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Registered Member #2694
Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97
I mean, some guys like me, perhaps need more structural guide, like that, for example Guide of that kind is real helpful, because in front of you in one place, the main points, oscilloscope screenshots and etc.
When I start building my first drsstc, I know nothing about all it and this material was a very helpful for me and for many other people, I think. Why not be able to make same guide for phase-shifted QCW? It's a needful thing now.
I know this, I was asking if the lower secondary impedance means that the primary MMC sees less reactive power for a given sparklength.
Yes, under some conditions which I believe hold true for Steve's design. The derivation is a bit involved so I don't think it is appropriate to this particular forum.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Great Project indeed!
I've also been fiddling with the phase shifted bridge idea for a while now. Steve you mention you hard switched at turn off on one side of the bridge. What I have been doing to avoid this is to ZVS on turn off by using the devices self capacitance and also a small amount of external capacitance to commutate the tank current while the switch turns off (typically < 100nF). A small amount of delay also has to be added before the other switch is allowed to turn on. This yields near perfect ZVS at turn on an off. Another nice perk of this type of drive is the ability to control the rise and fall time out to 100nS or 200ns making the whole inverter much less noisy.
My method doesn't use a CPLD of MCU of any kind just basic logic and RC delays for the ZVS timing, also use an Opamp and comparator to generate the phase delay based on a mod voltage. But I like the idea of using a PWM ramp seems easier.
Also love the idea of using the transformers to both match and current share very sneaky!
All the best, Eric
EDIT:
After reading more of this thread it seems you already do exactly this! I find that it's easy to make even the slower IGBT turn off with ZVS if the correct sized cap is placed in parallel with with device. So that the commutation is extended out as long as desired.
I guess what through me at first is that you said "hard Switching" even though your kinda not hard switching :P hehe
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Yes, thats right Eric. I experimented with adding snubber capacitors to the IGBTs to extend ZVS at turn off, but i got the cleanest results without any capacitance added there. The devices i used have very low switch off loss (really fast), so i decided to just leave it "hard switched". And as you say, the capacitors limit the maximum phase shift since you want the load current to charge them, and not the IGBTs.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Steve Ward wrote ...
Yes, thats right Eric. I experimented with adding snubber capacitors to the IGBTs to extend ZVS at turn off, but i got the cleanest results without any capacitance added there. The devices i used have very low switch off loss (really fast), so i decided to just leave it "hard switched". And as you say, the capacitors limit the maximum phase shift since you want the load current to charge them, and not the IGBTs.
Yeah that's kinda a down side to the phase shift as your limited to under full voltage out, but I guess at full 180 phase this shouldn't be a problem anyway because at that point it should be ZCS anyway. Although there would still be a small window in which turn off losses would be rather high.
So you say that because your switches have good turn off characteristics you just let them hard switch all the time at turn off with no ZVS? I'm curious to the problems you encountered with the capacitor snubbers in place? Did losses get higher? Or did distortion just increase?
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Really interesting Project and amazing results so far, congrats Steve !
I really like the idea of adding secondary capacitors ! What kind of capacitors did you use and how did you mount them to prevent arcing inside secondary ? Why so many high value caps instead of a few HV ones ? Any close pic of the capacitors assembly inside the secondary ?
I would like to apply this concept to my 572B VTTC and see if it improves output
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The capacitors for the secondary were B32672L8122J.
I was originally looking for some ceramic doorknob caps that would suit the job, but couldnt find anything reasonable. The long string of PP film caps seems like a real winner.
I simply soldered them face to face (sort of in a spiral pattern, 2 caps per "turn"). I was then able to fit this string of caps snugly into a PVC pipe that was about 1.4" ID (going from memory here). I think its important that the capacitor string be physically the same length as the secondary coil so that the voltage gradient along the cap string is very much the same as the gradient along the coil. This provides low field stresses inside of the coil, and keeps the capacitor string free from pesky breakouts that would destroy the thing (obviously, if it can make a 4 foot spark).
These are the only pictures i have of the MMC:
Hopefully thats enough info for you to recreate it for your VTTC. Im not sure if a coil will benefit from the added capacitance unless its streamer capacitance starts to exceed the self-capacitance of the coil. But, i think its a worthy experiment still!
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Thanks Steve for the detailed explanation and pics.
What are your secondary self-capacitance without the MMC, and your topload capacitance ? (to give me an idea of the secondary MMC value I should start with).
My VTTC secondary self-capacitance is 2.3pF, my topload is 5.3pF.
I'm thinking to use a string of these small K15Y-1 caps. Their Ø is around 15mm, which is fine, and their axial outputs are perfect for series connection.
Not sure I'll get any benefit from this, but definitely worth trying it !
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Steve Ward wrote ...
The capacitors for the secondary were B32672L8122J.
I was originally looking for some ceramic doorknob caps that would suit the job, but couldnt find anything reasonable. The long string of PP film caps seems like a real winner.
I simply soldered them face to face (sort of in a spiral pattern, 2 caps per "turn"). I was then able to fit this string of caps snugly into a PVC pipe that was about 1.4" ID (going from memory here). I think its important that the capacitor string be physically the same length as the secondary coil so that the voltage gradient along the cap string is very much the same as the gradient along the coil. This provides low field stresses inside of the coil, and keeps the capacitor string free from pesky breakouts that would destroy the thing (obviously, if it can make a 4 foot spark).
These are the only pictures i have of the MMC:
Hopefully thats enough info for you to recreate it for your VTTC. Im not sure if a coil will benefit from the added capacitance unless its streamer capacitance starts to exceed the self-capacitance of the coil. But, i think its a worthy experiment still!
What size doorknob caps do you need? I have some here I could contribute to your experiments. I have some 30KV and 50KV caps. I dont mind contributing you have contributed a lot so I could certainly hook ya up with either 2700pF or 1700pF, 30KV and 50KV respectively.
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