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Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Those bursts (pulses) look WAY too long. I can't magnify the image right now but does that say 500uS!?
Keep the secondary side leads of the GDT as short as you possibly can. That is where you'll get terrible parasitic oscillation of the gates. Primary side isn't nearly as critical.
Yes, that does say 500uS but the timing is adjustable. I will be changing it to 50uS in the code. I kept it at 500uS so that it would be easier to see in the image.
So for the Bridge, I guess that means I should put all of the IGBTs on one side of the heat sink then. That's good to know! Also, would connecting the GDT to the gates with traces on the board be better than connecting with wires? I will be using twisted pairs wherever possible, so hopefully that will help, but I'll keep them as short as possible.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi all,
Just another update:
I have decided to use 1/4" copper tubing for the primary coil (which, based on your suggestions, I have decided to make flat), and the 75nF, 12kV arrangement for the MMC that loneoceans suggested. Here is the input and output of JavaTC:
Input: Output:
The primary, secondary, and topload are all finished, including an old fuse holder for a tap--
Coils: Tap:
(Please excuse the mess--I'm in the process of reorganizing)
I also started working on my inverter board and mounting it to the heat sink. Pics will come eventually.
My main question this time around is if you folks could take a look at this updated schematic. Is there anything clearly wrong with it that needs immediate fixing? Or does it look ok? The main difference between this one and my last schematic is the addition of the J-K flip-flop for soft-switching the IGBTs. I based that part of the circuit on Steve Ward's DRSSTC-1 schematic.
Updated schematic:
Finally, regarding the inverter capacitors--What is the recommended value? I've seen snubbers used (small, maybe 1uF or less), and I've seen large ones (1000uF or greater). I expect the larger ones are for smoothing? Or do they serve another purpose (decoupling, perhaps)? I'm using a full-bridge design, so I'm not talking about the capacitors used in half-bridges. Any recommendations would be greatly appreciated!
Also, I mentioned it in my previous post but never really asked, does a 50uS on-time sound more reasonable than 500uS for the interrupter? I know that, according to JavaTC, it will take approximately 17.7uS for the energy to completely transfer from the primary to the secondary.
Thanks again for your help, guys. I think things are really coming along now.
Use twisted wires for any length you can, you can't (easily) twist traces.
Yeah, 50uS sounds better. I wrote a calculator for determining maximum safe OnTime for DRSSTCs, so plug your data into that to get a better idea. OnTime limit is related to current limit and MMC voltage rating.
The beefy caps are for smoothing, so that bursts that occur when the mains voltage is low aren't less powerful. Snubber sized caps are for snubbing!
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Use twisted wires for any length you can, you can't (easily) twist traces.
Yeah, 50uS sounds better. I wrote a calculator for determining maximum safe OnTime for DRSSTCs, so plug your data into that to get a better idea. OnTime limit is related to current limit and MMC voltage rating.
The beefy caps are for smoothing, so that bursts that occur when the mains voltage is low aren't less powerful. Snubber sized caps are for snubbing!
I have been using your calculator, and once again I'd like to congratulate you on it--it has been very helpful!
Using your calculator though, I'm still a bit confused about the derating of the capacitors. My MMC is rated for 12kV, so I've been setting it to 0%, but I'm not sure if that's what I should be setting it to. Anyway, that gives me a burst length of less than 70uS (which is how I came up with the 50uS figure). With 30% deration it gives me 48uS.
Thanks for verifying the capacitor bit. That's what I was assuming--there are often smoothing caps and snubber caps, but I think I'm going to skip the snubbers for now. Other than needing to withstand several hundred volts, though, is there anything else I should keep in mind for the smoothing caps? Or does that work the same way as standard smoothing caps after a bridge rectifier? I ask because a lot of things seem to work differently when connected to a DRSSTC :p
So the schematic looks good? I'm starting to set up the board, so I'll want to verify that the circuit is right before actually putting it together.
Thanks, glad you like it. I like to leave about 100V/cap overhead minimum for QUALITY HV caps, if you are using noname chinese stuff you might want to derate them by as much as 40% (lots of cheap "20kV" caps on ebay that are NOT able to withstand 20kV!).
I didnt check your schematic as I haven't used that flipflop, so I'll let someone else weigh in.
Keep all connections between the filter caps and the IGBTs as short as possible. In one of my early KW-class SSTCs I used 6" leads between bridge and caps and it caused a 30% loss of power.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
Thanks, glad you like it. I like to leave about 100V/cap overhead minimum for QUALITY HV caps, if you are using noname chinese stuff you might want to derate them by as much as 40% (lots of cheap "20kV" caps on ebay that are NOT able to withstand 20kV!).
I didnt check your schematic as I haven't used that flipflop, so I'll let someone else weigh in.
Keep all connections between the filter caps and the IGBTs as short as possible. In one of my early KW-class SSTCs I used 6" leads between bridge and caps and it caused a 30% loss of power.
Thanks for the help Sigurthr. I am using 940C30s for my MMC, so I'm thinking they should work ok. If not, I'll go ahead and buy some 942Cs like I had originally planned (they're rated for higher pulse currents).
For the smoothing caps, I generally see huge ones used across the bridge, even on full-bridge designs. Would there be a disadvantage if I used several smaller ones (For example, from an ATX PSU) strung together, provided they can withstand the voltage and have a high enough capacitance to lessen the ripple? Just curious if this is another thing that really needs a lot of thought put into it.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Matt Edwards wrote ...
RIFA or UPE is a great choice if you can find them cheap enough. Everyone i know generally uses them for the larger coils.
Thanks Matt, That's very helpful!
Anyone else have more thoughts regarding the schematic? At someone's recommendation I have re-done the feedback circuit and added OCD based on Steve Ward's DRSSTC-1 with OCD. I also added a second Schmitt trigger inverter. I know this will cause more delay, but I believe it's necessary, otherwise the signal would be inverted when it shouldn't be. Here's the updated schematic:
By the way, has anyone used CM300s for higher frequency (~200kHz) coils? Or would you recommend different IGBTs for that? I want to have a plan B in case my 20N60s aren't up to the task.
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
about the schematic.... from Steve Guide to DRSSTC Design in order to make the J-K to work properly the timing of the RC must be carefully chosen/calculated based on your coil frequency Read this
i'd say build this section of the circuit on a breadboard feed in the interrupter and feedback signals scope the output and tweak the RC value until it is working how it should be working
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
I've tried CM300s (600v) @ 190 kHz small coil, it worked, but heating is an issue, however, Phase lead compensator solved that issue. But i would recommend shifting to faster IGBT especially if you're not planning to play with "that" current level about 500 - 600 Amps or so.
Plus, 1 inverter on the input can still make the coil work, you just have to change the phasing of your primary by exchanging the CT secondary feeding your 74hc04. Lastly, you could use 74hc14 instead of 74hc04 so you can eliminate 1 IC and save space. thanks, hope this helps
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First SSTC design - Need some critique
