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Registered Member #56364
Joined: Mon Aug 10 2015, 10:07PM
Location:
Posts: 2
Hi all, just putting together my first DRSSTC based loosely upon Kaizer's DRSSTC1. It's using IXGN200N60A3s with a bus voltage of 315VDC, primary cap of 0.2uF at 4000V with about 6.2 turns on the primary right now and also using the universal driver 2.7. I'm doing low power testing into a dummy load placed in the center of the primary and some odd things start happening at about 45-50VDC on the bus. It rings up the current well and starts to drop back down, but after the input pulse from the fibre has gone dark, the voltage across the bridge keeps switching for an extra couple of cycles and then occasionally starts oscillating wildly! Yellow is voltage accross one of the IGBTs in the bridge, blue is the drive waveform coming out of the universal driver, and magenta is the voltage accross the burden resistor from the CT coil.
I've checked that the driver is not outputting anything to the GDT at this time, and the drive waveform before this looks fine... Any ideas on what's going on here? I'm guessing the gates are getting capacitively coupled into the collector/emitter junction whilst the GDT is blocking this high frequency allowing it to continue oscillating. There must be a way to stop this easily?
Registered Member #56364
Joined: Mon Aug 10 2015, 10:07PM
Location:
Posts: 2
So after a bunch of googling, I've tried adding some series resistance directly on the gate (not on the driver side as I thought most people were referring to) and that seems to have reduced/removed the crazy oscillations that were happening. However, the primary still continues to oscillate for a short amount due to small spikes on the gates at zero crossings of the primary current.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
I'm not an expert on DRSSTC but during the process of building a QCW I had it hooked up as a DRSSTC. I saw a similar effect ie a few cycles after the bridge had supposedly been turned off by the interrupter.
I was told by an experienced coiler that this was normal and was the result of the energy that remains sloshing about in the primary tank circuit. A more technical description I cannot give. But I also found several other waveform pictures on this forum that showed the same thing.
So I assume that it is normal.
I would look into the spikes on the IGBJTs. If they are real (and not due to the measurement system) they look to be larger than the actual applied voltage.
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New DRSSTC [Oscillation Issues]
