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Registered Member #4460
Joined: Sat Apr 07 2012, 08:31AM
Location:
Posts: 9
Hello,
I am having some trouble with my SSTC mini I've built. I'm not seeing what I would expect on my gate drive waveforms. I'm using ucc37322/1 with 5 ohms of gate resistance powered with 12 volts from an LM7812. I can't figure out why the amplitude is so small on the mosfet pins and why I have such a huge spike at the beginning of the pulse. The coil works OK when I use irfp260s, but if I swap them out for irfp460s, the output is much smaller than with the irfp260s for the same input votlages, an both mosfets fails after a few seconds.
Any pointers would be greatly appreciated!
EDIT: Sorry, I didn't mean to double post, can a mod please delete one of them? Thanks!
Without seeing your layout the first thing that comes to mind is you're not getting bipolar gate drive. Your waveform clearly shows a positive ON, but no negative ON half cycle. You're also getting ringing at the ON/OFF transitions. It is very possible one of your gate chips is dead. I would also check your layout and wiring as it is contributing to the ringing.
Remove the GDT from the driver, use a differential probe (or power the driver from batteries) and scope the output TO the GDT. You should see a 50% duty cycle 24Vp-p waveform, with each 12Vpk coming from one of the UCC chips. I usually just touch the antenna input with my finger for a convenient low current source of 60Hz, but if you've got a signal gen then feed that to the antenna input.
Registered Member #4460
Joined: Sat Apr 07 2012, 08:31AM
Location:
Posts: 9
Thanks for the pointers. I was using the scope incorrectly and now I see the proper ~24v peak to peak wavevorm on the GDT.
Now I'm even having trouble using the irfp260s. Using a variac, the coil operates nicely up to about 36V allowing me to draw small arcs from the topload with a screwdriver but then when I turn the variac up any farther, the current draw drops dramatically on my ammeter and I just get intermittent streamer strikes instead of constant output on my breakout point. If I leave it running in this state with the variac on max for too long the mosfets blow up.
Other than my sub par layout, could anything else cause me to have problems running at a higher input voltage like this?
Do I need to have a grounded Faraday cage around the driver board? Could the lack of one be causing this issue or would adding one help any? The driver board is in a semi open wooden box and the coil sits on top of that.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The IRFP460 supports 20A continuous, while the IRFP260 supports 46A. Excessive current is a possible cause of destruction. Absolutely, take the unshielded driver from under the coil!
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SSTC mini - strange gate drive wave form
