Author Topic: Tuning Phase Lead / Soft switching  (Read 683 times)

Offline Michael_DA

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Tuning Phase Lead / Soft switching
« on: September 21, 2018, 07:14:51 PM »
Hello people,
I am currently testing the primary circuit of my DRSSTC and just trying to set the phase lead correctly before the final tests. As inductors I have small 7-slot inductors between 10 uH and 70 uH. However, the phase does not change, no matter which inductance I install. In the picture you could see the bridge voltage and the corresponding primary current, but the switching time is more towards the current maximum than in the vicinity of the zero crossing. Nevertheless, I can find almost no voltage peak. I did the pictures with 300 V bus voltage and a current maximum of approx. 200 A (the used probes are not considered on the oscilloscope pictures). The differential probe used has a bandwidth of 25 MHz, so the peaks in the voltage should not actually be filtered out? So I've used a total of 20 uF snubber capacitors. How would you assess the situation? I'm scared to go up to 560V with the bus voltage because I do not want to kill the IGBTs. On the other hand, I can (despite the bad switching time) do not measure dangerous voltage peaks.
I would be very grateful for any advice / hints.
Thank you and kind regards from Germany,
Michael
« Last Edit: September 22, 2018, 10:06:27 AM by Michael_DA »

Offline Mads Barnkob

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Re: Tuning Phase Lead / Soft switching
« Reply #1 on: September 21, 2018, 10:43:09 PM »
Hi Michael and welcome to HVF

Since you observe no excessive switching spikes and before going into more details, did you check if skew between your channels is the problem? See page 10 in the guide attached here: https://highvoltageforum.net/index.php?topic=111.0
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Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #2 on: September 22, 2018, 11:27:39 AM »
Hello Mads,
that was a good tip, thanks!  :) I did not pay attention to this point.
I just looked for forum and the Internet for the keyword "skew" and found some interesting things. As soon as I'm back in the lab on Monday, I'll try to find the mistake. While searching, I noticed that I switched a high-impedance 1:2 voltage divider during the current measurement after the Pearson transformer so that the voltage on the oscilloscope does not become too high. I think that could be the mistake  :-X. This is definitely the first point I will investigate. If it does not, I'll probably compare the signal that the CT measures for feedback with the signal from my Pearson transformer. With the same cable lengths etc.

I'll get back to you after the tests on Monday! As soon as I have finished my exams and have more time, I will present the whole (hopefully working :D) DRSSTC here in the forum.

Thank you and kind regards,
Michael

Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #3 on: September 24, 2018, 08:28:47 PM »
Hello,

I used another current transformer today and the signal was ok. Apparently the IGBTs are switching pretty close to the zero crossing. Obviously it was not a good idea to reduce the output voltage of the current transformer again with a voltage divider.  ::)



However, a new problem has emerged. I started today with a bus voltage of 200 V and this increased to 560 V. Up to 500 V, everything went well. At 560 V, there was a loud bang. After the first shock, I was fortunate enough to find out that only the TVS diodes I mounted antiparallel to the IGBTs explode. The IGBTs were apparently not damaged. Here you can see the moment the diodes exploded.




By the time the diodes failed, the T_On time had already passed. Therefore, my guess is that the current flowing through the TVS diodes was just too high for them. Nevertheless, I'm still a little skeptical. I increased the voltage from 200V to 560V in 50V - 60V increments each, but the amplitude of the bridge voltage measured was a maximum of 204V (for all tested bus voltages). Therefore, I suspect that the diodes may have already cut the voltage here. But the current has risen sharply with every increase in voltage. However, this would be very strange, since I actually assumed that a diode should cut the voltage only from 442 V and I have always used 2 in series.
The diodes I use are this one: 1.5KE250CA   442 V and 1,5 kW bi-directional TVS Diodes
(https://docs-emea.rs-online.com/webdocs/0469/0900766b80469a6e.pdf)

Then I set the voltage again to 200 V (stupidly back to 200 V, but since I had not noticed that in the previous experiments, the diodes may have cut off the voltage at 200 V) and turned on the coil without the diodes.



The rectangles still look pretty clean. Are TVS diodes necessary in this case? If yes, which have you used successfully so far? I use these IGBT bricks (1200V / 300A) in a full bridge in a parallel circuit.

http://www.europowercomponents.com/media/uploads/2MBi300UC-120.pdf

Thank you and kind regards,
Michael


« Last Edit: September 24, 2018, 08:37:43 PM by Michael_DA »

Offline Hydron

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Re: Tuning Phase Lead / Soft switching
« Reply #4 on: September 24, 2018, 11:27:34 PM »
You do not need the TVS diodes, they are useful at clamping and taking out spikes on higher impedance nodes (e.g. the gate drive) but aren't much help on the rest of the bridge.

With 1200V IGBTs you're not likely to get anywhere near the rating even with 560V bus, so there is no reason to use TVS diodes even if they worked in this application. I would remove them and keep testing the coil at higher voltages. The waveforms may get a little less clean (ripples on top, some spikes at switching), but this is normal, and they look very good at 200V.

If you can share pictures of the coil I would be interested in seeing the bus layout - apart from the TVS problem it looks like you have good results and a very clean waveform.

Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #5 on: September 24, 2018, 11:42:23 PM »
Alright, thanks for the answer! That sounds good!  :) I've noticed by now that I've used much too undersized TVS diodes anyway with a breakdown voltage of 237 V (instead of the 442 V I was assuming). So no wonder it eventually crashed. ::) ;D

Sure, I like to upload some pictures of the setup here. I wanted to do that anyway in 1-2 weeks, when the whole coil is finished. Although I have already tested the coil as a SGTC, as a DRSSTC I have only used it without a secondary coil. But apart from a few little things, there is nothing left to do.  :)

Kind regards,
Michael

Offline Mads Barnkob

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Re: Tuning Phase Lead / Soft switching
« Reply #6 on: September 25, 2018, 08:25:50 AM »
However, a new problem has emerged. I started today with a bus voltage of 200 V and this increased to 560 V. Up to 500 V, everything went well. At 560 V, there was a loud bang. After the first shock, I was fortunate enough to find out that only the TVS diodes I mounted antiparallel to the IGBTs explode. The IGBTs were apparently not damaged. Here you can see the moment the diodes exploded.



It seems like its the reverse voltage at on-time switch-off, which is missing in the waveform, that could have caused the overvoltage. Just a comment on it as it seems to be missing here.

Good that you found the source of the skew and under-sized TVS, which I agree with Hydron on, they are not needed on the inverter bridge as your design should be so low inductance that only good snubber capacitors could solve transient problems.
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Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #7 on: September 28, 2018, 07:10:12 PM »
Here, as promised, pictures of our bus layout. A new heatsink for the IGBTs has already been ordered, this was only a provisional solution.

                           

Offline Mads Barnkob

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Re: Tuning Phase Lead / Soft switching
« Reply #8 on: October 01, 2018, 01:43:16 PM »
How much DC bus capacitance do you have from those 4 cans?

The copper busbar from DC bus capacitors to the IGBTs have much room for low inductance improvements! Instead of that huge piece flying over, you could just laminate between them with a insulating material and having them run parallel very close is perfect for the lowest inductance possible.

If you have noise problems, try using a large gauge wire for all the common ground bar wiring, you do not want that function limited by wire resistance :)
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Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #9 on: October 03, 2018, 08:59:06 AM »
Quote
How much DC bus capacitance do you have from those 4 cans?

The capacity of the bus capacitor is 3 mF. So we have voltage drops around  15-20 V in operation. We could probably increase that in the future. But it is not urgently needed or?

Quote
The copper busbar from DC bus capacitors to the IGBTs have much room for low inductance improvements! Instead of that huge piece flying over, you could just laminate between them with a insulating material and having them run parallel very close is perfect for the lowest inductance possible.

Yes that's true! We will definitely do that. :) As you can see on the scope below, we already have slight peaks or rather voltage dips. Could these come through a too small bus capacity? Or how would you interpret that? What is the reason for the phase shift between primary current (CH1) and secondary current (CH2)? What we also find very interesting would be a measurement of the secondary voltage. Has someone already experiences / possibilities with it? Suppose you leave a wire from a large high voltage divider 3 meters away from the breakout point of the coil and wait until a discharge hits there? Or should I prefer to open a new thread for this?



Quote
If you have noise problems, try using a large gauge wire for all the common ground bar wiring, you do not want that function limited by wire resistance :)

How do you mean? We've connected all the cases / heatsinks to a common star point (the copper bar to the left of the MMC). This is connected to the "protection earth" connection of our three-phase network. Do you think that we should build this connection with extra thick wires? So that the resistance of the ground connection decreases and the voltage drop over it is lower?

Thank you for your hints! :)
« Last Edit: October 03, 2018, 09:18:41 AM by Michael_DA »

Offline Mads Barnkob

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Re: Tuning Phase Lead / Soft switching
« Reply #10 on: October 03, 2018, 10:40:14 AM »
3000 uF is too little for such a large coil, are you sure your calculated voltage drop is only 15-20V? Because that is what I expect in my large DRSSTC with 6000 uF, check out the calculations here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/dc-bus-capacitor/

I know a couple of different secondary circuit discharge measurements that have been done.
1: Terry Fritz with a antenna: https://deanostoybox.com/hot-streamer/TeslaCoils/MyPapers/planant/waveant3.html
2: Hydron with a battery driven data recorder: https://highvoltageforum.net/index.php?topic=117.0
3: Uspring with battery oscilloscope: http://4hv.org/e107_plugins/forum/forum_viewtopic.php?153922
4: Greg Leyh on the huge "Electrum" SGTC, where he could sit inside the topload and do measurements on a analogue scope, but many sites with this information is gone, you have to find a mirror or get lucky on archive.org (pictures here https://www.lod.org/electrum.html )

Regarding the earthing wire, its more about impedance and inductance at the frequency that is a problem, so you need to optimize for that. I use a local ground rod just next to the coil, a long earth path through the 3 phase mains alone is not optimal. I tried to cover this topic extensively with a lot of examples here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/grounding-circuit-protection-and-emi/
« Last Edit: October 03, 2018, 02:47:21 PM by Mads Barnkob »
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Offline Uspring

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Re: Tuning Phase Lead / Soft switching
« Reply #11 on: October 03, 2018, 01:59:23 PM »
Quote
What is the reason for the phase shift between primary current (CH1) and secondary current (CH2)?
The phase between primary and secondary current determines how much and in which direction energy is transferred between primary and secondary coil. The average power is

P = pi * f * k * sqrt(Lpri*Lsec) * Ipri * Isec * sin(phi)

phi is the phase angle between primary and secondary current. In your diagram you can see initially the secondary current leading a bit before the primary current i.e. energy going from the pri to the sec. At the end that reverses. That's due to the fast discharge of the primary tank coming from the IGBT reverse diodes after the end of the burst. They empty the primary tank faster than the arc empties the secondary. So eventually secondary energy flows back into the primary.

Offline Michael_DA

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Re: Tuning Phase Lead / Soft switching
« Reply #12 on: October 14, 2018, 03:45:34 PM »
Sorry that I have reported back so late, but the last few weeks have been very stressful due to various exams, as well as the presentation and demonstration of our Tesla coil.

Quote
3000 uF is too little for such a large coil, are you sure your calculated voltage drop is only 15-20V? Because that is what I expect in my large DRSSTC with 6000 uF, check out the calculations here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/dc-bus-capacitor/

Yes you are right, the voltage drop is much higher. We will definitely replace the capacitor in the future.

Quote
I know a couple of different secondary circuit discharge measurements that have been done.
1: Terry Fritz with a antenna: https://deanostoybox.com/hot-streamer/TeslaCoils/MyPapers/planant/waveant3.html
2: Hydron with a battery driven data recorder: https://highvoltageforum.net/index.php?topic=117.0
3: Uspring with battery oscilloscope: http://4hv.org/e107_plugins/forum/forum_viewtopic.php?153922
4: Greg Leyh on the huge "Electrum" SGTC, where he could sit inside the topload and do measurements on a analogue scope, but many sites with this information is gone, you have to find a mirror or get lucky on archive.org (pictures here https://www.lod.org/electrum.html )


Thank you for listing the different possibilities. We will try using a field probe to measure the secondary voltage! :)

Quote
The phase between primary and secondary current determines how much and in which direction energy is transferred between primary and secondary coil.


That was really a very useful hint. We were asked exactly this question after our presentation and thanks to you we were able to answer it!  ;D

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Re: Tuning Phase Lead / Soft switching
« Reply #12 on: October 14, 2018, 03:45:34 PM »

 


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