energy as well.  I didnt take very good notes on what
> the strange behavior was without the resistors installed, but i seem to
> recall that it improved current sharing, and eliminated some issues i was
> having with improper switching on the first 1-2 cycles.
>
Hmm, now that you mention this I wonder if I've been seeing similar stuff
with our coils.  Sometimes when I'm watching the primary current on a scope
I'll notice that the first cycle will be very large or very small during
some of the bangs.  We have slow bleeders across the DC bus of our H bridge,
but not to its outputs, so there's nothing to reset charge on our tank
capacitor.  Maybe residual charge is the issue... I'll have to look into it.

I do have some, though it might be a few days before i have time to mess
> with posting them.
>
Awesome, looking forward to it.

-Mike
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