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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Some awesome copper work over here, and really admirable low-inductance design.
Regarding supply transients, I don't believe they've ever caused that much trouble as you guys believe.
You should be able to absorb them with TVS's and judge the dissipation from them; I don't think it will be a lot.
Regarding Jacob's ladders.. do you guys draw arcs directly from the transformer (no doubler, filter)? How do you keep ZCS then? I mean, spark behavior is quite chaotic.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
Some awesome copper work over here, and really admirable low-inductance design.
Thanks! As you maby know, we are both toolmakers, so the metal work comes easy.
Marko wrote ...
Regarding supply transients, I don't believe they've ever caused that much trouble as you guys believe.
The mosfets and the IGBT`s can avalance those spikes, at the expence of heat. I think the first bridge that blew, did so, trying to snub some really nasty spikes, by avalancing, which at that time were lower frequency, meaning that they were wider and more powerfull.
Marko wrote ...
You should be able to absorb them with TVS's and judge the dissipation from them; I don't think it will be a lot.
No doubt that would be possible, but only to some extent. From what we have seen, a TVS can only snub a 100% ringing down to 50%. This way, we don`t have to snub at all.
Marko wrote ...
Regarding Jacob's ladders.. do you guys draw arcs directly from the transformer (no doubler, filter)? How do you keep ZCS then? I mean, spark behavior is quite chaotic.
Yes, we drive them directly. ZCS is acheived by design. It is inherent in the series resonant topology.
Steve Conner wrote ...
Hi Finn & Daniel,
I'm also playing with those white Semikron IGBTs, and I've also noticed some fairly serious ringing on the bus even at low DC bus voltages. Maybe they are really fast
I think they are very fast I hope you will show the combination of gate waveform and voltage waveform that you end up with.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Yes, we drive them directly. ZCS is acheived by design. It is inherent in the series resonant topology.
From all I learned by now the supply ZCS's only into a capacitive load.
With resistive load the resonant frequency of the primary circuit is always going to be lower than this (I can be like 100% sure from my observations) and heavily dependent on load/arc size.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
From all I learned by now the supply ZCS's only into a capacitive load.
With resistive load the resonant frequency of the primary circuit is always going to be lower than this (I can be like 100% sure from my observations) and heavily dependent on load/arc size.
You have been into this before. Could you please describe in detail, (Schematic, pictures, spice file, scope shots) what circuit you made these observations in?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Finn,
While the slow switching of the gate might give pretty square waves it comes at the cost of increased switching loss. Luckily you can finely tune the thing to get pretty good ZCS.
I think the 50% overshoot you see is in fact very common when using these brick sized IGBTs. The reason being is that they have a pretty large internal inductance (relatively speaking when compared with a TO-247). And as you have found, using a snubber does help, but you still have something like 20-30nH snubber L and another 20-30nH (for newer bricks) or 50-100nH (for older ones like the CM300's i use) of IGBT inductance. I believe it is due to this that most brick datasheets give their example ratings with only 600V applied to 1200V devices (because the overshoot will almost always be large).
So as Conner said, you really should be tuning your gate R for best device efficiency and not just for looks (im adding that second part).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, if the stray inductance were mostly inside the brick, the ringing waveform should be bigger on the output terminal than on the DC bus terminals. Does that match up with what you saw, Finn?
As far as the seriousness of spikes goes, I believe that as long as the energy stored in the stray inductance (0.5*L*I^2) is small compared to the IGBT's avalanche rating, and the power dissipated through avalanching is small compared to the device's rated dissipation, it should be OK. Of course, for devices with no avalanche rating all bets are off! :(
I think the 124 series are faster still than the 123.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
You have been into this before. Could you please describe in detail, (Schematic, pictures, spice file, scope shots) what circuit you made these observations in?
Yes, but I'm rather going to open another thread than hijack this up. My ''observations'' involve royer converters and are about what I thought to be 'basic transformer theory'. And described behavior of your CCPS is 'odd' from that point of view. My idea of it was consistent with Steve's explanation so I thought I'm OK.
I never built anything like CCPS and can't by any way claim I have a clue what I'm talking about :(
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Steve Conner wrote ...
Well, if the stray inductance were mostly inside the brick, the ringing waveform should be bigger on the output terminal than on the DC bus terminals. Does that match up with what you saw, Finn?
Of course, for devices with no avalanche rating all bets are off! :(
Yes, I do beleive there was more ringing on the output terminals, but I am not sure, and right now the ringing is back bad, snubbers are frying the resistors and everything is sort of "hot". Have some debugging to do before i can answer that with scopeshots.
Semikron don`t mention the word avalanche in their datasheets. They say "Non punch through" though, with a little imagination that could be read as an avalanche ability. Anyone have an opinion about that?
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CCPS (Capacitor Charging Power Supply)
