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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
yay steve - nice toy over there.
I wonder how would it fare against a bigger stack of good old CDE942's?
Shortening their legs and soldering them tightly onto water cooled busbars might help them get rid of heat.
If you are already water-cooling the coil water flowing through same pipes could simply be passed around the cap.
Resonance between the resonant cap and its inductance to the mosfets. I had huge problems with this when attempting a higher power ZVS oscillator when using high speed gate drivers instead of the crappy low speed "gate drive resistors". I believe that using gate drivers makes the parasitic oscillations WORSE because its easier to excite that resonant mode with the faster drive. In fact it was so bad i gave up on it rather quickly and determined that if i was gonna do a ZVS circuit, i would do it properly instead of the "mozilli" driver :P.
Interesting points steve... when did you really run into problem? (power level, voltage..?)
I had most of trouble with diodes; I couldn't even put in an UCC directly, because voltage drop of most diodes is too high to turn them off safely.
My newest ideas are completely different from this though, I can tell more about them if you guys are interested.
I always intuitively kept drain inductance as low as possible, cutting the mosfet legs down and soldering the cap right onto them.
That wasn't a problem for power levels I was running, but if you have a large water cooled cap like that it becomes a bigger problem.
I wondered if you could simply keep the cap close to it's work coil, and make low current connection with the inverter over two fat coax cables.
Or, if you have enough water cooling, you could simply put the mosfets directly onto the cap and cool everything at once. I don't know.
My best shot of doing a ZVS inverter ''right'' would be to use PLL and a current fed fullbridge.
Hardest part is definitely designing the gate drive, which needs to be make-before-break (now I need to generate shoot-through instead of deadtime) and this makes use of an GDT difficult.
It may be solved through some crazy logic, but I don't really have any original ideas.
Feedback could simply be taken as voltage feedback from the bridge output.
I imagine circuit could suffer from same problem as royer but PLL could limit the frequency range.
I'm glad I got you 'big guys' interested, you are making an interesting discussion.
BTW, huben made some good work at project board - it may not be all that problematic as you guys made it appear. Just needs to get scaled up with some 1200V devices...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Those humps on the waveform do scare me though. I noticed the same thing on my class E SSTC, but never on royer yet even in Mhz range.
How did the failure happen though? Mosfets just badly overheated? (right one sort of doesn't look like it's heatsunk properly )
Thinking again, could the oscillations be happening because there is some deadtime between transitions? I mean, the feedback system of the royer isn't too perfect on itself.
Having lots of mosfets around I was tempted to try a royer IH, but my main problems are lack of a power supply (like 100..150V with lots of amps) and adequately sized heatsinks. I have only some small CPU heatsinks left around, so I think I'll have to find something on ebay or etc.
Oh, and too much simultaneous projects...
For, HFsstc freak, royer oscillator (Mazzilli driver, ZVS flyback driver) is used, adn this is basic schematic
Instead of a flyback a work coil in the air is simply used.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi all,
Yes, like Marko said, it is a ZVS/Mazzilli/Royer/whatever oscillator. The only unusual thing is that there is no centre tap on the work coil. I used a bifilar wound ferrite transformer connected in parallel with the work coil to "add" a centre tap. Maybe the stray inductances of this transformer are what's causing the problem. I don't know, I just wanted to try this to see if it would work.
I was running it off a variac, transformer and bridge rectifier, and what happened was that it stopped heating, started to hum, and blew a fuse. I just assumed one of the MOSFETs had popped. But when I got the thing out again, both MOSFETs were actually good! I think it just dropped out of oscillation which makes both MOSFETs turn on, shorting the power supply.
I don't know what those humps are either! :( But I do know that as I increase the voltage, the ringing you see on them takes longer and longer to decay, until it bursts into continuous oscillation of huge amplitude and everything RF sensitive in the neighbourhood freaks out... Again I'm not sure how this happens, but these parasitic oscillations are what limit how far I can turn it up.
In any case, this is about as much power as I'd care to use without water cooling. The work coil gets hot enough to char the wooden table, and the tank capacitor gets hot enough that it's painful to touch. I've stopped using it in case I damage the capacitor.
The MOSFETs run quite cool in spite of the terrible heatsinking, and all the magnetics run cool too. The DC bus choke is a stack of four iron powder cores that I got off Ebay. They are Micrometals type 52 material IIRC.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Yes, I noticed you have no center tap on the coil!
I'm not sure if transformer is any good though, the ringing could be due to it's leakage inductance resonating. Center tap to the work coil always seemed to work well - could be done by a small conductor since it carries relatively small current.
I myself try to get rid of ferrite anywhere I don't really need it!
I was running it off a variac, transformer and bridge rectifier, and what happened was that it stopped heating, started to hum, and blew a fuse. I just assumed one of the MOSFETs had popped. But when I got the thing out again, both MOSFETs were actually good! I think it just dropped out of oscillation which makes both MOSFETs turn on, shorting the power supply.
I managed to replicate exactly the same thing. With mosfets like IRFZ44, low power circuit always happily worked from 12V and I was lighting bulbs by magnetic coupling to the coil safely.
Then I tried the same thing with 2+2 (wanted ot add much more later) small 1.5 amp 500V mosfets, with high ON resistance (7 ohms or so). I used a much bigger work coil and a smaller cap;
As I powered the circuit up with just 12V, it shorted the supply the same way with no load at all!
It worked only if I applied the gate voltage first (then it starts to oscillate through gate drive resistors) and then apply the drain voltage.
Still circuit would sometimes drop out of oscillation if overloaded.
In an IRFP450 based circuit, I also blew a mosfet this way, by applying the drain voltage before gate power.
Now may be getting some insight why is this happening, since I made an actively-pulluped circuit which never dropped out of oscillation (yet).
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I don't know what those humps are either! :( But I do know that as I increase the voltage, the ringing you see on them takes longer and longer to decay, until it bursts into continuous oscillation of huge amplitude and everything RF sensitive in the neighbourhood freaks out... Again I'm not sure how this happens, but these parasitic oscillations are what limit how far I can turn it up.
Thats exactly what happened with my setup too. Seemed to limit me to about 30VDC which was a far cry from the 170VDC i was hoping to eventually use :P. Next time i will just generate the drive signals with an oscillator and forget the feedback/drive stuff. Shouldnt be too big of a problem with an induction heater.
Registered Member #1217
Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11
Hi, that was my video that melted the steel.... Steve Conner- absolutley right mate, those are celem conduction cooled caps in the tank circuit. They are the most critical component used here and also the most expensive. I have experimented with the full bridge and half bridge topologies. The Fairchild application note is right, a half bridge is better for reliability. I used a hgtg20n60a4d half bridge in my latest and best version. Much simpler and reliable. It draws 3.1kW continuously and melts steel without insulation now. My other youtube video shows the setup more clearly. I think anyone who can build a drsstc will have no problem building one of these. As for water cooling... ESSENTIAL. The copper work coil softens and colapses in less than 2 seconds with no water at 3kw. Also, if the water pump stops, the water in the coil boils in a few seconds and fires the silicone pipes off. Alot of people have been asking for schematics and i will try and find some time to draw some up. Regards Karim
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