Author Topic: Problems with my first DRSSTC  (Read 858 times)

Offline bozidar

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Problems with my first DRSSTC
« on: August 05, 2019, 10:04:50 PM »
Hello.
I'm making DRSSTC and I'm having some problems, so I hope one of you can help me.
First problem is Vge overshoot.In attached images you can see gate-emitter voltage (yellow signal), primary current (blue signal) and Vce (green signal).


This is core that I use for GDT and CT https://www.aliexpress.com/item/32857367831.html?spm=2114.search0104.3.15.968a6e5f9nOtT4&ws_ab_test=searchweb0_0%2Csearchweb201602_9_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536%2Csearchweb201603_53%2CppcSwitch_0&algo_expid=1e5a0c1a-4811-4a8e-926b-6c1f0134c046-2&algo_pvid=1e5a0c1a-4811-4a8e-926b-6c1f0134c046 .
I'm using calculations for GDT from this site: http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-i/.
For my GDT, I put only 4 turns of 5 twisted wires .I use 5 twisted wires  that are wrapped with aluminum foil and all this is put into the shrink tube (handmade) to minimise leakage inductance.
That is explaned here http://thedatastream.4hv.org/gdt_practical.htm
I tried to reduce voltage oscillations on gate-emitter by lowering leakage indutcance of GDT , but without much succes.
Then I bought this core https://export.farnell.com/epcos/b64290l0048x830/ferrite-core-cylindrical-34mm/dp/2355017?ost=B64290L0048X830&ddkey=https%3Aen-EX%2FElement14_Export%2Fsearch to try to get better results, but the results were the same.
I use 5.1 Ohm resistors with reverse diode and P6KE30CA diodes across the gates.

I put on an 11 Ohm resistor today and I got a lot better waveforms , but I don`t know is this resistor too big because rise and fall times are little slower(around 300nS) .



My second problem is that all the time i have had problems to make the coil  work properly. When I set the freq to 3kHz and 200us on time the coil did not want to start to work, and if i decrease freq or on time everything returns to normal. That happens on low voltage on Vbus ( 18 V ) and when I put 310V on Vbus the coil  will not work at all (of course ,with low on time, 10us ).When I connect a small cap (100pF) between the ground and one GDT of the primary output connector, the coil works again as it should ( 18V -Vbus ). I really don't know how to fix this and what can cause this?
I check all connections,igbts,caps etc and everythig is allright. That happens with low voltage (18VDC) on Vbus and also with full voltage ( 310 VDC ).(I just decrease supply voltage between 15V and 18V to keep Vge peak under 30 v).
I have already connected the coil to 220VAC twice and it worked properly but now it will not work properly again, only if I put 100pF cap as I mentioned before(with 18V on Vbus).
Therefore sometimes the coil works properly and sometimes it does not.

I made almost identical Loneoceans UD2.7, just with through hole components. In place of optic fiber receiver I'm using arduino nano with optocoupler . Arduino comunicate with my phone over bluetooth. I wrote code for arduino that generate impulses with variable frequency (1 Hz - 20kHz) and on time (1 - 200 us).
Specs:
Bridge : 4x IXGN60N60C2D1 (full bridge configuration)
Primary resonant frequency : 149 kHz
Secondary resonant frequency : around 160kHz
MMC: 4 strings in parallel of 4 in series WIMA FKP1 capacitors for 68nF at 8000VDC
The coil is not big , secondary is around 23 cm tall and 7,5 cm in diameter. I plan to set current limiter to 300A.
Sorry for my bad English and thanks for the help.
« Last Edit: August 05, 2019, 11:00:08 PM by Mads Barnkob »

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #1 on: September 23, 2019, 01:45:37 AM »
Not sure I can help with all the issues, but a couple things come to mind:

1) The gate-drive overshoot looks quite symmetric between rising and falling edges.  That's surprising with diodes across the series resistors, which usually make the waveforms asymmetric.  If most of the gate-drive-transformer leakage inductance is on the primary side, that would make a bit more sense.  With only 5 wires on your transformer, is the primary a single wire?  I'd suggest 8 wires, 4 twisted pairs, with one half of each twisted pair going to the primary gate drive.  (I saw that suggestion somewhere here - using Cat5 cable.)  Either of your cores should be fine.
2) I tried WIMA FKP1 capacitors initially on my latest coil.  They failed quite quickly.  These were 22nF 6kV parts running at +-4kV peak at 80kHz.  They degraded quickly until a couple parts ruptured their cases.  I rebuilt my entire MMC with Chinese induction cooker capacitors, 0.33uF 1200VDC, 630VAC.  Being wary, I started with an abuse test of a pair of parts, running at +-1200V 75kHz, 1% duty cycle bursts.  After this ran fine for 5 days straight, I increased the voltage/current to +-1700V.  Ran a couple more days with little sign of degradation!  Increased again to +-2050V.  After a day there, their capacitance had dropped about 5%, which is likely getting close to catastrophic runaway failure.  That's at 170% of rated peak voltage!
3) I'd suggest experimenting at MUCH lower than 3kHz pulse rate.  3Hz would be better.
4) The 100pF capacitor to ground suggests some noise coupling issue.  Grounded shielding around the control circuitry and common-mode chokes (large ferrite beads) around all connections from control electronics to power electronics helps too.  Even though everything is transformer-isolated, transformer capacitance causes high-frequency current pulses.  Common-mode chokes make those slower with lower peak current.
David Knierim

Online Phoenix

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Re: Problems with my first DRSSTC
« Reply #2 on: September 23, 2019, 07:37:55 AM »
Hello

Are you only using 4 turns for your GDT? I would use way more turns, the GDT of my DRSSTC has 14 turns for example. Your second core should be fine, I am using the same N30 material.

Greetings
Phoenix

Online Hydron

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Re: Problems with my first DRSSTC
« Reply #3 on: September 24, 2019, 12:21:49 AM »
4 turns sounds a little low, but remember it's at 150kHz or so - it probably doesn't need many to work well and you don't want to go overboard, as it makes leakage inductance worse. The issues I can see in the waveforms are not what I'd expect from too few turns - note that there isn't any droop in the tops of the waveform.

As for the OP's issue - I haven't seen quite as large an overshoot before, and certainly not one symmetrical like that when the speedup diode is included. Maybe try a small resistance in series that is not bypassed by the diode? Also could be worth playing around in ltspice or the simulator of your choice to try and recreate the waveform and then play around trying to solve it.

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #4 on: September 24, 2019, 10:03:02 PM »
Thanks for answers,
Today I tried to wind the GDT again , so I put 9 turns of CAT 5 cable on N30 core , replace 11 ohm res with 15 ohm and replace uf4007 diodes with STPS2H100 schottky diodes and I got a waveforms like this.(first img)
This is with signal gen connected to the feedback input but with feedback CT connected the coil will not work. So I guess that the feedback CT is problem. How I can fix this? I`m using 1:625 turn ratio for feedback.(second img) Is this good or not?

Offline Fumeaux

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Re: Problems with my first DRSSTC
« Reply #5 on: September 24, 2019, 10:40:36 PM »
Isn't that a 1:1 Transformer? The white wire has 1 Turn on each other Core. You would need the brown wire to have a single turn on the middle core and the white wire one turn on the blue wire core.

That might be the source of your problems

Turns out it is not, my bad.
« Last Edit: September 24, 2019, 11:21:14 PM by Fumeaux »

Online Hydron

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Re: Problems with my first DRSSTC
« Reply #6 on: September 24, 2019, 11:17:24 PM »
There is a crucial bit of info missing from the picture - the primary is a single turn (not shown) through the central toroid, and the twisted wires seen on the outer toroids are two ~independent secondary outputs.

With this known the 1:625 makes sense, however I can certainly see why it could also be interpreted as a 1:1 from the pic shown!

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #7 on: September 25, 2019, 12:36:44 AM »
Sorry,
I forgot to mention that I use brown wire for OCD (24 turns) and blue for feedback(25 turns)  and the primary wire (one turn , not shown in the picture) passes through the core with white wire (25 turns) .

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #8 on: September 25, 2019, 04:19:22 AM »
Several thoughts:

Are you certain that the CT is connected with the correct polarity?  At low power, it shouldn't hurt to try reversing it just to check.

Looking closely at the last working scope image, the duty cycle is asymmetric at the start.  I'm not sure if this is typical for UD2.7 or not.  To me, it implies there is some DC offset.  I'd suggest switching back to your signal generator, and dial down the amplitude to 50-100mV P-P.  That should be enough to generate gate signals.  If it doesn't, then scope through the UD2.7 signal path to find the issue.  For asymmetry, a couple possibilities are that D1 and D2 aren't opposite directions, or that the quiescent voltages on the + and - comparitor inputs (IC8 pins 2 and 3) aren't close to the same (issue with the values of associated resistors).

At 155kHz, 4 turns should be fine for gate-drive with the cores you selected, but 9 won't hurt either.

The gate drive voltage is lower than most people run, but should be fine until you push primary current to the IGBT's peak rating or past.  If you are worried about the negative undershoot getting too large, either decrease the TVS voltage for the GE clamps or add a smaller resistors (2-5 ohms) in series with the gate resistor shunt diodes.

Good luck!
David Knierim

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #9 on: September 25, 2019, 04:55:26 AM »
One more thought on possible issues with CT feedback not functioning:

(BTW, hopefully continuing my last reply isn't what is banned as double-posting.  I can't figure out how to edit my last reply.)

Do you have a bleed resistor across your primary MMC array?  If not, the UD2.7 could be driving the IGBTs to the state that has the same voltage as is left on the MMC array previously.  That would induce no current, so no signal to reverse the gate drive. (Which is one reason to prefer self-oscillating controller circuitry.)

David Knierim
David Knierim

Offline Mads Barnkob

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Re: Problems with my first DRSSTC
« Reply #10 on: September 25, 2019, 08:40:33 AM »
One more thought on possible issues with CT feedback not functioning:

(BTW, hopefully continuing my last reply isn't what is banned as double-posting.  I can't figure out how to edit my last reply.)

Do you have a bleed resistor across your primary MMC array?  If not, the UD2.7 could be driving the IGBTs to the state that has the same voltage as is left on the MMC array previously.  That would induce no current, so no signal to reverse the gate drive. (Which is one reason to prefer self-oscillating controller circuitry.)

David Knierim

A simpler and quicker fix for the MMC residual charge, is to place a 5-10K power resistor across the output of the inverter, this removed instability problems with my large DRSSTC.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #11 on: September 25, 2019, 09:02:33 AM »
I only use signal gen as feedback input because when I connect feedback CT nothing works( also with reversed wires ). I used 4 turns on my previous core with high AL value (11400 nH/turnĀ²). I'm using power supply with adjustable voltage (14v to 24 v) , so I adjust voltage to somewhere around 17 v to keep Vge overshoots under 30v and I use the same power supply for Vbus. ( 1.5KE27CA TVS is connected across gate emitter). I connect 10Mohm res across every capacitor( 4 strings of 4 in series).

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #12 on: September 26, 2019, 04:54:35 AM »
10Meg on 68nF is a 0.68 second time constant.  That may not be discharging in time.  Mad's suggestion of a 5-10k power resistor across the bridge output would be a good idea.  Perhaps that is the only issue - residual charge on the primary capacitors.  1-2 watts should be enough for the resistor - would cover 310V bus at 10% duty cycle.

If that doesn't fix things, then I'd suggest my previous signal-generator experiment. I understand (I think) what you did previously with the signal generator.  I'm suggesting a new test to debug the UD2.7 controller with signal-generator input, if residual charge isn't the only issue.  Feeding the UD2.7 current sense input with a sine-wave from the signal generator, ramp the voltage from 0 until barely seeing gate-drive signals.  A signal should start appearing around 25mV P-P.  By 50mV, the gate drive should be reasonably symmetric.  If significantly more signal-generator output voltage is required, then I'd set it to barely-enough and scope through the UD2.7 signal path from current-sense input to gate-drive output.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #13 on: October 01, 2019, 10:42:31 AM »
At yours suggestion I put a 6 kohm resistor ( I found a few 3w and one 5w and connected them in parallel ) across bridge output and I continued testing with signal gen connected to the feedback input. Unfortunately  the signal began to appear just over 200mV P-P and the signal at the gates was not symetrical. Than I connect CT feedback instead of signal gen and increased the supply voltage to 24V ( also 24V on Vbus ) and the coil works just with not symetrical gate voltages as you can see in attached images. I noticed that the  signal is symetrical at the end where the current (feedback voltage ) is higer. So can i just reduce the number of windings on CT so that I get more voltage at the feedback input?
Are these negative peaks dangerous for transistors?
« Last Edit: October 01, 2019, 10:44:20 AM by bozidar »

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Re: Problems with my first DRSSTC
« Reply #14 on: October 01, 2019, 01:39:15 PM »
If you are using a diode to bypass the gate resistor on turn-off then a larger over-shoot of gate voltage in the negative direction is expected. If you ignore the over/undershoot (only consider the flat portion of the waveform) then the voltage should be symmetrical. Finally, fewer turns will give a stronger feedback signal which may be useful if oscillation is a bit iffy at lower currents, but you do need to be careful not to over-do it and cook the driver!

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #15 on: October 02, 2019, 05:07:29 AM »
After reading your results, I examined the UD2.7 schematic some more, especially the biasing of comparator IC8 (TL3116).  Your 200mV result is actually reasonable for the circuit.  The negative input will be pulled down by 16uA across R27 into the forward drop of diode D1.  (I hadn't previously noticed that R27 bias current, so was using the R3/R4 voltage divider as the presumed DC quiescent level of IC8 negative input.)  I think that's less than ideal, but appears to be inherent in the UD2.7 design.

One option is as you suggested, lower the current-transformer turns ratio.  As Hydron already noted, make sure the output current doesn't get above ~1A when you ramp up power.  (At higher bus voltage, this 200mV issue may be insignificant.  The coil current will ramp up quickly, so soon overdrive the 200mV level enough to make symmetric 50% duty-cycle gate waveforms.)

Another option is to remove 100k resistor R27 and place it in parallel with D1 and D2.  That will make the UD2.7 respond to current sense signals less than 200mV, and improve gate drive duty cycle symmetry.

Others here have more experience with gate-drive undershoot.  From what they say, I gather it's not too concerning.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #16 on: October 08, 2019, 09:53:21 AM »
After my last post  when everything was worked , yesterday when I turned it on again nothing worked as it should. So I just don't know what to do  :(
That's happening from beginning , one day it works and when I try the other day it doesn't work properly. Only works if I connect signal gen.Than I reduced feedback CT turn ratio to 1:450 but results were the same.
In attached img can be seen gate signal ( yellow trace )with CT  connected to the feedback input. ( blue trace - current) It's weird that things get a little better when I touch one of the feedback inputs with a screwdriver  :o
What can cause this behaviour , driver , CT , GDT or something else?

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #17 on: October 09, 2019, 04:57:19 AM »
It looks like the first gate-drive half-cycle isn't always generating enough primary current to start the oscillation.  One of your earlier scope images that zoomed in to the start of a burst (when it was working) showed about 1V of current sense on the first gate-drive half-cycle.  That should be sufficient, especially with your test showing that 0.2V P-P was enough to get some gate signal.  It's hard to tell on this latest overview scope image what the difference is between the bursts that function vs. the ones that don't.  If it is possible to zoom in on the start of a failed burst, that would be great.  Especially helpful if compared to a good burst (as most in your latest scope image), and if the two H-Bridge outputs can be included in the traces.

For fixes, the only thing that comes to mind is to change the resistors you added (at Mads suggestion) from across the H-Bridge output to two separate ~5-10k resistors, one from one output to the VBus + supply and the other from the other H_Bridge output to the VBus - supply.  You'll need to try both ways (swapping which H-Bridge output is puilled up and which one down).  One connection should make it not work at all, and the other will double the current on the first half-cycle.  Hopefully double-current will get it above the threshold for the UD2.7 circuit and cause it to reliably start oscillation.
David Knierim

Offline Mads Barnkob

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Re: Problems with my first DRSSTC
« Reply #18 on: October 09, 2019, 09:09:19 AM »
It's weird that things get a little better when I touch one of the feedback inputs with a screwdriver  :o
What can cause this behaviour , driver , CT , GDT or something else?

It is clear that you have a too weak feedback when you can get better performance when using yourself as a capacitively coupled antenna on the feedback.

I would look at all components from feedback op-amp -> diodes -> CT

Did you rule out phase error? Reversing primary connections (remember to place CT on the low voltage end of LC circuit), phase of CT, phase of GDT. It is possible to get unstable operation when driving things off-off or out-of-phase just from the leakage energy through the system.
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Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #19 on: October 10, 2019, 08:42:21 PM »
I checked the diodes and replaced chinese chip LT1016 with MAX9013EUA (from farnell). The results are still the same. :(
I also checked phase of CT and phase of GDT.
In attached img can be seen some waveforms (filename of first 7 img is the frequency and on time).
At 1.9kHz and 196us everything is almost good and in attached img you can see what happens at 1.9kHz and 200us.
In last 4 images you can see bridge , driver etc.
Hope this is not too many pictures  ;D

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Re: Problems with my first DRSSTC
« Reply #19 on: October 10, 2019, 08:42:21 PM »

 


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Laci
October 12, 2019, 02:12:59 PM
post Re: 220ac to 440dc 5a boost convertor
[Voltage Multipliers]
rustedone
October 12, 2019, 10:43:48 AM
post Re: DRSSTC with litz-wire primary and 40 x TO247 IGBT H-Bridge
[Dual Resonant Solid State Tesla coils]
ScottE
October 12, 2019, 07:38:47 AM
post Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
[Electronic circuits]
davekni
October 12, 2019, 06:16:12 AM
post Re: DRSSTC with litz-wire primary and 40 x TO247 IGBT H-Bridge
[Dual Resonant Solid State Tesla coils]
davekni
October 12, 2019, 05:52:51 AM
post Re: so far video
[Spark gap Tesla coils]
MRMILSTAR
October 12, 2019, 05:13:28 AM
post Re: Problems with my first DRSSTC
[Dual Resonant Solid State Tesla coils]
davekni
October 12, 2019, 04:41:19 AM
post Re: so far video
[Spark gap Tesla coils]
thedoc298
October 12, 2019, 02:58:19 AM
post Re: so far video
[Spark gap Tesla coils]
thedoc298
October 12, 2019, 02:56:17 AM
post Re: so far video
[Spark gap Tesla coils]
MRMILSTAR
October 12, 2019, 02:38:43 AM
post so far video
[Spark gap Tesla coils]
thedoc298
October 12, 2019, 12:26:44 AM

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