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Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Guys,
this is my small DR-SSTC (3.2" x 8.2" coil dimension)
secondary is 3.2" OD, my primary is 5.6" ID (using awg#8 TW wire)
this coil works GREAT with secondary base feedback.. with 220Vac in //fullwave rectified and filtered with 1140uF 400V ElCap...
now I tried primary feedback..as usual, I try first with lower input voltage (100Vac 1000Watt stepdown transformer)... everything works great! compared to the secondary fb with 100Vac in, THIS one has longer streamer length... now when I powered this thing up with 220Vac line, I instantly get arcing between middle-half of secondary to primary....AND IS THICK!! I still dont have the OCD ckt made yet (Im the OCD for the meantime )
my question, is it not worth doing primary feedback with "short" coils such as this??
and is it possible to suggest for a minimum h/w ratio? or should I "widen" the primary coil even more?
as I have read from the past, widening primary Diameter will let the IGBTs do MORE "work"!
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
Hi, according to my PSpice simulations I think that for DR-SSTC primary feedback is always the best choice because it allows to achieve almost perfect zero-current switching. On the contary, I have noticed that with feedback on the secondary base current the phase error can lead to swith the bridge with a current of some %, even 10% of the maximum value...
Anyway, this is the world of simulations, I have no "real life" experience...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Try tuning your primary to a somewhat higher frequency, hopefully this should cure the arcing.
The reason is: A DRSSTC has two resonant frequencies, and it can oscillate on either one, depending on the tuning and feedback method. If it oscillates on the lower one, flashovers between primary and secondary are more likely.
By tuning the primary slightly higher than the secondary, you can make it prefer the higher resonant frequency.
Note, if you were using secondary base feedback before, it worked the other way around! Tuning the primary LOWER than the secondary makes the system oscillate at the higher frequency. So maybe this is why your coil was working before you messed with the feedback.
I discovered this with my PLL driven DRSSTC, and Richie Burnett did some experiments that support it. It was easy to stumble across, because the PLL can be set to drive at either frequency, irrespective of how the coil itself is tuned. It would run beautifully at the upper pole frequency, but burn itself up from flashovers if run at the lower pole.
Unfortunately, Steve Ward's latest tunings achieve an efficiency boost by oscillating at both frequencies simultaneously! The PLL can't do this.
Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Hey Steve!
thanks for that info... yes your right about secondary base feedback , the primary was tuned at a lower Freso than the secondary (220KHz pri and 260KHz sec) .....
I guess I would be making another lower value pulse cap for the primary :) to increase Freso..
Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Steve,
I agree its MUCH easier to to take a FEW turns BUT im into 4.7turns and a smallish 4.5uH on my LCmeter... Im afraid to go lower. though I have not blown any IGBTs yet (dont want to either)
better make a smaller value cap(and go to all the trouble again)
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Question regarding primary winding feedback
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