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Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Recently, ive been using multi-conductor shielded cable for my GDTs driving CM300 modules at 35 to 65khz. Yes you heard right, GDT driving big IGBT bricks, and its just as fast as all that high-side driver crap i developed. I did notice issues with my fiber-optic receiver, but they seemed to go away completely when giving the Rx a shielded box. That and the fact that it was also dependent on streamer conditions made me believe that the noise issue was from the tesla coil, and not coupled through my GDT (unless this is a secondary effect).
So while id agree that a tri-axial conductor would be more ideal, i have had success with simple multi-conductor cable with the shield as the primary. I am using a discrete bridge of P and N mosfets driving the GDT primary at 24V (48Vp-p). That along with 20uF of DC blocking caps seems to provide excellent gate drive to all 4 CM300s from a single GDT (2" ferrite core).
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
"Step-down" gate-drive via a 2:1 or 3:1 GDT is a very good way to drive large die devices or parallel banks of devices where total capacitance is high. You could even use a mini H-bridge running off 50V or 100V to drive IGBT bricks via ferrite stepdown GDTs.
The drive voltage on the power side is stepped down by the turns ratio, and the drive current is stepped up by the same ratio. So you have plenty of current available to combat the "Miller" effect.
A side effect of the turns-ratio is that stray inductance on the primary side also becomes reduced by the square of the turns-ratio when transformed to the power side. This means that a small amount of stray inductance on the drive side of the GDT is far less damaging to performance than the same amount on the IGBT side. So... The moral of the story is to mount the step-down GDTs right at the IGBT bricks and make the primary leads to the driver stage be the longer twisted-pairs because some inductance can be tolerated here!
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GDT Arcing problem :'(
