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Frosty90

Wed Nov 19 2008, 12:30PM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi everyone,
After the fun i had with my first SSTC (which blew up) I've decided to build another now that i have the time, but would much appreciate some wisdom from others more experienced.
The original coil was powered by an H-Bridge of IRFP460s, and I have plenty left so my new coil will also use these. My main question though is how much power can I expect to put through these, while still having them not explode? my original coil was a 2.7" by 12" coil and i managed 12" sparks and sometimes upto 15/16". Is thisabout the limit i can expect with 4 IRFP460s? I am planning to use a 4" diametre coil this time and a largish toroid, but will the max spark size be abit underwhelming for the coil size? I have deliberately decided not to go double resonant, as I like high duty cycle firey sparks.

Also, my old coil would run fine on continuous mode, but would heat (the mosfets) alarmingly when run with an interuptor. I thought that this may be due to the interuptor switching off in the middle of an rf cycle, and forcing the mosfets to switch the full peak current. So how can i avoid this this time? Is there any generally 'accepted' method for detecting the zero crossing of the rf cycle to switch the interuptor then? I havnt been able to find anyone who has bothered (i suppose most people move to DRSSTC), any suggestions?

Cheers,
Jesse

101111

Wed Nov 19 2008, 01:14PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

For the interrupter problem I think you could just a AND gate, one that is connected to your output of your frequency generator or feedback and one to the interrupter and the out pin to the driver to the MOSFETs. This is just a guess though.

Dr. H.

Wed Nov 19 2008, 02:17PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Good day sir. With those fets I've managed 24 inches but that using an interuptor and a second torroid at around 1800VA. I doesn't sound right the fets to be heating up when using the interuptor. Is it possible that your interuptor is picking up interference ?

Here is the thread about my IRFP460 coil

Cheers

Wolfram

Wed Nov 19 2008, 04:15PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I have managed about 22'' with a H-bridge of IRFP460-equivalents and a secondary slightly smaller than the one you used. Bridge powered from halfwave rectified mains. Power was somewhere between 2 and 3 kW, I think.

Design of the coil depends on how you want to run it, interrupted or halfwave/fullwave rectified mains. If you are running it interrupted, you can't have too high coupling, as there is servere risk of flashover (in my experience). If running halfwave rectified mains with a smooth toroid, you can get away with higher coupling, as the risk of flashover is much smaller. This seems to lead to very long sparks compared to secondary size.

Anders M.

MRacerxdl

Thu Nov 20 2008, 03:41PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

For some High Power SSTC's, a Interrupter Sincronizer is good, but it sounds like you have a problem with your interrupter getting some noise.

Dylan

Thu Nov 20 2008, 10:49PM

Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95

I third the motion about your interrupter picking up noise, I doubt the heating is due to lack of interrupter sync.

As to your question about interrupter synchronisation:
The basic idea is clocking a flip-flop with a feedback signal , and using your interrupter signal as the flip-flop data in, your synchronised interrupter signal is then available at the flip-flop outs.
The problem with this simple approach is that at the beginning of the interrupter burst there is no feedback signal to clock in the pulse.
This is generally resolved by using the interrupter signal to also manipulate the preset and clear inputs of the flip flop.

Note that there is no reason you cannot add this system to a fixed frequency SSTC, you could simply add a primary current CT, and the flip-flop synchroniser to provide your gate drive "enable" signal, but continue to use your fixed frequency oscilator (or secondary feedback) signal for your gate drive "data" signal.

Hence giving you a primary current synchronised SSTC.
You could of cource also add in primary overcurrent detection.
And basically you will end up with a classic DRSSTC driver, with one modification, namely providing fixed frequency (or secondary feedback) to the "data" input of the gate drive section in place of the primary current feedback.

If you really want to push the limits of whats possable with a given SSTC configuration I see no reason why this setup would not be beneficial as it will give you all the benefits of primary current limit controll and sychronisation.

I havn't done it myself yet, but Im going to give it a go sometime, maybe someone else already has and can comment?

Frosty90

Sun Nov 23 2008, 03:43AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi,

Thanks everyone for your advice. In terms of my interuptor picking up noise, how could this be causeing heating in the mosfets? They run cold in continuous mode (with half or full wave rectified DC bus), so how could noise on the interuptor be causing heating if syncronization isnt really a problem? Anyway I'm building my new coil in a sort of 'modular' fashion, so i can easily change things like interuptors, gate drivers etc with ease, so i plan to do alot of experimentation.

Dylan; thanks for your info on using the flip-flop. I thought this would be the way to do it, but i think for now I will just go with a basic interuptor, and if it causes problems i will build a syncronised one.

Cheers,
Jesse

LithiumLord

Sun Nov 23 2008, 03:04PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

>so how could noise on the interuptor be causing heating if syncronization isnt really a problem?

Noise in the interruptor signal will cause a serie of unipolar strobes that will cause some mess independant of the drive type. Also fixed frequency sux for long sparks as the system starts up well undertuned.

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