High Voltage Forum

Tesla coils => Dual Resonant Solid State Tesla coils => Topic started by: sjsimmo on November 20, 2018, 05:31:36 AM

Title: My First DRSSTC
Post by: sjsimmo on November 20, 2018, 05:31:36 AM
Hello again,

I'm working on a DRSSTC, but am having some problems. Any help would be appreciated  :)

The current Specs:
- FGY75N60SMD full bridge
- 400x90mm secondary, ~2000 turns
- 7 turn primary
- primary resonant frequency ~140khz
- my own driver design (I know, should have just used one of the UD designs...)
- 500hz interrupter 10-200us on-time (will later be replaced with a fibre optic based midi interrupter)


The driver:
The driver is primarily based on the onetesla Gen-1, but I have modified it to include OCD.



The problem:
I am currently operating the coil with the secondary not in place, but primary connected.
When I inject a 100-200 kHz square wave into the driver feedback CT input, the gate drive waveforms seem ok, and the circuit operates as normal, OCD works when current is applied to the OCD CT inputs, system shuts off at zero crossing point when the interrupter goes low etc.

However, when I connect the CT's and apply voltage to the bus, the circuit starts oscillating below 12VDC, but at the wrong frequency! It seems to oscillate at about 250-300 khz (maybe the 2nd harmonic  ??? ). In addition to this, the output of the GDT becomes horribly triangular. placing an oscilloscope probe on the primary coil, shows that the primary circuit voltage isn't ramping up, but is just a constant AC (as expected, considering it's not oscillating at the resonant frequency). The same thing happens as input voltage is ramped from 0-120VDC input (the waveform just grows stronger with larger input voltage).

On a side note, when I tried applying 24VAC to the bus, without slowly ramping the voltage up, the UCC37321 blew up violently :-[ . I have ordered replacements (not sure when they will arrive) , but am not sure why this happened...

I don't have any scope shots of GDT outputs, primary current etc. at the moment, as I blew up the driver before thinking to save them...

Anyone have any ideas/suggestions?

thanks in advance, sjsimmo[/attachment]
Title: Re: My First DRSSTC
Post by: Laci on November 20, 2018, 07:14:56 PM
Hi,sjsimmo!
Did you try swapping the feedback polarities or the IC's blew up before you could?I also see a few things to complain about,like the OCD full bridge rectifier,which seems to be mains rated frequency for me(am I right?)and the capacitors around the UCC driver IC's.Another possible problem can be the way you added the R6 resistor to the feedback,because the interrupter and the Q1 OCD MOSFET might influence the signal.Also just one pair of those MOSFET drivers is probably not enough to properly drive a full bridge,maybe try paralleling them only if you find a solution to the problem.I definitely recommend the MOSFET output stage driving system of the UD's! :)
Title: Re: My First DRSSTC
Post by: Mads Barnkob on November 20, 2018, 08:32:47 PM
I agree with Laci on the full bridge rectifier on the OCD, you need some fast small signal diodes here.

The UCC drivers should be good enough to drive a full bridge of TO-247 IGBTs, so as long as the GDT is made properly that should work ok.

There is a only a few things can make UCCs explode that way. Driving them into a short circuit or maybe a saturated GDT? If there is no load connected to its output, they can oscillate into MHz region from their own output capacitance and burn down.

Steve Ward has written on his site that all his early designs are not to be used as they are full of flaws and bugs. They are only there to keep a track of history. You should at least use the UD1.3b or UD2.1 if you are driving bricks. I never tried building his earlier designs, so I got nothing to add with experience on those.
Title: Re: My First DRSSTC
Post by: sjsimmo on November 20, 2018, 11:12:10 PM
Thanks for the replies :)


Laci, you are indeed right about the rectifier, I'd never thought about the fact that it would have to be high speed (obvious now that you mention it)... I'll fix that up when I get the chance, however it shouldn't have anything to do with my current problem.
The feedback circuit, is pretty much a direct copy of the oneTesla driver. Interestingly the R6 resistor is the one part of that circuit that I've never understood... (I've attached their circuit to the post). I believe it may help the system start oscillating, but this is just a guess.

I'm confused about the capacitors around the UCC (what's the potential problem with them?). I might try paralleling a couple drivers, when I get them, to see if it helps (I've ordered 10 of each :) ).


Mads, I was wondering about the UCC failure, It could possibly be saturation, as I think I only tested the circuit with a 200kHz feedback signal from a function generator (much higher than the 140 kHz resonant frequency!). It could also be short circuit, as I was scoping the IGBT gates at the time, and it is possible that the probe's ground could have bumped something... I was also thinking that it could be caused by a ultra high frequency coming from the feedback, due to the sudden change in bus voltage (not sure if this makes sense)?


I've also tried swapping the feedback CT's polarity. This resulted in the primary voltage waveform which I've sketched in an attachment to the post. (only going from memory, so it's not by any means accurate...).


Any other thoughts on why the circuit was operating at double the resonant frequency?
Title: Re: My First DRSSTC
Post by: sjsimmo on November 23, 2018, 03:48:47 AM
The replacement UCCs arrived today :), so I've done some tests with a 100 kHz feedback signal (no voltage on the bus).

Without the IGBT gates connected (GDT is connected), the output of one of the UCCs looks like this (yellow trace):

This would indicate that the transformer isn't saturating (with a 500hz interrupter repeat rate, the UCCs are drawing less than 1mA average) unfortunately, I can't scope the output of the GDT without the IGBT gate connected, because the probe's impedance seems to cause the UCCs to go into ultra high frequency death mode (they didn't blow up, luckily)...

However, once I connect the IGBT gates, the UCC output waveform becomes significantly worse (green trace):

average UCC current draw is now about 8mA. Looking at the IGBT gate waveform is slightly better, but still not very good (green trace):


The GDT has 4 10 turn primaries in parallel (20 turns on the secondaries), on this core: https://au.element14.com/epcos/b64290l0674x038/ferrite-core-t38-13-5uh/dp/2673406
The wire is rainbow ribbon cable, separated into individual conductors, then twisted in 3s with 1 primary and 2 secondaries in each bundle. The gate resistors are 6.8ohm (no gate diodes). Unfortunately I don't have access to a camera at the moment, so I can't give you a picture of it  :(

Does it need to have 2 ICs paralleled, or do you think it's my GDT that's causing the problem?

Thanks again,
- sjsimmo



Title: Re: My First DRSSTC
Post by: Mads Barnkob on November 23, 2018, 06:39:47 AM
All those waveforms look good, however it is a bit overdamped once you connect the IGBT gates. Try to use a lower value gate resistor.

See this for future troubleshooting:  http://www.richieburnett.co.uk/temp/gdt/gdt2.html

Sent from phone.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 08, 2018, 09:51:47 AM
Sorry for the late reply (yr12 head start program has been keeping me busy)...

I swapped out the 6.8ohm gate resistors for 3.3ohm ones today, but the IGBT gate waveform looks almost identical ???:
 [ Invalid Attachment ]
I guess the slow response must be caused by the driver ICs or the decoupling capacitors... the decoupling capacitor on the UCCs output is a 3.3uF multilayer ceramic, and the decoupling capacitors on the UCC power pins are 47uF electrolytic with 0.1uF multilayer ceramic parallel. Are these components ok? It may also be the long leads to the power supply...

I've also swapped out the OCD rectifier for 1n5819 diodes (the same are used in UD1.3b)  :)

Do you think it's ready to have  power on the bus again, or do I need to address the IGBT gate waveform first?

Thanks for the help so far :)
- sjsimmo
Title: Re: My First DRSSTC
Post by: Laci on December 08, 2018, 12:52:41 PM
I think 0.1uF of ceramic capacitor is not enough for UCC decoupling,since the electrolytics are bad for high frequency filtering.I always used at least 1uF polypropylene capacitors,but ceramic capacitors are also good.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 09, 2018, 06:05:02 AM
Thanks for the advice Laci, I don't have any larger value ceramics/polypropylenes in stock at the moment, so I tried swapping the electrolytic caps for 220uF ones, still no difference... I've now decided that I'll deal with that problem later when I've got some bigger ceramics  ;).

I also put voltage on the bus again today (still with no secondary coil). It works! At 10VAC the circuit starts to oscillate, with the current smoothly ramping up in a sine-wave. and at 50VAC with 60us interrupter on-time, the circuit even triggers the OCD (currently set to ~50A) :). My only theory as to why it wasn't working previously, is that one of the UCCs was already half dead. Before they blew up, the GDT output wasn't particularly symmetrical, and wasn't reaching as high a peak-peak voltage as it is now. I believe that this non-symmetry must have caused the circuit to operate at the wrong frequency.

After about 5 minutes operating with those settings (and 500hz interrupter frequency), the heat sinks got only slightly warm. One odd thing is that one IGBT got slightly warmer than the others, does anybody have any idea why this is?  ???

I think the next step will be to add the secondary and create some lightning  :D

Edit: attached scope shot of primary current waveform.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 10, 2018, 02:18:59 AM
EDIT: Disregard this post, turns out there was a dodgy solder joint on the collector of one of the IGBTS :D
Sorry for double post,

I just scoped the Bridge output (still no secondary), and something isn't right (yellow trace, green trace is primary current)...


It would seem that the bridge isn't outputting a symmetrical square wave  ???. Any ideas as to what could be causing this?

All gate waveforms seem ok. The bus voltage is consistent, but one half of the H-bridge has a consistently incorrect output waveform.

Here's another scope shot showing each half of the bridge on separate channels (referenced to bus negative, 12VDC input):
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 10, 2018, 09:18:24 AM
It seems like half of you bridge is not switching properly.

So either you have a problem with the driver not outputting a good drive signal or 1-2 of your IGBTs are bad.

Try to switch the leads around from GDT to bridge, if the bad switching IGBTs move to the other pair, the problem is in your driver circuit and if it stays the same, its your IGBTs thats something wrong with.

Check gate resistors, diodes etc as well, they can also be damaged without exploding, f.ex. a gate resistor going half-open will have a very high resistance and it could look like what you are seeing.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 10, 2018, 11:13:38 AM
It's fixed now as I said in the previous post, it turned out to just be a dry solder joint on an IGBT collector. The heatsinks are even cooler now, and the driver is outputting a nice square wave. The current also ramps to 50A in half the time (30us at 50 vac input). I'm now testing it with the OCD set to 100A, and everything still seems to be working fine.  :)
On a side note, is it normal for the MMC to buzz at the interrupter frequency? Mine is buzzing quite audibly, proportional to the primary current. I checked the capacitance and they're all still 0.1uF. Since the circuit is working fine, I presume that this is ok, but it would be nice to have a second opinion.  ???
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 10, 2018, 12:25:15 PM
I misunderstood the edit mark and thought it was a new problem you described :)

The buzzing and to some extend that you can also feel it on your smallest hair on the back of the hand is normal effect of switching high frequency current, it is much more audible than you really imagine. You will however not be able to hear it once you have sparks flying with a sound level approximately as a hovering helicopter.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 11, 2018, 12:03:54 AM
I've decided to upload some of the scope shots, to show how it's currently operating.  :)
Firstly, the bridge output and primary current (~20VDC on the bus):
 [ Invalid Attachment ]
Secondly, the IGBT gate waveform and primary current:
 [ Invalid Attachment ]
and Finally the primary current at ~75VDC on the bus, triggering the 100A OCD:
 [ Invalid Attachment ]

Do these waveforms look right? My only concern is that it looks like it's switching close to peak current, rather than zero current... This could of course just be my measuring equipment/methods, or the fact that it was been tested at low voltage. :-\
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 11, 2018, 09:13:49 AM
Your waveforms does look fine for low voltage testing.  You even have quite low voltage switching spikes for just testing at 20VDC (or your oscilloscope is not fast enough to pick up the spikes!). Normally you would see very large switching spikes that are 200% of the input voltage, this is due to IGBT output capacitance being constant no matter the input voltage, so as the input voltage rises the spikes ratio to inverter output gets smaller and smaller.

If you are concerned about skew in your measurements, check out this guide: https://highvoltageforum.net/index.php?topic=111.0
Title: Re: My First DRSSTC
Post by: sjsimmo on December 12, 2018, 11:45:08 AM
Thanks for the feedback :) . The lack of spikes is probably just because I'm using a 20Mhz USB oscilloscope (the 'Bitscope Micro').

I added the secondary and succeeded in first light yesterday, but didn't record it till today. Here's the video :D :

Is it normal that the coil is only outputting 8-12cm sparks at 100A? The OCD triggers at above 60VAC input (set to 100A) causing spark length to decrease significantly, so maximum spark length is at 60VAC, 200uS interrupter on-time. I've tuned the primary coil as best I can (starting from a value determined using the function generator/oscilloscope method, then adjusting as far as about 1/4 turn each way (in 2cm increments), until I found the point at which I could have the highest bus voltage without triggering the OCD.

Also, I'm using the garage walls as RF ground (in addition to mains earth). From what I've read, I believe this should work well, but I'm not certain...

Do you think it's ready for turning the OCD up to 200A? (I will eventually set it to 300-350A, once I'm sure everything is operating correctly).
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 12, 2018, 01:15:30 PM
Congratulations on first light!

100A is pretty low for a DRSSTC and I would not expect much happenning at such a low energy, unless it was very long on-times in the ms.

It looks like your coil is VERY close to the metal sheet wall, be aware that this will affect the resonant frequency of the secondary circuit ALOT! You need it to have a certain amount of free space in order for the calculated frequency from f.ex. JavaTC to be true.

Your low performance could also be from bad tuning, which could be the above cause of that, else experiment some more with the tuning point of the primary coil and get some more current flowing :)
Title: Re: My First DRSSTC
Post by: sjsimmo on December 12, 2018, 11:57:02 PM
Thanks for the advice.
The video makes it look closer to the wall then it actually is. The actual distance is about 1 metre, But I'll give it a little more space, to see what happens. I'll also do the function generator/scope check on the secondary resonant frequency, to see how much it's affecting it.
After that, I'll try turning up the current  :D

On a side note, I'm glad to see your still working on the DRSSTC design guide. It's a brilliant resource for beginners like me :)
Thanks for the advice.
The video makes it look closer to the wall then it actually is. The actual distance is about 1 metre, But I'll give it a little more space, to see what happens. I'll also do the function generator/scope check on the secondary resonant frequency, to see how much it's affecting it.
After that, I'll try turning up the current  :D

On a side note, I'm glad to see your still working on the DRSSTC design guide. It's a brilliant resource for beginners like me :)

EDIT 1:
I turned the current up to 200A! Spark the length is now 20-25cm after tuning, with ~100uS on-time and 180VAC input (limited by OCD firing again).  :)

The bridge is now getting noticeably warm after ~30 second runs, so I think it's time to look at fixing the IGBT gate waveform again.

EDIT 2:
I fixed the gate waveform!  :). Turns out Laci's original thought was correct, The UCCs weren't powerful enough. I'm now using 2 paralleled (stacked on top of each other), and the waveform looks much better now (about 0.5uS 0<->20V rise/fall time):

Interestingly, fixing this waveform caused the tuning point on the primary to become much closer to where I originally thought it should be tuned (which was done using a function generator to find resonant frequencies). I believe that the slow gate waveform must have been causing some phase-shift...  ???
The maximum voltage I can put into the bus (without the 200A OCD firing at 100uS on-time), is now 125VAC, indicating that the current must be ramping up faster with the better waveform, and the bridge is also slightly cooler now. Spark length is now approximately 25-30cm, although I don't have any pictures yet.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 17, 2018, 06:49:52 AM
A quick question about the FGY75N60SMD IGBT... What's the maximum current I can run these at?
I've had a look around, but can't find anybody actually using them... (I saw Hydron's review of them, but he didn't specify how much they could handle in DRSSTC use). Since the FGH60N60SMDs are 180A pules rated, and are good for 300A, I figure the 225A pulse rated FGY75N60SMDs should be good for 350A... Does this sound accurate, or will things explode?

Also, I tried using a strike target today, and managed to get 45cm arcs at 170VDC bus, 80uS on time, and 200A peak  :D
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 17, 2018, 12:26:08 PM
You will have to do the calculations yourself :) It is the power dissipation and your permissible temperature rise of the die that will limit how high a peak current you dare to push through it. It is however always fun to see if the theoretic limit is more or less than the real world experiment.

Maximum current for a IGBT through power dissipation calculations: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/
Title: Re: My First DRSSTC
Post by: sjsimmo on December 18, 2018, 02:08:57 AM
Thanks for the guidance :)

I just finished doing the calculations, and got the following values for my coil at 350A peak, 100uS on-time, 1000BPS:
- Fmax1=1290kHz
- Fmax2=984kHz
These values seem about 5 times bigger than what I was expecting. Am I doing something wrong, or are these IGBTs invincible?
 [ Invalid Attachment ]

Thanks again,
-sjsimmo
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 18, 2018, 09:47:51 AM
1000 BPS is really extreme to calculate with, if you want long sparks I think its better to use longer on-time to transfer more power and maybe maximum 500 BPS, a 1 kHz tone is not that pleasant to listen to anyway :)

I looked at this datasheet: https://www.mouser.com/pdfdocs/FGY75N60SMD1.pdf

Did you extrapolate "Figure 15. Switching Loss vs. Collector Current" to get the real mJ losses for the higher current? Or where did you get those low values? Just by eye it looks more like it should be 10-30 mJ (before derating)
Title: Re: My First DRSSTC
Post by: sjsimmo on December 18, 2018, 10:45:01 AM
I'm confused now, that is the datasheet I was using...


The calculated current at switch on is 11.6A, and switch off current is 22.7A. These values are both below the minimum current shown on the graph of figure 15. extending the graph gives a switching loss of about 0.25mJ and 0.4mJ at 80 degrees celsius respectively, or am I reading the graph wrong?  :-\


These values can then be multiplied by 0.2, giving the final values that I wrote down.


The reason for using 1000BPS for the calculations, is because I want to midi modulate it eventually, and I figure that 1kHz should be a sensible limmit... (or is this actually too high?) ??? . As for on-time, I might make that a bit longer, at 200A anything above 80uS has little effect on spark length, but I suppose at 350A this time will grow (although by how much, I'm not sure, since I can almost double bus voltage still.)
Title: Re: My First DRSSTC
Post by: Mads Barnkob on December 18, 2018, 12:50:45 PM
I'm confused now, that is the datasheet I was using...


The calculated current at switch on is 11.6A, and switch off current is 22.7A. These values are both below the minimum current shown on the graph of figure 15. extending the graph gives a switching loss of about 0.25mJ and 0.4mJ at 80 degrees celsius respectively, or am I reading the graph wrong?  :-\


These values can then be multiplied by 0.2, giving the final values that I wrote down.


The reason for using 1000BPS for the calculations, is because I want to midi modulate it eventually, and I figure that 1kHz should be a sensible limmit... (or is this actually too high?) ??? . As for on-time, I might make that a bit longer, at 200A anything above 80uS has little effect on spark length, but I suppose at 350A this time will grow (although by how much, I'm not sure, since I can almost double bus voltage still.)

You are right, I jumped the first step there.

I will try your numbers in a excel spreadsheet I got on my PC at home, I will take a look at it tonight.

Edit: I used a bit more conservative extrapolations on the graphs, noticeably 6V CEsat and Ton 30mJ, Toff 20mJ, (derated by 0,05% on-time and 0,2 for pulses) and I am getting 144 kHz

Title: Re: My First DRSSTC
Post by: sjsimmo on December 19, 2018, 12:19:32 AM
Thanks for doing the calculation :) 

It's interesting to see how much effect changing those values had on the maximum frequency, especially since they're all from reading the same graphs. I guess it's a pretty big guessing game once we go outside the values shown on the data sheets...

So should I proceed to turn up the current until I either get 350A, or an explosion? (going by our two wildly different results...)
Out of curiosity, how much can an IGBT overcurrent failure affect other components? Is it likely for example, to kill the UCCs?

I'm also a little concerned about the fact that my gate waveform still has about 500 nS rise/fall times. Whilst this is a lot better than before, I'm not sure what effects it will have on reliability at higher peak currents. Should I be concerned about this?

EDIT: I've attached a scope shot of my gate waveform (1uS/div)
Title: Re: My First DRSSTC
Post by: Fumeaux on December 22, 2018, 10:25:25 PM
EDIT: I've attached a scope shot of my gate waveform (1uS/div)

You might already know about this page: http://www.richieburnett.co.uk/temp/gdt/gdt2.html
If not, it looks like your waveform is overdamped so i guess you should reduce the inline resistance.

The rest of the topic goes over my head so that's that.
Title: Re: My First DRSSTC
Post by: sjsimmo on December 24, 2018, 01:15:00 AM

Yeah, I've seen that page. It's a really good resource  :)


I've previously tried reducing the gate resistance from 6.8R to 3.3R with no effect (it's still using the 3.3R). This resistance change was before I doubled up the UCCs which had dramatic improvements, resulting in the waveform in my previous post. Previously it was more of a sawtooth wave. I'd think that needing less than 3.3R would be very unusual, considering I'm using a TO-247 IGBT full-bridge.
One thought that occured to me is that maybe I need to run 3 UCCs in parallel, but this also seems a bit ridiculous, given the IGBTs that I'm using.

On a side note, the biggest spark so far is now 68cm to a grounded object with 300A peak current, 240VAC input and ~50uS on-time. (increasing on-time causes the OCD to fire constantly, may need tuning?)
Title: Re: My First DRSSTC
Post by: sjsimmo on January 19, 2019, 08:08:58 AM
So I had my first IGBT failure recently  :(

One of my IGBTs has always noticeably run warmer than the others (for unknown reasons), and I suspect this to be the cause of the failure. The coil had not been run in over a week, and so had been packed up since the last full power run. I ramped the coil up to 240VAC input. Everything seemed normal for about 5 seconds. However, before i could turn the interrupter from 10uS on-time up to 50uS on-time, one half of the bridge failed short circuit (Gate, Collector and Emitter now read ~0 ohms on both IGBTs), blowing up the 3A slow-blow fuse in my variac. Neither of the IGBTs have any sign of physical damage on the package. Is it normal for IGBTs to fail without any visible damage, and when not operating at full-power?

Thanks again for your help, everyone :)
Title: Re: My First DRSSTC
Post by: Hydron on January 19, 2019, 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