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Messages - Hydron

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1
Dual Resonant Solid State Tesla coils / Re: My First DRSSTC
« on: Today at 11:07:22 AM »
The 75N60s were fine hard switching 225A (their rating), I suspect you could get away with a bit more than this with resonant switching but haven't tested them at higher currents than this. They won't be as resilient when grossly overloaded as a brick, but if you pop a set then they are also cheap to replace!

Edit: I wrote this reply based on the first page, didn't see page 2. Comments still stand but it sounds like you've already done some testing to failure :P

2
Thank for sharing your design here - looks like another good option (I especially like the phase shift adjustment - I tried to do a similar thing on a UD2.x design but didn't quite get it right) and the price seems very reasonable for complete tested board.

I agree that info on the FPGA implementation would be very interesting - if you are able to share anything it would be appreciated.

As for open vs closed source, it is a tricky one. If you do a bunch of hard work (both with initial design and subsequent user support) then having someone rip it off and sell to people with no support doesn't seem fair.
At the same time, open source allows me to take something and improve/modify it to do exactly what I need it to, rather than stick to a fixed design. The UD3 work being done is a good example of this - I will be taking Netzpfuscher's work and using it both to run on my coil (after I modify both the hardware and software design), and also as a way to learn about using an RTOS on ARM (I am more experienced with hardware design than coding).
Given this my personal opinion would lean toward making things open-source when practical, but I certainly understand both views!

3
Dual Resonant Solid State Tesla coils / Re: (DR)SSTC II
« on: January 07, 2019, 10:23:38 PM »
Sorry about the confusion about bus vs capacitor ESL - I should have specified I meant the combination of the two. The bus layout in your photos (including the latest) is still a high inductance layout which I would expect to cause both spikes and the "sawtooth" waveform.

For best results the positive and negative rails of the bus should be wide parallel planes, and the emitter-collector connection between the two transistors in each leg should be as short and fat as possible. I suspect some of your noise is caused by this bus inductance, in addition to the sawtooth output voltage waveform.

An easy way to fabricate such a bus (if you are familiar with PCB design tools) is to make it out of a PCB, however you would need to consider clearances and copper thickness (e.g. 1oz is probably not OK for a 1000A peak coil, but may be fine for ~100A if traces are doubled up and designed appropriately). If you are not familiar with PCB design tools and requirements then copper sheeting can be bought on ebay etc, and works well with IGBT bricks and the appropriate mounting screws/hardware and some labour cutting out the appropriate areas and holes. I would suggest that a PCB based design is by far the best way to go for TO-247 IGBTs however.

4
Dual Resonant Solid State Tesla coils / Re: (DR)SSTC II
« on: January 07, 2019, 08:09:49 AM »
I assume that the "saw tooth" output is due to too much ESL/ESR in the DC link/bus capacitors - I have seen similar (though not as bad) on my larger coil at high currents.

As for spikes on the CT output, I have also seen this - it's worse with lower burden resistance/higher ratio CTs, and my Pearson current monitors show less than my DIY CTs. I've attached an example of this - red trace is a low-ratio pearson with an attenuator (helps even further), blue is a DIY CT:

This is still a decent performance from the DIY CT - ignoring the minor sensitivity difference they both give very similar output with the exception of more noise on the initial transitions where the IGBT switching isn't quite as soft as it is later. This difference in noise performance can also be seen between Pearson models with differing V/A ratios.

5
Woah, nice work! Will have a look at the design when i get a chance. I've already got a low frequency VNA (aka FRA) and it's super handy, including for measuring tesla coil bits.

6
Dual Resonant Solid State Tesla coils / Re: TO-247 IGBT for DRSSTC
« on: November 11, 2018, 11:51:56 AM »
Well I was thinking of using two half bridge boards together each as one half of a full bridge.  Are there any issues with having the two half bridges on different boards, as long as the input power connections are tightly coupled?  Or do they need to share the same DC Link capacitors on the same board?  It's harder to do the layout for 8 transistors and a full bridge on the small board, but I give it a go.

Dan
Laci's reply brought my attention back to this - I have done some thinking about this exact issue recently and have come to the conclusion that different boards are fine - you will get the full primary tank current flowing between them on the power connections, but this is a nice sine wave - no switching transients etc so just size the cable big enough and you're OK, no need to worry about anything other than big enough and not unnecessarily long (it will simply add to the primary inductance). The power connections between the boards are also another place to locate the primary CT - in my case with multiple parallel bridges and a split MMC, this turns out to be the most convenient location, with no high voltages (like will be present on both ends of the primary coil with a split MMC) to worry about.

Laci - I would pay more attention to the thermal resistance number than the small difference in Icm in this case - the 30 part is much lower than the 20 part, which indicated a larger die inside. And worst case if you pop the 30s you can try again with something different!

7
Laboratories / Re: Moving to a new lab!
« on: November 10, 2018, 04:51:13 PM »
Nice equipment!  How do you like the Rigol scope?  I was going to get one but I heard some mixed reviews.

Did you ever get the solar panels set up?

That Rigol was probably the best value low end scope up until a year or so ago, but I'd look at the 4 channel Siglent SDS1000X-E units as an alternate option now - have not used one but it sounds like there are some very significant improvements vs the Rigol for not much more money.

8
Dual Resonant Solid State Tesla coils / Re: Next Gen DRSSTC
« on: November 07, 2018, 09:50:32 PM »
Thanks! I'm about to order some boards to upgrade my big coil (in NZ) to a UD3 to use when I'm there over Xmas. Will take some pics/video and also some topload current waveforms to share.

9
Nice boards, I assume they are all homemade.

How did you do the silkscreen? And a little funny with a mirrored silkscreen sitting on the opposite side of a surface mount component board :)
I'm guessing that silkscreen is toner transfer? If so, nice trick, never tried it when doing my own PCBs.

10
I've just finished laying out a couple of boards for the bridge and gate drive of my larger 160mm DRSSTC which I'll be upgrading when I'm in NZ over the (southern) summer. These may be of interest to others, so I've uploaded all the design files to github, and also put a couple of 3d renders below:

https://github.com/Hydron-4hv/Big_DRSSTC_Bridge_PCBA

https://github.com/Hydron-4hv/Big_DRSSTC_Gate_Drive_PCBA


Notes:
- Github README has the majority of explanation in it
- The schematic pdfs have a 3d model in them, open with acrobat reader to view
- Original files are in Altium format, I may be able to export to something else if needed (KiCAD is unlikely though, sorry!)

I'd welcome any suggestions/criticism, these are going off to JLCPCB in the next few days and obviously I'd like them to be correct!

I may have some more PCBs coming up too, but they are likely to to be more specific to my design (though I will share them anyway).

11
Electronic circuits / Re: Eaton PowerWare 20 kVA UPS teardown
« on: October 16, 2018, 01:15:17 PM »
Aha, I recognise a lot of stuff in this video.

A few years back I came across the toss-out pile of a UPS company that was moving premises and scored a bunch of PCBAs that match what Mads has here (along with a lot of other goodies) - nice to know what they were actually out of!

12
In theory commutation losses are minimised by always switching ON at zero current, and OFF while current is flowing. This avoids any diode reverse recovery, as the commutation from diode to IGBT only happens at zero current, with zero reverse recovery. In practice, Alan's coil isn't zero-current switching that well (part of the earlier discussion), and mine is in it's prototype stage, with sub-optimal layout etc.
If you have a look at the waveforms I posted then you can see that alternate half-bridges are phase shifted (switches each cycle), this spreads the losses out. Alan's is just phase shifting one bridge, but the driver he is using can alternate them like what can be seen in my pics.

13
I'd have to examine the buck more carefully (my understanding of these beasts changes over time) but I remember at one
point tuning the ZVS in DRSSTC mode and then checking it during QCW mode and being surprised that it still looked in tune.
I mentioned it to loneoceans and he was not surprised at this.
Yeah I don't see any reason why the ZCS (not ZVS) tuning should change between QCW/standard interrupter modes - hence why I said I was surprised. I'd need to look at the maths regarding the phase lead technique in use - maybe it ends up being peak current dependant (though this would also apply in normal non-QCW mode), or possibly the switching noise throws it off.

14
Hmm, I'm surprised that the optimum ZVS point changes so much over the ramp - in a normal DRSSTC it doesn't seem to shift much. Did your other coil (the buck one) do the same thing? Would probably need to do some simulation to get to the bottom of it, though at some point if it works then it's not worth chasing too much more.

As for the alternative phase shift mode, the "special gate drive techniques" thing is a bit overblown - as long as your gate drive transformers have enough turns on them to handle half the QCW frequency (i.e. about 150kHz and higher) then they should work fine. You can always test it at low bus voltage, if there are no issues then it'll work at higher power too.

15
Interesting scope caps, thanks for sharing. Looks like you're seeing ringing up in the 10s of MHz on switch-off (~20Mhz), similar to me, though you are also seeing nastiness on turn-on due to poor zero-current switching.

I've attached another capture I dug up from previous testing with a very big spike well over what the IGBT is rated for. I'm not 100% sure how real some of the worst ringing (at about 70MHz, right after the big spike) is though, as I know that the differential probe I was using has it's own issues (input leads are a bit long and ring with the input capacitance at around this frequency).

It looks to me like the green trace is nominally zero-current switching for the whole burst, with the blue trace being phase shifted to perform the ramp. I'd expect the ZCS trace to be fairly clean, but it looks like it's switching a little late and you're getting spikes on that one as well due to diode reverse recovery at turn-on. I'm not familiar with the driver you're using, but the idea of a fixed inductor phase-lead and an adjustable RC delay following it seems sensible, and probably just needs some tweaking (delay seems too long to me - maybe take that extra capacitance out and reduce the resistance). I have actually adjusted phase lead with a coil running a few times - normally I'd use a dummy-load (cast iron pan or something with high losses) in place of the secondary or simply nothing in place (current can ring up _fast_ with no load though), and be very careful what I'm touching. The UD3 driver I'm playing around with lets you do it remotely though, which is very handy!

The other thing I'd look at is using the alternative phase-shift mode:
Quote
2. Mode in which shifted half-bridge change places every feedback cycle.
This is a very advanced mode that provides even better heat dissipation balancing, and without the necessity for interrupt to occur. It requires good attention to gate drive quality(isolated drivers are recommended, but was successfully tested with GDTs too). For activating it, set jumpers to (AltShift = 1, PSkip = 1).
This is what is seen in my earlier screenshot - it spreads the losses from hard-switching between the two bridges. This won't solve the spike problem, but is worth using if it works.

16
I can take some data for sure. It would be helpful to know what would be most informative for you. I can currently do things like CE (with an isolated scope) and primary current.

By detailed would you means images like the one you posted with a few cycles of Vce and primary current? And doing this at points towards the front, middle and end of the "ramp".

I have no idea how to make simultaneous Vce measurements of two transistors. Since each is at a different potential. references each to ground seems like it could get pretty confusing quickly.
My Vce measurements were done on the lower transistor in each half-bridge, so they shared the same reference potential (-ve DC bus voltage). I was running off a bench supply so no issues grounding the bus - to do the same on mains one would have to isolate the bus or the scope (preferably the former unless the scope is designed for it).

To get the actual bridge output waveform (as seen by the LC primary circuit) I just turned on the maths trace with a subtraction operator.

And yeah, similar sorts of captures was what I was thinking, as you suggest in front/middle/end of ramp. I was asking as it'd be interesting to see how similar the spikes you're seeing are to what I get. Don't inconvenience yourself making measurements - was mainly asking in case you already had some scope shots, or already had the scope/coil setup anyway.

I would not stress too much about getting perfect ZCS - as long as the current isn't too high when you switch the losses will be minor, especially if switching is a little early rather than late.

Edit: I agree that not many people have both side by side - it's valuable to have the perspective of someone who has built both, especially when it agrees with the plan for my coil :P

17
Solid state Tesla coils / Re: Strokes caused by Tesla coils
« on: October 11, 2018, 05:01:06 PM »
No idea, but I'd say it's unlikely to be the cause of a tesla-coil related demise, unless someone manages to get an RF burn through their head!

The biggest danger is definitely the large mains transformers used by bigger SGTCs (capable of amps at HV, insta-death), followed probably by big HV caps on larger SGTCs. Maybe also voltages used in VTTCs (not very familiar with them unfortunately).

Other than that, mains is probably the most dangerous part of solid state coils - everything else might be able to give you a nasty RF burn but is unlikely to kill you. For something like a QCW the fright and stumble/fall from being hit by one is probably more dangerous than a spark from one hitting the body (which, from experience, isn't bad at all).

As always, look to crazy russians on youtube for an idea of what people have gotten away with: https://www.youtube.com/channel/UCJeTqOt4xDuL2imX7zYnJlw (e.g.
/>
Any reason why you asked about strokes in particular? It wouldn't hurt to start a thread on safety, but this is very specific!

18
Thanks for the scope caps and link - will have a read tomorrow. Would also be interesting to see the detailed waveforms too (when convenient), possibly at different parts of the total on-period? I haven't seen too many real waveforms from QCWs posted, so would be of general interest as well as for this thread.

19
Turn-on vs turn-off transients depends on what type of modulation you're using for your phase shift.
- "Leading-edge" modulation, where you modulate the time between the zero-crossing and turning on the IGBTs, will result in reverse-recovery losses when the current is commutated from the free-wheeling diode to the IGBT, and this may cause turn-on spikes, but turn-off (at zero crossing) would be cleaner.
- "Trailing-edge" modulation, where you switch the IGBT on at the zero-crossing but then off early (modulating the time spent on, rather than off), will not have any losses from reverse-recovery and a clean turn-on, but Vce spikes at turn-off, as current flowing in the loop between the two IGBTs and the bridge output has to commutate from the turning-off IGBT to the free-wheeling diode of the opposing IGBT _very_ quickly.
- Many tried-and-tested IGBTs, in particular the FGH60N60SMD and family, have significantly lower turn-off losses than turn-on (taking into account the diode reverse-recovery), so trailing-edge has been the method normally used.
- This all assumes non-modulated switching occurs at zero-current or close enough for minimal losses and Vce spikes.

(decided to make this a generic summary - maybe of use to others. BTW your IGBTs seem to be pretty similar in turn-on/turn-off losses assuming the datasheet is accurate)

I have no idea what type of modulation your driver is using - would you be able to share some scope captures of the primary current and the output of the half bridges?

See my attachment for an example of trailing-edge modulation and some _really_ bad spikes on turn-off (I think this may have been from some of the faster IGBTs i'd tried). The cyan maths trace is the voltage seen by the bridge - see that it starts each cycle adding energy then switches off after a period.

Edit: Try googling "IGBT active clamping" to find out what I was describing. There are some fancy proprietary schemes, but can be done with just a TVS and a diode. If I find a good reference I'll link it here. With a DRSSTC you'd need to also be careful that the active clamp doesn't lead to shoot-through - it delays turn-off.

20
I'm not convinced that TVS diodes will make a lot of difference just put straight across the IGBT - if you look at voltage vs current, at reasonable currents the voltage rises quite far above the rated breakdown spec. I may still put in provision to fit some on my design though if space permits - worth a go given how easy it is.

My plan for a TVS was not to clamp directly across the IGBT, but to build an "active clamp" circuit - by placing a TVS of appropriate voltage (i.e. somewhere above bus voltage but below Vce(max) ) between collector and gate; if a turn-off spike goes above the TVS voltage it will bypass enough current to partly charge the gate back up to flow appropriate current and clamp the voltage at approximately the TVS value. This only works because the spike is caused by the turn-off speed being otherwise too fast and uncontrolled during part of the IGBT turn-off and commutation process (something that is very hard to fix with a bigger resistor without increasing losses significantly). There are a few ways to do this active-turnoff (e.g. using a charged capacitor instead of a TVS) - I will need to have a look at the options.

I am a bit surprised that you are seeing spikes on both hard and soft switching transitions - I am seeing nice clean waveforms with soft and nasty spikes with hard. What IGBTs are you using? And do you have a link to the PC boards?

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