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DRSSTC driver tests

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

Hi all,

Terry, I'm glad to hear your stuff is working successfully.

My DRSSTC driver is coming along well too. I did a bench test last night at
reduced voltage and current using just a single resonant circuit.


I used two "Miniblock" IGBTs in a half bridge running off 30V dc at about
30A peak current and a frequency of around 180kHz. The IGBTs seemed happy
and switched impressively fast just with the ordinary GDT arrangement. I got
best results with the deadtime turned down to minimum (about 500ns)

By fiddling with the PLL tuning and "Clairvoyance" controls I was able to
get more or less ideal zero current switching. Clairvoyance is a set and
forget adjustment that "looks ahead" to compensate for the delay in your CT,
gate driver, and IGBTs. I should probably call it something else since the
PLL just predicts when the current zero will happen based on the timing of
the previous zero crossings and an initial guess supplied by you when tuning
it up. It's no more paranormal than getting up at 7.30 to make sure you
arrive at work by 9.00 ::)

When these controls are set up properly the VCO idles very close to the coil
resonant frequency so it locks instantly. The cleanest waveforms seemed to
be got with the current slightly leading the voltage so that is how I set it
up. I tried pulling the resonant frequency by putting metal objects in the
coil and the PLL seemed to track it fine.

The two screenshots with multiple overlaid waveforms show that the zero
current switching is held well throughout the burst. These overlaid displays
are probably a good way of tuning and monitoring a DRSSTC. All you do is
display the H-bridge output and primary current together, and trigger the
scope off either one of them. The persistence of the scope screen does the
rest. If the voltage and current wander out of phase at any time you should
see it.

You can also see the current limiter working in one screenshot, clipping the
primary current to 30A.

There is a schematic but it's out of date, the current limiter and interrupter wiring are not shown-


I'll post an updated schematic tomorrow, and I'm also going to try driving
two coupled resonant circuits and see what happens. The PLL might not be so
happy with the dual resonance. Rather than a real Tesla resonator I'll use a
second identical coil and capacitor, for safety reasons. I think I can add a
spark gap and 240V halogen lamp in series across the capacitor to simulate a
streamer load.

Once I'm happy with the results from this it's a case of adding some
shielding and increasing voltages and currents by a factor of 10 :-o

Steve C.