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Registered Member #1596
Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65
So I switched to GDTs and finally got my drive doing something. Major problem I am having, rectifying HV diode heats up very fast and after it heats up, the secondary current drops significantly. It’s a cheap 0.5A 15kv diode off ebay. Ammeter starts at 80ma and goes down to 30 in a few seconds.
Voltages have not been measured yet, but it looks like around 10kV idle (sparks over 1 cm distance).
What could be a reason for it? I hope its not a diode working as a ballast resistor when voltage across the arc drops to 2kV and other 8kV need to go somewhere.
I also blew up the bridge once again. If you look at gate voltage waveforms, you can notice spikes which can lead to IGBTS being opened at the same time. Is any snubber or filter circuits I should use between GDT and IGBT?
Forgive me my stupid questions, but this is the first time I am doing power electronics… Any input is highly appreciated!!!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Waveforms look pretty good. Ive fixed the gate drive overshoot issue with something like 10 ohms in series with the GDT primary.
If your diode is not rated for ultra-fast recovery times, then it will never work well for this circuit, and will always overheat from reverse recovery loss. Since you said it was "cheap" im guessing that its not a suitable diode.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
baleworker wrote ...
Primary current
Why everyone gets these asymmetrical current waveforms? I have NEVER been able to produce symmetric waveform, even though my driver IC was a SMPS controller w/flip flop in it (meaning the outputs have exact pulse lengths and dead times)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hint: They look pretty symmetrical at half power. Maybe the asymmetry only happens after the "butt cheeks" have touched, and the cure is to never allow them to touch.
BTW, I hope we aren't helping baleworker out in his day job. He looks remarkably well funded with machine tools and digital scopes, and he never mentioned what the power supply was for.
Registered Member #1596
Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65
Dr. Quit The Dr. Thing! wrote ...
BTW, I hope we aren't helping baleworker out in his day job. He looks remarkably well funded with machine tools and digital scopes, and he never mentioned what the power supply was for.
Haha... my day job is computer programming, but I am a freak and spending a lot on tools and equipment for many of my hobbies. I built my shop over a few years, getting most of the stuff cheap, off ebay etc.
Waveforms do look symmetrical, though you can see the current always swings in positive direction a little bit less than in negative.
Steve, what recovery recovery time would you recommend for operation up to 200kHz?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I used diodes that are rated in the low 10's of nS recovery time, 50nS would be great. You could probably stand a little more, but really it will make it less efficient.
Most HV diodes are "standard" recovery, meaning they often take many uS to recover, this will eat up most of your power at 200khz. If you are using a microwave oven diode, then thats absolutely the worst thing ever, unless of course it came out of the newer switch mode microwave power supplies.
Id suggest getting a bunch of UF4007's or there are also some other types, i used to get the SF1600 from newark, there may be some BYV types too. It should be relatively easy to find 1kV 1A ultrafast recovery diodes on the cheap. They should mention avalanche ratings, which is helpful to string them up because you dont have to worry so much about balancing issues. The diode with the least junction C will take the most reverse voltage, so it will just begin to avalanche a bit to self balance. Still, i usually go 50% extra voltage rating on my HV diode strings.
Registered Member #1596
Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65
Replacing diodes did help with their overheating, but they were not the reason for the current drop. It's a capacitor in series with primary. It overheats and cause primary current to drop!!! How to solve this? What caps should I use? I tried putting 10 smaller caps in parallel and it did not help at all, all 10 overheat like crazy!!! Are there are any special caps for high current applications? Please help!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
You have some junky caps then!
I use a CDE 942C20P15K i think, its their 2kVDC 150nF metal foil and film cap. Its rated like 13A RMS, i pushed it to twice that for awhile without it asploding :P. You need a pretty serious cap for this converter, not any polyester junk. It also has to have a pretty good VAC rating, preferably 600V or so! I guess this seems like no big deal to me because ive got boxes of high quality PP caps just laying around here :P. So go spend 5 bucks and get a decent part.
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SLR tested! Works! Wave Forms, need help!!!
