Author Topic: Building a bigger DRSSTC  (Read 2299 times)

Offline ritaismyconscience

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Building a bigger DRSSTC
« on: April 12, 2020, 04:20:10 AM »
Hi,
I'm planning on building a bigger coil using more paralleled TO-247 parts. Bricks are really expensive for me, I can buy about 150 TO-247 IGBTs for the price of a single CM300 brick and I can buy 85 of them for the same weight (shipping from China is fairly expensive). I have an aluminum heat sink which can fit a full bridge of 7 paralleled IGBTs for 28 in total, and I should be able to switch 1750A peak current. (I checked and the parts have positive temperature coefficient, they'll all be on the same heatsink, and I'll arrange my IGBTs for current sharing). I'm guessing I might need to use a different driver because the gate capacitance is quite big (around 160nF for 7 IGBTs).

Here's what I came up with:


Are there any problems with my plan?

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #1 on: April 12, 2020, 06:01:34 AM »
Paralleling TO247 IGBTs can work.  My DRSSTC has sets of 10, 40 total for the H-Bridge.  There are lots of pitfalls as well.  Mine is designed for 3500A peak, but I haven't ran it past 2600A yet.  Also designed for 450 bus voltage, but I'm mostly using 300V so far.  Average power should hit 10kW eventually.  Mostly running it at 3-3.5kW so far.  Planning to run 6kW this year if our local (Portland Oregon) Maker Faire actually happens in September.  So I may yet end up with a bunch of charred electronics.  My DRSSTC is here on this forum:
https://highvoltageforum.net/index.php?topic=798.msg5332#msg5332

Start with some Spice simulations of your basic topology, and later with parasitic inductances added.  Your biggest challenge may be improving construction techniques, especially parallel copper planes for VBus to the IBGTs and local snubber caps.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #2 on: April 12, 2020, 06:45:45 AM »

Here's my bus design which should make inductances a bit less than with my other coil.

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #3 on: April 12, 2020, 08:11:44 PM »
If I understand correctly, the sketches are front and back of a two-sided copper-clad board.  If so, it looks great.  Will the IGBTs solder along the upper and lower edges?  That's the way mine are.  What's the purpose of the rectangular cutouts in the back-side copper?  Is that clearance for the bulk cap screws?  If so, four small holes, one for each screw, would be better than two larger cutouts.

Such boards can be made with knife or dremel-tool cuts in raw copper-clad boards - much cheaper than buying ECBs, and much easier than home etching.

Another suggestion:  Consider spending more time using and experimenting with and learning from your first coil before starting another one.  Run it at higher repetition rates to see how that affects spark length and tuning.  Scope more nodes through your circuit to understand affects of parasitic inductance, etc.  My 40-IGBT version was my third coil and sixth drive system.  (I'd made multiple iterations of electronics for my first two coils, improving on what didn't work well.)
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #4 on: April 13, 2020, 05:41:42 AM »
I can't really get farther with my first coil because of space and noise concerns. I'll probably have to take it somewhere else, which is kinda hard right now with the coronavirus outbreak. Right now I'll probably just get started building the bridge and the driver for my second coil (the parts will probably also take a while to come here since I'm buying from China).

I'm probably going to use the same IGBTs that I used last time. I don't know why, but some of them seem to show a voltage drop when tested with my multimeter.
I have enough space on my heatsink for a few diodes too, and I think I can use a few RURG8060 diodes.

Here's the circuit I came up with for driving the gates. Any problems? (Checks out on my circuit sim)

I'll probably have to determine the gate resistor values experimetnally

Another thing I want to ask about is the ferrite cores. I'm trying to use smaller ferrite cores because they're less expensive to ship. I'm wondering if I can get away with using a ferrite core with dimensions OD 31mm and ID 19mm. The one I'm using right now has OD 48mm and ID 30mm.

Also where did you find IGBTs with 350A peak current rating?
« Last Edit: April 13, 2020, 07:10:48 AM by ritaismyconscience »

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #5 on: April 14, 2020, 06:26:43 AM »
The best way to see if your gate-drive ideas work is with simulation.  Include starting and stopping the gate drive waveform.  Once it looks good stand-alone, simulate all four driving an H-Bridge.  (The IGBT models I've found simulate slowly and/or have other issues.  I usually simulate with FETs, even when the final circuit will use IGBTs.  But that requires manual evaluation to make sure there's enough switching dead-time to handle the IGBT's slower turn-off time, since IGBTs are primarily minority-carrier devices, so have base storage time.)

For GDT, the requirement is that magnetization current isn't large enough to cause much voltage drop across the driver chip outputs.  Meeting this requires reasonable inductance, and especially avoiding core saturation.  More turns can compensate for a smaller core, but more turns adds leakage inductance.  If you stay with a floating gate-drive buffer, then leakage inductance is less important, as the GDT will be driving much less capacitance.

BTW, IGBT bricks are just multiple parallel devices, IGBTs and diodes, in one package.  Bricks typically include individual resistors for each gate internally, and separate source pins for gate-drive return.  Driving bricks directly through GDTs is common here, so should be possible with paralleled TO247 devices.  However, I have no experience with that, as I have floating buffers.  I'm using isolated gate driver chips rather than transformers, however, to feed the buffers.  The isolated driver chips add dead-time between turn-off and turn-on.

Concerning your specific ferrite core, I can't say much without specifications.

My IGBTs are rated for 240A peak at 15Vge, including safe hard turn-off from that current.  For soft-switching, I'm running them past their peak current specs, and at 18Vge.  (Most designs here run bricks a bit past their peak current specs as well.)  I'd initially tested one part with 500A pulses.  Just finished a more aggressive stress test running one part with ring-down waveforms of 650A peak, repeated at 30Hz.  No heatsink, so the IGBT case is 138C.  After about 25 hours running this way the IGBT fried shorted.  I haven't dissected it yet to see if the IGBT or diode actually fried.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #6 on: April 16, 2020, 07:59:09 AM »
I bought another one of those FPGA boards, but this time I bought one with phase lead. The seller claims that they just replaced all the logic (like the 74xx chips) with the FPGA. I'll upload the user manual sometime later (seller says he'll email it to me).

Here's the interrupter circuit:


Edit:
http://s000.tinyupload.com/?file_id=01763471399166613607

^schematic
« Last Edit: April 17, 2020, 01:40:47 AM by ritaismyconscience »

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #7 on: April 19, 2020, 10:35:43 PM »
I'm trying to figure out what power supply to use. Would a 24V 1A switching power supply work for this? I'm worried about noise getting into the power supply and destroying it.

I guess a "dumb" iron core transformer would probably be much more reliable.

Also, I don't think a GDT would work. The CM600DY brick has about 180nF gate capacitance. Each of my IGBTs has 30nF capacitance according to datasheet, so 14 of them would have 420nF, which is much higher than the brick.
« Last Edit: April 19, 2020, 11:10:58 PM by ritaismyconscience »

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #8 on: April 20, 2020, 12:14:22 AM »
My DRSSTC uses a used Dell laptop supply, 19.5V.  Cheap switchers from EBay can be sensitive to EMI.  Good ones are fairly robust.  Common-mode chokes can improve robustness.

Your IGW75N60T parts are at the high end of TO247 IGBTs for gate charge, 450nC for 0-15 gate swing.  The brick you reference (CM600DY) is towards the low end, 2700nC.  Others are higher, such as CM600DX at 4500nC.

Brick gate-charge specifications apply to each IGBT, not the pair combined.  Compare against 7 of your IGW75N60T parts at 7 * 450 = 3150nC.  Using a GDT directly is a reasonable option, as is direct drive as I chose for faster switching.  One down-side of at least the simpler direct-drive designs is that IGBT gate drive swings from 0V to +24V (or 0 to +18.5V in my case), rather than -24V to +24V.  This uses less gate-drive power, but doesn't add dead-time to prevent momentary simultaneous conduction of high and low-side IGBTs.  I use isolated-gate-driver chips, which include adjustable dead-time as a feature.  You will need to include dead-time somewhere in your gate-drive circuit.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #9 on: April 20, 2020, 12:57:42 AM »
It looks like I probably could use GDTs, but I need 4 of them probably (for each quarter). Also, my driver boards use the FDD8424 which can't source a lot of current (50A pk). I'll still probably need a second stage to drive the GDTs

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #10 on: April 20, 2020, 04:39:15 AM »
Discrete FETs for gate-drive have significant advantages, especially improved clamping in the OFF state.  Take a look at UD2.7 schematics for an example - a 9V gate driver coupled to dual FETs.  Discrete TO220 FETs could provide lower impedance.

Rather than four GDTs, I'd suggest a single large one with many twisted-pair windings.  It would be ideal if you could use 6 or 8 IGBTs instead of 7.  Then your GDT could have 8 outputs driving 3 or 4 IGBTs each, or even 12 or 16 outputs driving 2 IGBTs each.  That handles issues with voltage drop on the source leads between the different paralleled IGBTs coupling into gate drive.  Of course, it means needing more gate damping resistors and parallel fast-turn-off diodes.  (The IGBT bricks typically have multiple internal gate resistors.)

Make each GDT output one wire of a twisted pair.  After winding each twisted pair, re-twist the primary leads as a pair to the driver, and the secondary leads as a pair to the IGBT(s).  Thus, if you have 16 GDT secondaries, you'll also have 16 GDT primaries, which all wire in parallel to your discrete FET driver buffer.  Of course, make sure the phasing is correct for each output.

Although not popular here, EE core sets work well.  My SSTC GDT has 8 outputs and 8 paralleled primary windings on an EE55 core, two turns per winding.  Here's a couple EBay links for suitably EE55 cores, but you can find other sources too:
https://www.ebay.com/itm/E5521-Ferroxcube-E55-E-EE-EE55-Ferrite-Cores-transformer-AL-6300-1set/251024599965?epid=1372843764&hash=item3a723b6b9d:g:bxsAAOSwF7lb6rmf
and
https://www.ebay.com/itm/Qty-4-FERRITE-E-CORES-H7C1-EE-55-55-2-NEW-2-PAIRS-or-4-PCS-TDK-H7C1-77-23-Z/310352364736?hash=item48427144c0:g:sD4AAOSwEVZaXmTF
Two turns is sufficient on this core as long as your gate-drive buffer has low output impedance (and a sufficiently-large series capacitor if you include such).  You could use three turns for margin.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #11 on: April 20, 2020, 09:35:59 PM »
Looks like it's possible, but I still need more current than my driver can provide. Output stage mosfets on my driver are only good for 50A peak. I guess I could parallel them to get 100A, but my simulation says I need about 160A peak assuming 0.35  ohm gate resistance (or 10 ohms per IGBT). I also don't think the copper traces on the driver board are going to act as a good heatsink.

One problem I see with SMPS is that there's a Y capacitor between mains and the output ground. At 80khz you would pull quite a lot of current through that capacitor.
« Last Edit: April 21, 2020, 01:44:11 AM by ritaismyconscience »

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #12 on: May 16, 2020, 11:45:31 PM »
Looks like my stuff came



IGBTs def look suspicious, I tried testing G-E capacitance. You can see the results under each transistor. It could simply be that the one on the left is a newer version of the other two (you can see the middle transistor is real but has a significantly lower capacitance than the right transistor).



Seems to be some sanding marks on the suspicious ones. None on the real ones.



Back side looks different too (3 on top are questionable, two on bottom are real)

Edit:
I found some pretty good evidence of sanding and remarking:


Take a look at the date code markings. Top one definitely is sanded, you can see the ridge isn't as big as the one on the bottom, which is real. Will be messaging the seller. Anyways, I bought these for the same price as the other ones I used in my big TC, but the other seller actually sent legit parts.

« Last Edit: May 17, 2020, 01:16:48 AM by ritaismyconscience »

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #13 on: May 17, 2020, 02:25:05 AM »
It's hard to tell counterfeit for sure unless you can measure an out-of-spec parameter or check the lot code with the vendor.  (We've had cases of lot codes that the real vendor never produced.)  The different packaging could be just a different packaging vendor.  Some companies use multiple vendors for packaging the same die, especially for high-volume parts.

The low capacitance could indicate counterfeit, or possibly a newer version of a real part.  As semiconductor processes advance, it becomes possible to make smaller die with the same current capability.  Usually thermal performance suffers, however, especially transient thermal capability.  You could search for any updated specification with higher thermal resistance, or dissect one of each and compare die size.  Of course, a smaller die could also easily indicate a counterfeit part.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #14 on: May 17, 2020, 03:14:25 AM »
I think these are fake still.
I also bought some other parts that had the same pattern (horizontal sanding marks) These diodes turned out to be fake as well (capacitance of 204pF vs 760pF for real part, off by a factor of 4). I guess I got lucky the first time I bought, as those parts were genuine.

Is there a simple way to test the current rating? I don't have any fancy test equipment at home.

Edit: sellers insist they are real, usually with fake parts seller just gives up because doesn't want to get a fraud complaint. Seller says that they are sending out newer parts now. Anyways testing the reverse recovery time should be really easy.
« Last Edit: May 17, 2020, 04:28:59 AM by ritaismyconscience »

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #15 on: May 17, 2020, 05:21:44 AM »
Current testing isn't terribly difficult, although it will be hard without being able to capture two simultaneous  single-shot waveforms.  The simplest is to bias the IGBT constantly on with 15Vge (to match specified voltage drop) and apply collector-emitter current.  Gate voltage can come from any random source, even a couple half-dead 9V batteries that give ~15V.  Here's the circuit I often use:


Parts can be rearranged too, and values can be adjusted.  The basic idea is to make an L/C ring-down test that hits the intended peak current on the first half-cycle.  Here I use 20uF and 20uH to make a 1-ohm resonance.  Charging C1 to 220V would ideally produce 220A peak, but due to losses may be around 200A.  Scope Vce to see if voltage drop is as expected, and check diode voltage on the second half-cycle.  Scope voltage on the L1-C1 connection to see the actual ring-down amplitude and frequency.  Presuming your capacitor is accurate, frequency will give a measure of impedance.  To find peak current, average start and end voltages for the first half-cycle, then divide by impedance.

For the inductor, I usually just wind an air-core coil with some scrap of wire.  The capacitor is best to be polypropylene - a motor-run capacitor or DC link or whatever.  It doesn't need to be rated for AC use for this short test.

The hardest part is getting a clean switch closure.  If you have any TRIACs around, a BTA16 will handle 500A for such a short test, and produces a clean waveform as long as voltage is high relative to a diode-drop.  If not, prop/tape/... a pair of wires, one against a board and the other a cm or two above it.  Hit the top wire down with a dowel or plastic rod or whatever.  The hard fast impact will often make a reasonable fast switch closure.  (Using relatively large C and L values for low frequency makes switch closure less critical.)

BTW, I use this ring-down test much more often to test inductors.  Same circuit except the IGBT and its gate supply are missing.  The ring-down waveform will start with a flat-topped distorted sine wave due to inductor core saturation, then ring down to more of a sine wave.  Measure area of a half-cycle, divide by 2, and that's the volt-second capability of the inductor (or transformer winding).
« Last Edit: May 17, 2020, 05:34:11 AM by davekni »
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Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #16 on: May 18, 2020, 03:18:40 AM »
Ended up figuring out what the parts were.

Seller ended up admitting that they were the 50T60. I tested a real 50T60 part, and I confirmed this (G-E capacitance was 12nF for the 50T60 part, similar to the fake 75T60s).

Ended up agreeing on a partial refund (since I did not want to ship the products back to China).
« Last Edit: May 18, 2020, 03:47:00 AM by ritaismyconscience »

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #17 on: May 23, 2020, 04:14:52 AM »
Some pictures of my PCB so far:






Any problems?

Offline davekni

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Re: Building a bigger DRSSTC
« Reply #18 on: May 23, 2020, 07:41:41 PM »
Nice low inductance!

When I've had to stack capacitors for space constraints, I run copper foil up the sides to reduce inductance of the long leads.  You should be fine as is, though, especially since it looks like you plan to bolt a couple electrolytic caps directly to the board.
David Knierim

Offline ritaismyconscience

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Re: Building a bigger DRSSTC
« Reply #19 on: May 26, 2020, 02:01:33 AM »
Done with the full bridge:

Top view:


Bottom view:


A closer picture of part of it:


Transistors are actually 50T60 (I wonder if I can run it at 200A each or 1400A total). Also, I'm using 3 x RURG8060 as antiparallel diodes, do I need more?

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Re: Building a bigger DRSSTC
« Reply #19 on: May 26, 2020, 02:01:33 AM »

 


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July 02, 2020, 10:47:18 PM
post Re: How to prevent secondary oxidation / Rust
[Dual Resonant Solid State Tesla coils (DRSSTC)]
johnf
July 02, 2020, 09:03:20 PM
post Re: How to prevent secondary oxidation / Rust
[Dual Resonant Solid State Tesla coils (DRSSTC)]
DJthefirst
July 02, 2020, 08:01:03 PM

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