Hi Michael,
> >
> > Also agree, so its not quite the same as a spark gap clearing an arc.
> > Basically the energy will go to both the arc and back to the DC bus
> > simultaneously. You can control an H-bridge to "clamp" its output
> > (to nearly 0V, really 2 junction drops) which will keep it from
> > recharging the DC bus. This would be like a spark gap that doesn't
> > quench, but is also very, very, low loss.
>
> I have previously given some thought to this. Presumably, the way to
> 'clamp' the output at the end of a burst would be to change the phase of
> the
> drive to one side of a full-bridge so that the IGBTs are still driven (and
> hence there is no need to try to run dc though GDTs), but no additional
> energy is injected into the primary tank circuit.
>
> Does anyone do this? Does it work 'better' or 'worse' than simply ceasing
> to drive the IGBTs?
>
I do use this technique in my latest DRSSTC drivers (not published anywhere
yet). I use it mostly for current limiting, so that instead of terminating
the gate drive upon current trip, i simply let the primary "coast" by
leaving off 1 half-bridge (the diodes commutate). This allows the primary
to ring with only the damping the secondary, and not regen to the DC bus.
I very much like the results of having this added control of the bridge.
> > Viewing the primary and secondary currents on an oscilloscope can
> > be very useful to get some intuition on tuning.
>
> Do you have tips on how I could do so with minimal risk to equipment or
> body? The obvious thing would be to use current transformers, but is there
> more to it than that? For example, does the DSO need to be in a Faraday
> cage? Are there transients (particularly on the secondary) that need to be
> blocked?
>
I dont take any unusual precautions. Use current transformers. You can
make your own with a ferrite toroid, 100 or so turns of wire, a burden
resistor and a BNC output. It will probably only have a few MHZ of
bandwidth, but i find the phase shift to be insignificant compared with a
pearson or ion physics CT at <100khz. For higher frequency CTs i just use
less turns (sometimes just 10 to 33 or so), the response time was measured
to be less than 20nS.
You may get lots of switching noise on your CT. I found a 330pF cap on the
scope input (using a BNC splitter to rig it in there) helps filter out the
noise. Its likely that you could put a cap across the burden resistor at
the CT and get the same result.
When i refer to secondary current, i mean the current at the "ground" end
of the coil, so the voltage should be low there. Of course, a TC that arcs
to ground will "bounce" the ground voltage, which might be harmful to the
scope, so its best to keep the CTs well insulated for that reason.
Steve
>
> MBD
>
>
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