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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
With a couple of more hacks, I finally got the bridge up and-a hummin.
With 1.2uS combined delays from switching command issued to switching accomplished, it is still possible to acheive switching without overshoot and ringing, as I have documented with this little video.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Really excellent work Finn! The video is very informative. I would recommend anyone interested in Solid-State Tesla Coils to check it out.
It shows very dramatically how eliminating the reverse-recovery spikes of the co-pack diodes eliminates the resulting overshoot and ringing on the voltage waveform.
The only thing you have to watch out for is advancing the timing too much! Eventually you will see little steps form at the beginning of the voltage waveform, right after the transition. This is a little step downwards right where all the ringing used to be. If this step exists this indicates that the IGBTs are turning off before the zero-crossing of the current. The length of the step indicates how far ahead of the current-zero the IGBT turned off. This is generally bad for slow IGBTs because of current tailing when interrupting large collector currents. In practice it equates to increased switching losses at turn-off.
So, switching too late you have, reverse-recovery spikes, lots of ringing and turn-on switching losses. Or switching too early you have turn-off switching losses due to current tailing in the IGBTs. Obviously the sweet spot is where the IGBTs turn off with almost no current flowing at all, and the next device turns on right at the zero current crossing or very soon afterwards.
I shall follow your further developments with interest.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
GeordieBoy wrote ...
Really excellent work Finn! The video is very informative. I would recommend anyone interested in Solid-State Tesla Coils to check it out.
It shows very dramatically how eliminating the reverse-recovery spikes of the co-pack diodes eliminates the resulting overshoot and ringing on the voltage waveform.
The only thing you have to watch out for is advancing the timing too much! Eventually you will see little steps form at the beginning of the voltage waveform, right after the transition. This is a little step downwards right where all the ringing used to be. If this step exists this indicates that the IGBTs are turning off before the zero-crossing of the current. The length of the step indicates how far ahead of the current-zero the IGBT turned off. This is generally bad for slow IGBTs because of current tailing when interrupting large collector currents. In practice it equates to increased switching losses at turn-off.
So, switching too late you have, reverse-recovery spikes, lots of ringing and turn-on switching losses. Or switching too early you have turn-off switching losses due to current tailing in the IGBTs. Obviously the sweet spot is where the IGBTs turn off with almost no current flowing at all, and the next device turns on right at the zero current crossing or very soon afterwards.
I shall follow your further developments with interest.
-Richie,
Thanks.
I was just about to ask about the "dreaded current tail", one concept that I have not fully grasped. I will take a closer look at the waveforms to investigate this further.
Thanks!
Dr. Spark wrote ...
Nice work sir!
The video documentation is great. Only issue I had is there was this face in there that could scare the newbie’s to death!
Still thinking a snubber with diode/resistor would be a safe measure just in case. Will not switch in 99% of the time, but that one time when things get crazy.
Cannot wait to see your Fatboy ripping indeed!
Spark On, Christopher
You are absolutely right, but you know there is a bit of Clint Eastwood in me, and I couldn´t resist going "whole hog" with that video. I will use at least TVZ´s as snubber fer sure.
I am planning a test snubber, where I use a string of TVZ´s, one with a small value at the top, so like 1000V, and a 7.5V one on the top. When this string starts to conduct, there will be 7.5 volts across the top TVZ, and enough current to drive a LED with associated resistor. This could be used to show if there are overshoots, and at what input voltage level, Could even make a small battery powered board with a 555 to make longer, more visible diode flashes.
But sure, the production version will have belt and braces protection.
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
Nice video Finn. I will have to apply this to future projects.
Your relatively quick progress with this and ultimately its successful implementation got me thinking. Why did mine have so much phase shift variation? I went through my junk boxes and found the test setup from years ago, hooked it up, and sure enough, there was several degrees of phase change just by varying the current. Since yours did not behave this way I looked into it further. I had tried many different resistors and CTs but I always used the same small variable inductor. Since it was the only constant I removed it from the circuit and checked it with my LC meter and the inductance variation was nowhere near linear. I took the can off and, to my surprise, found shorted turns! Arggg, this whole time I thought the circuit was the problem! I recall accidentally running it without a burden a few times; I must have flashed over a few of those tiny turns... so much wasted time!
I’m glad you brought this concept back into light or I may never have revisited it, thinking my original results where accurate. I can’t wait to try it out now… Also, if you wanted to prevent the problems caused by too much phase lead, you could always use a simple phase comparator and leds. Maybe even a three led arrangement in a lead-0-lag configuration. You could also incorporate a shut down circuit feed from this comparator.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
i don't know, or it good idea, but or someone try LC (like in induction heater) topology for DRSSTC ? One of mine friend says , that if you using LC in series, you have large current in IGBT without any sense. So it can solve LC circuit...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Finn, looks sweet! My only worry with these lead networks was that, being differentiators, they would amplify any high frequency noise floating around the circuit. But if it works it works, I won't argue with a working circuit.
Oh, and if newbies find your face scary, they're too weak to face terrifying sparks from a Tesla coil, so I say bring it on "I don't use snubbahs any moah"
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Steve McConner wrote ...
Hey Finn, looks sweet! My only worry with these lead networks was that, being differentiators, they would amplify any high frequency noise floating around the circuit. But if it works it works, I won't argue with a working circuit.
Oh, and if newbies find your face scary, they're too weak to face terrifying sparks from a Tesla coil, so I say bring it on "I don't use snubbahs any moah"
Actually, the shunt resistors across the lead burden inductor are there to dampen noise. With the buss ringing like the liberty bell, initially the noice creates havoc in the circuit. Well, really it operates stable enough even then, but the scope is full of "tall grass" on all signals right at IGBT turn on time.
But when ZCS is dialled in, scope becomes nice and calm.
This will keep the whatshammacallthem? the EMC guys? happy.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
all,
I now converted the motor to transformer gate drive. An intermediate 80V bridge drives 2pcs. 4:1 gate transformers. I have boasted that I don´t use snubbas anymore, but to not appear foolhardy, I put 1000V strings of TVS'es.
This brings this cluttered look, Hats off to Linas for his clean looking bridge.
The motor is then dressed up with the box of controll electronics, the much fabled Hammertone "Predikter", and then it looks like this, where you can also catch a glimpse of the bench load we use to test the motor against:
With 15ohm gate resistance, the sweet spot was met, where there is virtually no overshoot, particularly when power is cranked up!
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"Thumper"
"I don't use snubbahs any moah"
