Author Topic: GDT keeps on killing IGBTs  (Read 6384 times)

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #80 on: March 29, 2020, 12:06:03 AM »
I tried doubling the capacitance and halving the primary inductance. It still works, I ended up with a resonant frequency of 151kHz. It's a bit too bright outside to see the sparks, so I'll test some more tonight.

Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #81 on: March 29, 2020, 12:47:38 AM »
Did you turn off pulse-skipping mode?  Or is that not possible?  At half inductance and the same frequency, you'll be running about twice the current, so much more likely to high current limit and pulse-skipping mode, which may fry more IGBTs.
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Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #82 on: March 29, 2020, 12:51:38 AM »
I turned off pulse skip mode. I'm now using the normal mode, with pulse width around 100-200us. I'm using the single pulse mode too, mainly to avoid more frying.

Edit: blew the transistors again, this time more violently.





Seems like the overcurrent might not be working. I'll scope that next.

Edit 2:
So it looks like the exact 5 IGBTs survive each time. It's always the 4 IGBTs on the left and the one on the top right. All the other 3 fail. I think this might be because I'm using 550V TVS, and maybe the IGBTs on the left got "lucky" and got the ones that triggered on lower voltage. I should probably switch to 440V. However, nothing bad seems to have happened yesterday, and I tested it at higher voltage. Maybe the extra current contributes to higher voltage spikes?
« Last Edit: March 29, 2020, 06:59:55 AM by ritaismyconscience »

Offline Mads Barnkob

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Re: GDT keeps on killing IGBTs
« Reply #83 on: March 29, 2020, 09:59:16 PM »
First some props to Davekni, you put a good amount of time into helping people! 8)

Should I lower the overcurrent? I have it set at 500A right now. I'm using 8 IGBTs in total, so it should work (each of them is rated for 225A pk in the datasheet)

I have never had success with pushing TO-247 packages to 500A, what IGBTs are you even using? Did we make it to page 5 without a part number? Did you do any calculations on the IGBT you are using, like looking at Fmax2 using my guide? http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/

Could it have been overcurrent? I think the current rises quite a bit when the secondary shorts. I've heard the OCP might be slow and take a few cycles to actually trigger. In that case 500A might be too high.

Also, for my next project, I might use the 120N60 IGBTs, which seem to have 600A peak current in a TO264 package. Would this work? Here's the datasheet: https://www.mouser.com/datasheet/2/240/Littelfuse_Discrete_IGBTs_PT_IXG_120N60B3_Datashee-1591813.pdf

OCD/OCP, if a exact copy of the Steve Ward design, has to use a CT+shunt resistor where you get a voltage for your limit around 2.5V If you get out to the boundaries of the op-amp, like setting your potentiometer to 5V reference for the maximum OCD current, you have basically disabled the OCD as there is no working range for the op-amp anymore.

The 120N60 looks like a re-packaged ISOTOP IGBT, properly same die size, I would not expect this to do much better than a TO-247 part. I would think the limit of this package is still 500Apeak. But do the math, calculate Fmax2!
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #84 on: March 29, 2020, 10:51:57 PM »
I'm using 2 paralleled TO247s, so the current should be 250 through each.

Also the driver I'm using seems to use a 9V supply for the op-amp, so 5V should work. See reply #39 for info about driver.

Think I told you guys I was using the 75T60 IGBT on one of the previous pages...
« Last Edit: March 29, 2020, 10:56:07 PM by ritaismyconscience »

Offline Mads Barnkob

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Re: GDT keeps on killing IGBTs
« Reply #85 on: March 30, 2020, 12:06:18 AM »
I found it on page 2 : https://highvoltageforum.net/index.php?topic=959.msg6538#msg6538 you are right, you mentioned it at the bottom of the post. This is why I prefer all details are given in the original post, looking through pages of thread for all the basic details is hell.

Paralleling IGBTs is not the same as doubling their current rating! For reliable operation you should never assume that they can handle more than 50% more. You need to match the IGBTs, have them on same heat sink, be sure their gates are driven identically etc. if you want to break that figure. All the figures in a IGBT datasheet is given for hardswitching, you can not use any of those numbers for current, losses or frequency for a softswitching Tesla coil, you have to calculate that yourself.

What is your OCD set at? 5V? it was just an example I have on the UD2.1b as it uses 5V on the op-amp. Did you test your OCD input with a frequency generator?

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Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #86 on: March 30, 2020, 02:10:26 AM »
Mads, thank you for the compliment, and especially thank you for moderating this forum!  I was thrilled to come across it last September.

"So it looks like the exact 5 IGBTs survive each time. It's always the 4 IGBTs on the left and the one on the top right. All the other 3 fail. I think this might be because I'm using 550V TVS, and maybe the IGBTs on the left got "lucky" and got the ones that triggered on lower voltage. I should probably switch to 440V. However, nothing bad seems to have happened yesterday, and I tested it at higher voltage. Maybe the extra current contributes to higher voltage spikes?"

Yes, most spike voltages are roughly proportional to current.  IGBT capacitance is voltage-dependent, and other IGBT parameters depend on voltage and current, so it's not completely simple.  In particular, the low-frequency (wide) voltage spike at the end of the enable pulse caused by bulk-capacitor to bridge wiring inductance is quite linearly proportional to current.  That would be my first guess for this latest frying.  Uneven current sharing is also a possible cause as Mads pointed out.

Yes, reducing the TVS voltage would be a good idea.  If the voltage spike has too much energy for a lower-voltage TVS to survive, at least a fried TVS diode would provide information that there was an over-voltage event.  (Although it's also possible that an over-voltage event is secondary, caused by the first IGBT frying, before the opposing one(s) fries.)
David Knierim

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #87 on: March 30, 2020, 06:13:52 AM »
So I tested the overcurrent by setting it to around 25A. It looks like there isn't a way to turn off pulse skip with this driver:



You can see that the current starts ramping down, then the driver turns on again until it trips the overcurrent again.

I'm running everything at about 2 BPS right now ( I manually press the single pulse button), so I got a really high value for the frequency. I used 200us pulse width when the IGBTs blew up (at around 90% of max voltage)
« Last Edit: March 30, 2020, 06:20:37 AM by ritaismyconscience »

Offline Mads Barnkob

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Re: GDT keeps on killing IGBTs
« Reply #88 on: March 30, 2020, 05:00:11 PM »
Thank you Dave :)

I still think you have a problem with smashing those IGBTs with way too much RMS current, I really encourage you to do the Fmax2 calculations and you will see exactly if you are misusing them or not. How about them being "fakes" or 2nd, 3rd and even "bad" batches that did not meet factory standards and was sold out the back door?

If you suspect the driver to be the problem and there is little to no help in the translated manual, you should spend a few days to build a UD2.1b, you can even do this with single side components all they way: http://stevehv.4hv.org/leadcomp/UD2_1revB/

I used output MOSFETs are IRF540 and IRF9540. http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iii/
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Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #89 on: March 30, 2020, 06:46:02 PM »
A documented driver is a good idea.  If you want to continue learning about this driver, while you have over-current set low, scope the gate-drive outputs of your driver.  It may be tricky to get all the information with a two-channel scope.  Can you get into a stable mode, perhaps w/o secondary, where the pulse-skipping is consistent from one firing to the next?  It would be interesting to see how this driver runs pulse-skip.
David Knierim

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #90 on: March 31, 2020, 01:33:16 AM »
Some pictures:


^GDT waveform vs voltage at capacitor


^Same thing, but at 10us/div


Left side gate drive and right side gate drive


Bridge output (looks like I need to increase phase lead)


Bridge output vs current measured with 1:1024 CT with 10 ohm resistor. 1 div is 100A. I don't know why, but looks like the OCP is trying to keep the current at 100A, not 25A. I tried it with OCP set to 50A, and the CT said the driver was trying to keep the current at 200A. If this is true, looks like I blew the IGBTs with overcurrent. That's probably also why 100nF worked but 200nF didn't.
« Last Edit: March 31, 2020, 01:41:36 AM by ritaismyconscience »

Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #91 on: March 31, 2020, 04:58:16 AM »
Thank you for the scope traces!  Do you know if both gate-drive outputs of your driver board always switch together?  I'm guessing so from the traces, but it's a bit hard to tell.  Pausing only one half of the H-Bridge makes a more efficient pulse-skip mode.  It makes the H-Bridge output go to ~0, rather than to inverted, so makes the current ramp down more slowly.  Your scope traces show current ramping down fairly rapidly during pulse-skip periods, leading me to guess that the two gate-drive outputs are the same.

There are two possible issues with this form of pulse-skipping.  First is the same bulk-cap to H-Bridge wiring inductance we've discussed already.  It's now being driven multiple times in a row.  If the skip pattern happens to hit the resonance of that inductance and your local H-Bridge snubber capacitance, the local VBus voltage peaks at the bridge will get higher.  However, the one scope trace of an H-Bridge output doesn't show evidence of such a problem.  What current was running for the H-Bridge output scope trace?  It may be worth scoping an H-Bridge output more as you set current limit higher again.

The other possible issue with pulse-skipping is the positive ring on the gate-drive signal when transitioning from negative to 0V.  (For example, see the upper trace of the scope plot labeled "Left side gate drive and right side gate drive".)  If that spike is enough to momentarily turn on an IGBT, before the opposing one is off, it would cause current shoot-through.  (The IGBT turning off will do so more slowly than normal, as it's gate voltage is going from positive to 0, not to -18V.)

Yes, there does appear to be an issue with current limit scaling.  That could be the entire issue.

Phasing appears to be about at 0 degrees.  It is best to have a little bit of phase lead, so IGBTs turn off slightly before current reaches 0, allowing the remaining bit of current to cause the voltage swing before the opposing IGBTs turn on.  So, yes, slightly more phase lead would be ideal.
David Knierim

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #92 on: April 01, 2020, 01:51:05 AM »
I fiddled around with the phase lead a bit:

I got it so that it switches before the peak.


Looks like there's still quite a bit of ringing, but I'm testing this at low voltage.


Here's a scope shot of the bridge at 5us/div instead of 2us.

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Re: GDT keeps on killing IGBTs
« Reply #93 on: April 01, 2020, 04:05:20 AM »
Are the sine-wave traces of the primary voltage, the junction between the MMC and primary coil?  Presuming so, it looks like a bit too much phase lead, although that's definitely better than phase lag.  Ideally the H-Bridge voltage steps on top of the primary voltage would be just 5-10 degrees or so before the peak.  Looks like perhaps 25 degrees now.  Would need to zoom in closer to tell.

Yes, the low voltage makes IGBT capacitance higher, so ringing tends to be worse.  Lower current may change the behavior of your phase-lead circuit, since the driver board's input impedance isn't linear with voltage.
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Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #94 on: April 01, 2020, 04:15:41 AM »
I'm still trying to figure out why the overcurrent isn't working as intended.



Here's a scope shot with the output of the transformer connected to the OCP and another CT I made. I expected the first one to have a higher voltage because it should be equal to the CT voltage plus the 1V diode drop. That doesn't seem to be true, though. The flat sine wave is OCP signal, 1 div is 0.5V

Edit: Improved phase lead:





« Last Edit: April 01, 2020, 05:36:06 AM by ritaismyconscience »

Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #95 on: April 01, 2020, 09:31:49 PM »
Phase lead looks great now!  Double-check that when you get back to full current, as phase lead may be current-dependent given the driver's clamped feedback load.

If I recall correctly, you are using 10 ohms on your scope current transformer.  The driver's over-current looks more like 1 ohm plus two diode drops.  Perhaps that's the scaling issue - a different driver load resistance than what you are using in calculations.
David Knierim

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #96 on: April 02, 2020, 03:48:55 AM »
The datasheet says 10 ohms and the resistor on the board says 100 on it, so I'm pretty sure that's not the problem.

I'll check with a multimeter to make sure.

Edit: Checked, measured 10 ohms. I'm using 3 cores for my feedback and OCP transformer. It looks like this:

Could the feedback be stealing current from the OCP?
« Last Edit: April 02, 2020, 07:34:26 PM by ritaismyconscience »

Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #97 on: April 02, 2020, 09:45:03 PM »
Yes, I was thinking about that last night.  It's not directly stealing current.  Rather, the phase-lead circuit is increasing the load impedance on that current sense output, which is likely enough to cause the large first-stage current-sense transformer core to saturate, which is reducing signal to the over-current second-stage.  Given your phase-lead patches, it's probably best to make completely separate current-sense transformers for feedback and for over-current.
David Knierim

Offline ritaismyconscience

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Re: GDT keeps on killing IGBTs
« Reply #98 on: April 03, 2020, 03:49:51 AM »
So I did a bit more testing. This time, I shorted the OCP input which should disable it.

Here's a plot of bus voltage vs peak current:


Something interesting happens above 100V, the driver shuts off the output at 900A even though the OCP was shorted. I think it might be acting as an antenna or something.



Test at higher voltage. Notice the shorter pulse width. Both are 500A/div. Anyways, I'm pretty sure my TO-247 IGBTs are not designed to withstand 900A.

I guess if you look at the datasheet I posted on reply 39, you can see that the FB and OCP pin both go to OCP_n, which probably is a pin on the FPGA. I think this is how the driver figures out there's too much current. Interestingly, the red OCP led did not light up. Also, the datasheet says the max FB current is 900mA, which seems like too good of a coincidence.
« Last Edit: April 03, 2020, 04:17:52 AM by ritaismyconscience »

Offline davekni

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Re: GDT keeps on killing IGBTs
« Reply #99 on: April 03, 2020, 04:21:56 AM »
There is something very odd about the current traces after shut-down.  I'm doubtful that the driver is shutting down due to normal over-current, especially since the LED doesn't light.  Perhaps it's something about the IGBTs getting too hot to function, but without permanent failure.  Probably incremental damage that will eventually lead to failure.  Silicon die can often survive 275C for short periods IF the resulting thermal-expansion mechanical stress doesn't crack die-attach or bond wire connections.  Perhaps your IGBTs are packaged in a way that survives such internal temperatures.  The biggest surprise to me is that the IGBTs at that temperature aren't increasing conductivity in the off-state to the point where they go into thermal runaway.  I don't have experience with hot IGBT die, however, rather with ~60V ASICs that I've designed and tested to failure.  Perhaps the hot IGBTs are somehow shorting-out the gate-drive signal, or not turning on even when it is high.
David Knierim

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Re: GDT keeps on killing IGBTs
« Reply #99 on: April 03, 2020, 04:21:56 AM »

 


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