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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
It's an OLTC, so the device will only be switched at 100bps or so, and turn off speed is not critical. In turn-on, IGBTs are about equal to FETs, and the IRG4BC20UD ultrafast IGBT I am using has a risetime of 17ns, which is practially nothing.
Note to myself: Put a de-q-ing diode behind the charging choke to stop oscillations beteween tank cap and choke.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Quote "Q: What kind of drive circuit do I need? A: Unlike a SSTC, where you turn the MOSFETs on and off at several hundred kHz, in the OLTC you charge the tank capacitor up and then turn the IGBTs on. The primary current starts to oscillate just as it does in a spark-gap coil. The positive half-cycles flow in the IGBT part, and the negative ones flow in the diode. When the waveform reaches its first "notch", the IGBTs are quickly turned off, trapping the energy in the resonator, where it will hopefully burst out in a nice display of sparks. You can then recharge the tank capacitor and fire again. "
..so the IGBT will have to oscillate at 1 MHz, even if you switch it on at 100 bps. Even it is ultrafast i doubt that its current capability is high at this freq.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
True, the current through the IGBT oscillates at a frequency at 1MHz, but this does not mean that the IGBT has to switch off during the half-cycles at which the diode is conducting. It only needs to turn off for quenching, which is probably after 10 cycles or so, which corresponds to 100KHz. This is well in the range of what IGBTs can do.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think that joe doh should be able to get this to work, and I'm looking forward to seeing what it does!
The bang energy in a DRSSTC is always "As much as your resonator can take without flashing over" and we find for efficient streamer growth it all needs to be delivered in 100-300 microseconds. These days I think it makes sense to rate DRSSTCs the same way as they rate radar transmitters, in terms of peak power, pulse length and PRF.
Jimmy Hynes, Steve Ward and I have used the action integral to estimate the current capability of IGBTs in our Tesla coils. We never knew it was called that, but I'm pretty sure that's what we were doing. I think Jimmy uses a thermal equivalent model in PSpice these days, but that's really not a million miles away from the action integral.
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Is it feasible to miniaturize OLTCs?
and we find for efficient streamer growth it all needs to be delivered in 100-300 microseconds. These days I think it makes sense to rate DRSSTCs the same way as they rate radar transmitters, in terms of peak power, pulse length and PRF.