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Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Dr. GigaVolt wrote ...
You should be getting risetimes of 25-50ns easily and almost perfect pulses.
I doubt that - as mentioned in the first post, the fets are IXFN48N50 which are even heavier than mine (IXFH30N50, as an off-topic - absolutely brutal fets, especially for you can get here for the mere $2!) - and I'm getting around 100ns full swing (from -8 to +12v) - and that is with drivers connected directly to the gates without any GDT.
Registered Member #1383
Joined: Mon Mar 10 2008, 11:09AM
Location: Wollongong, NSW
Posts: 27
Cheers, I hope it is the cores, it certainly would explain a lot. I'm trying those Jaycar cores now, fingers crossed hey, everyone must be getting sick of seeing this thread!
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068
Cotlone wrote ...
Cheers, I hope it is the cores, it certainly would explain a lot. I'm trying those Jaycar cores now, fingers crossed hey, everyone must be getting sick of seeing this thread!
Actually, I stand correct. Those cores are probably just fine. I dug out my old PSPICE simulations of the Plasmasonic (been a long time), and simulated with your parameters. They actually look like the waveforms you are getting.
Based on this, my advice is to go with what you have. Optimize the windings to get as best waveform as possible. You're not going to get better. Your switching lots of power and at high frequency.
The MOSFETs will get pretty damn hot as you mentioned, but you're running CW remember. Not a DRSSTC. Make sure they're well heatsinked and plenty of airflow (fans)
But also, you want your voltage to high enough so you saturate those MOSFETs. Up to about 12-15V. If you are getting less voltage, then try reducing turns, but also leakage inductance will affect this as well.
Registered Member #1383
Joined: Mon Mar 10 2008, 11:09AM
Location: Wollongong, NSW
Posts: 27
Ah, ok then thanks a lot I'll work on that. Dan, just out of interest is there any chance that I might be able to have a look at your simulation to help me understand whats going on? I've made a simulation of my equivalent circuit but I haven't modeled the cores. I'm far from a PSpice guru, it nevers hurts to learn though. Alternatively does anybody know of some MOSFETs which are a little less brutal in the miniBLOC / SOT-227B / ISOTOP package (same as the IXFN48N50)? Thanks.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Actually, things are not as bright. The heat level will drop the maximum current dramatically so even 20amps (there is no ZCS but there IS the magnetizing fully-reactive current, remember?) through the primary will result in a blast. The core replacement is definitely of no use, so you'll have to mess with the drive electronics a bit - try quad separate GDTs with a more powerfull drive (some 14a IXYS drivers will work great there) or just get rid of the GDT completely (like I did - I hate that core/winding/driver overload mess :) ) - those fets should be really great and you'll achieve really huge sparks with them - just it will take time and money, but it's still a good idea!
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
LithiumLord wrote ...
Actually, things are not as bright. The heat level will drop the maximum current dramatically so even 20amps (there is no ZCS but there IS the magnetizing fully-reactive current, remember?) through the primary will result in a blast. The core replacement is definitely of no use, so you'll have to mess with the drive electronics a bit - try quad separate GDTs with a more powerfull drive (some 14a IXYS drivers will work great there) or just get rid of the GDT completely (like I did - I hate that core/winding/driver overload mess :) ) - those fets should be really great and you'll achieve really huge sparks with them - just it will take time and money, but it's still a good idea!
14A IXYS drivers aren't need here. They may MOVE heat dissipation to the gate resistor (total heat dissipation DOESN'T change), but performance wise they aren't going to do much for this particular design.
So just how did you drive your circuit without the gate transformers?
I've uploaded my quick PSPICE circuit. Simply rename .ZIP to .SCH. (Its NOT a ZIP file)
Registered Member #1383
Joined: Mon Mar 10 2008, 11:09AM
Location: Wollongong, NSW
Posts: 27
Awesome, thanks very much I'll check that out. I had it running tonight and was getting some decent sparks (best I've had yet), and the music was pretty reasonable. The only thing is that it only tended to work when I held my hands in a particular orientation around the primary (I discovered when tuning), it seems to be independant of tuning. The heat sinks did get nice and warm but cool down pretty quickly with the aid of forced convection.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Dr. GigaVolt wrote ...
14A IXYS drivers aren't need here. They may MOVE heat dissipation to the gate resistor (total heat dissipation DOESN'T change), but performance wise they aren't going to do much for this particular design.
Heh, I'm talking about the fet heat level, not the driver one. You will get a higher drive current and therefore shorter rise/fall times, so switching losses will decrease. Actually I never tried that out, but I guess that an improved drive, preferably using separate quad gdts (one per gate) with such a high current may make good results even on such a heavy gate.
Dr. GigaVolt wrote ... So just how did you drive your circuit without the gate transformers?
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PlasmaSonic II based SSTC, with poor performance
