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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I built a boost converter for my DRSSTC that I'm now troubleshooting. It is a basic design, basically a copy of Steve Ward's very first 5kw boost. Just a boost with no snubbing, powered by a TL494 driver with a few modifications to give current and voltage simultaneous control using the two correction amps.
So the problem is that the switch-off of the transistor causes a huge voltage spike, 3-4 times the output voltage. One approach is to put a capacitor (seems to be 0.1uF+) drain-to-source, but this is a huge power waste. I put a string of 3 300V TVS diodes (the IGBT is 1200V rated), and the TVS diodes got so hot the solder holding them on melted. This was during a 400V output run into a 67 Ohm resistive load powered by rectified 110 Mains input, which equates to a 2.4kW test run. They melted off within about 10 seconds.
So, is there any good way to solve this problem? I played with gate resistances and it did not affect it at all. I currently have no gate resistor. Tried 2 ohm and 12 ohm, all exactly the same ringing magnitude.
Pictures attached are the ringing (drain to source) at 10uS/div, 1uS/div, and 0.1uS/div, and the boost converter
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
I'm guessing that is your output capacitor and rectifier right on your IGBT? You either need a faster diode or a slower fall time for your switch. Or a snubber. You could try a small RC snubber and see if it helps any.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
The diode and small filtering caps are directly at the igbt... I can try a different diode, although I'm already running an ultra-fast diode. Also I can't really get switching must faster considering I have no gate resistance
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I got it working now. The biggest help was that I realized I'm using a half-bridge for the single transistor, which means there is already a diode in the appropriate place inside the brick, which means the diode to transistor inductance is as close to zero as possible. Also added more local output capacitive filtering.
Seems to be doing well, I have now tested it through 5kW at 600V. No more ringing and no hot TVS
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Boost Converter Ringing
