High Voltage Forum

Tesla coils => Dual Resonant Solid State Tesla coils (DRSSTC) => Topic started by: ritaismyconscience on February 21, 2020, 02:41:03 AM

Title: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 21, 2020, 02:41:03 AM
Hi,
I have a problem with my gate driver, as it randomly kills IGBTs. I've already had to replace 4 of them due to this problem.

Here's what my circuit looks like:

(https://i.imgur.com/wx6YvTu.png)

Here's a couple pictures of my setup:

(https://i.imgur.com/nJUOsxS.jpg)

The brown and green wires go to the GDTs

(https://i.imgur.com/4jRGUd7.jpg)

A picture of the waveform with 2 44nF capacitors simulating the gates of the 4 IGBTs being driven by the GDT:

(https://i.imgur.com/U8nWNUl.jpg)
(Scale: 10V/div & 2us/div)

The driver circuit is a chinese version which uses a CPLD, but the output stage uses the same parts as the UD 2.7 (It uses the FDD8424)
Here's a picture of it:

(https://i.imgur.com/SHYDFGZ.jpg)



I think I might need to swap the TVS diodes for a lower voltage one. Anything else I need to change?

Edit: Unrelated question: I heard that you should use a fast diode instead of relying on the internal diode in the IGBT. Would using RURG8060 work, and would I need to put the diode on a heatsink? Thanks.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 21, 2020, 04:42:34 AM
I see that your gate driver is shielded.  Is there a connection between the shield and gate-driver input power V- somewhere near where the power comes into the shield?  FPGAs can get confused by stray field spikes.

There could also be a bug in the FPGA code causing high-frequency oscillation or other problems with the gate drive waveform during operation.  It's difficult to catch waveforms during the failure event.  If your scope has a pulse-width trigger mode, try looking for short gate-drive pulses.

Without knowing the circuit, it's going to be difficult to find the problem.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 21, 2020, 06:11:32 AM
I'll try replacing the TVS diodes and I'll try adding 24V Zener diodes.

Edit: I came up with this. Will this work?

(https://i.imgur.com/kycJIbb.png)

Also, I looked at other people's schematic, and they used 5.1 ohm resistors on each gate instead of doing what I did. Does this matter?
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob on February 21, 2020, 02:44:44 PM
The whole idea of a gate resistor is to have it as close as possible to the gate of the IGBT/MOSFET in order to quench ringing between input capacitance and parasitic inductance in the gate drive circuit.

You can not just move it to the primary side of your GDT and expect the same function. What you have done is entirely different.

I can not quite make out from your pictures, which wires goes where, but generally you have way too long leads going everywhere, have everything as short as possible. Everything else is just asking for even more inductance and "antennas" to pick up noise from spark EMI.

Your GDT is not killing your IGBTs, your changes to the gate drive circuit and far-from-optimal wiring is killing your IGBTs
Title: Re: GDT keeps on killing IGBTs
Post by: Hydron on February 21, 2020, 08:00:10 PM
It's also a good idea to add a speedup for the turn off resistance to help avoid shoot through etc.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 22, 2020, 03:39:10 AM
Mads and Hydron are likely right about the real cause of IGBT frying:  clean short wiring and gate damping at the gates with speed-up diodes for the turn-off edges.  (I was too focused the controller per your question.)

One other issue that could be causing frying is too little lead-compensation in the controller.  The gate drive signal should ideally switch just before the zero-current times of the primary resonant circuit.  Too far off from that is hard-switching of the IGBTs, which fries them.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 22, 2020, 06:49:22 AM
Haven't tested it with voltage yet, so it's probably not the controller's fault
Title: Re: GDT keeps on killing IGBTs
Post by: dj.cosmo.esq on February 22, 2020, 07:41:18 AM
Hey just a little input I have the same board which has given me hell on Earth (kinda). My question to you is are you using one GDT? Cause I have found some Chinese to English translations that say you are supposed to parallel the outputs since one is driving the GDT and the other is driving nothing. I ended up making 2 GDTs to drive 4x cm400ha-24h but being impatient I ran it off if the GDT1 and blew up the MIC chip on GDT2 but the IGBTs are ok. How are you getting your feedback signal? I was using a signal generator (fail) but Dave had a good idea and I went searching for a Kickstart circuit which actually was attached to a induction heater. Ive been trying to find the firmware to look at for the clpd board but no luck yet.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 23, 2020, 05:43:17 AM
My question to you is are you using one GDT?

I'm using 2 GDTs, one to drive each half of the fullbridge. Seems to work so far.

How are you getting your feedback signal?

Haven't tested it with voltage yet. However, using a 1:1023 feedback transformer should work.

Also, I fixed my driver and I saw this on my oscilloscope:

(https://i.imgur.com/Hq7nre9.jpg)

Does this look normal? For some reason the voltage spikes to 30V at the start
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 24, 2020, 04:37:33 AM
When you say "haven't tested it with voltage yet", are you saying that the IGBTs fried with no collector voltage, only gate drive?  That would be extraordinary.

Was the scope trace taken at the IGBT gate-to-emitter?  Normally there is undershoot there, but not overshoot.  Typically the gate series resistor is high enough for critical damping with the GDT leakage inductance for the low-to-high transition.  For high-to-low, a diode bypasses some of the gate drive resistance, making it underdamped, generating undershoot.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 24, 2020, 07:08:30 AM
Looks like I must have connected the scope leads backwards then, because it probably caught the negative cycle. Anyways, would -30V across gate and emitter be enough to kill my IGBTs? It looks like my zener diode circuit I came up with was too slow to deal with the spike.

When you say "haven't tested it with voltage yet", are you saying that the IGBTs fried with no collector voltage, only gate drive?  That would be extraordinary.

Yes, but it's probably because I messed up the circuit in my first attempt. I've also used a hammer to reveal the die, and there aren't any burn marks.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 25, 2020, 04:23:50 AM
(https://i.imgur.com/KPkcpQ6.jpg)

Looks like you were right, here's the waveform on the positive cycle.

Anyways, is -30V going to kill my IGBTs or should I not worry about it?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 25, 2020, 06:20:04 AM
Mads has said in other posts that -30V is fine for momentary undershoot, even though it's beyond the IGBT's specification limit.  I have no personal experience with such.

The zener diodes are likely fast enough, but not low-enough impedance to clamp the gate signal.

To avoid damaging your circuit, especially once bus voltage is applied, be careful about grounding sensitive nodes to your scope.  Even if the scope is floating (not connected to line ground), capacitance from the scope "ground" to the line power can be enough to disrupt circuit operation.  So, "ground" only low-impedance nodes that don't have intentional AC signals on them, such as the low-side emitters or high-side collectors (bus voltage - and + nodes), not gate signals or bridge outputs.  If you "ground" a floating scope to any high voltage such as + bus voltage, be careful not to shock yourself from the scope case!
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 25, 2020, 06:27:40 AM
Mads has said in other posts that -30V is fine for momentary undershoot, even though it's beyond the IGBT's specification limit.  I have no personal experience with such.

To avoid damaging your circuit, especially once bus voltage is applied, be careful about grounding sensitive nodes to your scope.  Even if the scope is floating (not connected to line ground), capacitance from the scope "ground" to the line power can be enough to disrupt circuit operation.  So, "ground" only low-impedance nodes that don't have intentional AC signals on them, such as the low-side emitters or high-side collectors (bus voltage - and + nodes), not gate signals or bridge outputs.  If you "ground" a floating scope to any high voltage such as + bus voltage, be careful not to shock yourself from the scope case!

I'm going to use a second current transformer to monitor the current and I'm going to stick a wire through the gate transformer to keep track of gate voltage. This way my scope is completely isolated from the voltage.
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob on February 25, 2020, 08:28:28 AM
Anyways, is -30V going to kill my IGBTs or should I not worry about it?

http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/

Quote
Maximum Gate-to-Emitter Voltage (VGE)

The gate voltage is limited by the thickness and characteristics of the gate oxide layer. Though the gate dielectric rupture is typically around 80 volts, the user is normally limited to 20 or 30V to limit current under fault conditions and to ensure long term reliability.

It is normal practice to drive IGBTs at +24VDC with the Steve Ward universal driver and clones of it that most use. The reason for the gate voltage limit is not so much for protecting the gate itself, it will first break down at some 80 Volt. Higher gate voltage means higher currents can be conducted through the Collector-Emitter. We take advantage of this by pushing the gate a little over its rated voltage to allow us to conduct higher currents through the IGBT, at the cost of higher switching losses. As explained by manufacturers in the following.

It is important to note however that IGBTs exhibit relatively high gain even at high gate-emitter voltage. This is because increasing the flow of electrons by increasing the gate-emitter voltage also increases the flow of holes. The gain of a high voltage power MOSFET however is very insensitive to gate voltage once fully on.

Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 26, 2020, 05:55:55 AM
Thanks for all of your help, looks like my setup is working now

Here's what the waveforms look like on 25VAC (the bus voltage is double the AC voltage, so the bus voltage is 70V DC)

(https://i.imgur.com/AVzoDM9.jpg)

1 div = 20A current

This is my first time building one of these, so I might need more help in the future
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob on February 26, 2020, 04:01:18 PM
This is my first time building one of these, so I might need more help in the future

Congratulations on having it oscillate correctly. Before you ask, low voltage testing can have seemingly large switching transients due to output capacitance of the IGBT being high at low C-E voltages. So do not fear large switching transients below some 100-200V DCbus

We are always here to help you with your Tesla coils, you provide a good amount of information and pictures to make it easier to understand your problems  ;)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 27, 2020, 03:27:07 AM
I tested it at half bus voltage today (170V) and I got this:

(https://i.imgur.com/g05S30n.jpg)

There seems to be some switching spikes (1 div = 100A). Does this look normal?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 27, 2020, 05:27:56 AM
Those transients do look high.  The actual current through the primary resonant circuit likely doesn't have such current spikes.  It's more likely capacitive or inductive coupling in your scoping current transformer or its wiring.  Do you have a picture and any details of your sense transformers, both the one used for scoping and the one for feedback?  What load resistor are you using on the scope transformer?

It's hard to tell in the picture, but it appears that the spikes occur shortly after the current zero-crossing points.  The H-Bridge switching is softer if done just before current zero-crossing (by increasing lead compensation in the gate-driver circuit).  That way the Tesla primary current makes the voltage transition during the IGBT dead-time.  Lack of dead-time would be another cause of large switching spikes.  Do you have a sketch and/or images of how your gate-drive transformers and resistors/diodes are wired now?
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 27, 2020, 07:22:17 AM
(https://i.imgur.com/j6WhaLX.jpg)
^ Feedback transformer (1:1023 for both)

(https://i.imgur.com/6nrGQnL.jpg)
^ 1:1023 current transformer + 10 ohm resistor (connects to scope)

(https://i.imgur.com/eBEsCLR.png)
^ Circuit design

(https://ae01.alicdn.com/kf/HTB1fSyBXI_vK1Rjy0Foq6xIxVXa5/Tesla-coil-DRSSTC-SKP-jump-pulse-driver-Yunbao-new-arc-extinguishing-controller.jpg_q50.jpg)
^ A picture of the driver I'm using. It doesn't seem to have phase lead compensation. Is it possible to modify the circuit to add this?
Title: Re: GDT keeps on killing IGBTs
Post by: johnf on February 27, 2020, 07:16:42 PM
My god
what a mess
you need to twist the transformer output wires together 3 or more twists per cm
this helps stopping picking up stray signals and or radiating signals into circuitry it shouldn't.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 28, 2020, 06:24:53 AM
Thank you for the images.  I'm presuming the gate-drive schematic sketch with the 5.8-ohm resistors is replicated to drive 4 IGBTs total from two GDT outputs of your controller.  That schematic looks good.  If gate-drive undershoot voltage gets too high, adding a lower-value resistor in series with the diode can help, at the expense of less dead-time (slower turn-off).

As John said, twisted pairs of wires would be good for both the GDT leads and current-sense leads.  For scope monitoring, a coax cable would be even better.  Keep in mind the pairing of the GDT leads even when adding the resistor/diode sets.  The resistor and diode sets should be close to the IGBTs, with the other emitter lead of the GDT secondary in close proximity to each set.

I can't tell from the image if your 10-ohm CST load resistor is wire-wound or not.  Wire-wound resistors have relatively-high parasitic series inductance, which could be amplifying high-frequency signals (ie. spikes).

I like your litz-wire primary winding - thought I might be the only person using it.  Litz isn't very important for the leads, but is useful within the coil itself to reduce losses.  For the primary inductor and MMC leads and H-Bridge output wiring, twisted pairs aren't practical due to the relatively-high voltage.  Keeping the wiring as close to each other as practical (but still far enough to not arc over) is helpful.  Reduces stray fields to couple into control wiring (current sense, gate-drive, etc.) and reduces parasitic inductance (increases effective coupling factor to your Tesla secondary coil).

Concerning phase lead, I'm surprised there's no adjustment.  The FPGA is likely implementing some phase lead, but not sure how it would know how much is needed for your IGBTs and wiring etc.  It's hard to design an external phase-lead circuit without knowing the controller's input schematic.  I'd suggest getting the wiring as clean as possible (twisted pairs), then some more scoping to see if phase-lead is truly insufficient.  (Do you have two-channel scoping capability?  Then you could scope current and H-Bridge voltage simultaneously, making phasing easier to measure.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on February 28, 2020, 07:32:19 AM

I can't tell from the image if your 10-ohm CST load resistor is wire-wound or not.  Wire-wound resistors have relatively-high parasitic series inductance, which could be amplifying high-frequency signals (ie. spikes).


I scraped away some of the ceramic and the inside looks black. I'm pretty sure my resistor is not wirewound. The resistor used by the CPLD board is SMD so it's probably thin film.

(https://i.imgur.com/IBt99c8.jpg)

^What's inside.

Reduces stray fields to couple into control wiring (current sense, gate-drive, etc.) and reduces parasitic inductance (increases effective coupling factor to your Tesla secondary coil).

Right now, the primary connection wire is quite long, as I still have to tune it so I don't know where to cut it yet. Probably around 50cm extra.

I'd suggest getting the wiring as clean as possible (twisted pairs), then some more scoping to see if phase-lead is truly insufficient.  (Do you have two-channel scoping capability?  Then you could scope current and H-Bridge voltage simultaneously, making phasing easier to measure.)

I have the 1744A oscilloscope. As far as I know, there's two ways to display both traces, ALT and CHOP. CHOP is way too slow (250kHz), and ALT gets the traces from two different runs and puts them together. Perhaps I could press A and B to display A+B on the oscilloscope, then try to reconstruct A and B? I don't have a digital scope, which is a problem (also, I don't have proper oscilloscope probes). Someone who knows more about analog oscilloscopes might be able to help me here.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on February 29, 2020, 05:16:03 AM
Chances are that the circuit behaves the same each time an enable pulse is received.  Presuming so, and that you have a stable trigger setting on the scope, then dual-channels should work fine.  Even chop works well if the scope's persistence is long enough to cover many trigger events.  Alternate is fine too.  If the circuit behaves differently each time, then that's something to chase first.  (I started engineering before digital scopes, and worked for a scope company, Tektronix, so am familiar with analog scopes.)

I understand your reason for long primary wires.  To mitigate the effect of the long wires, I'd suggest running the leads parallel to each other, close together, but at least 10mm apart to avoid arcing between them.  Use some cheap tape to tack the coil leads to a scrap of plastic (piece of PVC pipe for example) or other such insulating item.  That will minimize the excess lead inductance with its associated stray fields.  Placing the primary coil away from your drive circuitry, at least for this initial testing, will eliminate coupling from that coil as a possible issue.

Without scope probes, it's especially important to use coax (ideally) or at least twisted pair wiring from whatever you are measuring to your scope.  Route the scope connection(s) directly away from your circuitry, not adjacent any other wiring.  Gate drive and current-sense wiring needs to be twisted pairs too.

I can't see for sure from your pictures, but another issue that probably needs addressing is VBus inductance (power to the H-Bridge).  That needs to be extremely low, even lower than any of the other wiring.  Most designs have polypropylene capacitors directly at the IGBT terminals (high-side collectors to low-side emitters).  Using individual TO247-packaged IGBTs adds some challenges to low-inductance interconnect.  (My DRSSTC uses 10 TO247 parts in parallel for each switch, 40 total.  Designed a custom ECB just for the power connections - IGBTs and VBus capacitors.  I'd suggest at minimum some form of copper plane for VBus-, such as copper foil.  Or, better, use a piece of raw un-etched ECB material, with one side for VBus- and the other side for VBus+.

Finally, using TO247 IGBTs, make sure the gate-drive emitter connections are at the IGBT emitter leads, separate from the emitter power connection.  Higher-power IGBTs usually have two separate emitter terminals just for that purpose - one for power and one for gate-drive.  This is referred to as "Kelvin connection".
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 01, 2020, 05:42:26 AM
I fixed the wiring today and I replaced the FB and the GDT wires with coax cable. Here's some pictures:

(https://i.imgur.com/XhJPUNc.jpg)
(https://i.imgur.com/lFwBJRu.jpg)
^ the blue wire is for the OCD, so twisted wire is probably good enough for this

I just wanted to make sure I was doing everything correctly before I insulate the wires.

Some more stuff about the board: The thick green and red wires go to the + and - of the capacitor bank. (My red wire is a bit long because I needed to connect it to the other side of the capacitor bank for current sharing. My capacitor bank is made out of 14 1000uF 200V capacitors arranged 2s7p to double the input voltage. There's also more decoupling capacitors on the back side, and I made sure to drill holes in the PCB to connect the copper on the two sides together with thick wire. I also reinforced the thinner parts of my board with copper strips and lots of solder. The giant blue capacitors are some snubber capacitors I got from China, they're 1uF 1200V.

Here's some pictures of the board layout
front:
(https://i.imgur.com/9ZXNTfw.png)
back:
(https://i.imgur.com/oTgHqsb.png)

I made a couple minor changes to these, but that shouldn't affect anything that much.

(https://i.imgur.com/nT4UaQv.jpg)

(https://i.imgur.com/WtFMS2K.jpg)

The capacitor bank is made out of some chinese induction heater capacitors rated for 50kHz at 800VAC. I'm using 48 of these 300nF capacitors in a 12s4p configuration. The primary coil is made out of thicker litz wire

(https://i.imgur.com/omABh4t.jpg)

Here's the dummy load I'm using for testing.

No scope waveforms today, it's a bit too dark outside for testing.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 01, 2020, 06:44:56 AM
Informative set of pictures!  Great to see that you do have a circuit board for the IGBT power interconnect.  (It's clear that you need a larger soldering iron for the heavy wires and copper planes.  I bought a cheap 150W iron with 10mm-base-diameter tip for such work.  Use it with a light-dimmer as it doesn't have temperature control.)  Added copper foil can help too if you run high duty cycles.  I do the same thing.  The induction-cooker capacitors work great for the MMC - again the same as mine (after other parts failed).

Good to see some local film capacitors for VBus.  The long wires to your bulk caps will eventually cause trouble at higher power.  (Mine were relatively short, but still required better pairing.  Otherwise the IGBTs see a voltage spike at the end of a burst, when the H-Bridge changes from using power to returning power to VBus, reversing current in the wiring to VBus bulk caps.)

The coax should help.  Yes, twisting should be plenty for the over-current connection.

What are the individual wires above the litz-wire primary coil?  Be careful they don't touch multiple windings, as the magnet-wire insulation won't handle the primary coil voltage.

It looks like you have some finer litz wire in some of the interconnect, but coarser in the primary winding.  Am I seeing that correctly?  Litz isn't important in the general wiring, but is useful within the coil.

Keep your wood dry!  Damp wood will form arc tracks and eventually burn at Tetsla primary voltages.

Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 01, 2020, 07:27:03 PM
I've conducted another test, this time at 75% the final bus voltage(which is about 250V)

(https://i.imgur.com/2iW89T4.jpg)

1 div = 100A

There seems to be less spikes compared to last time.

To figure out when the transistors were switching, I came up with this:

(https://i.imgur.com/4zwYxaO.jpg)

It doesn't seem to work very well, it picks up a lot of noise. Also I still can't figure out how to display both traces on the oscilloscope, because when I turn the trigger level, this happens:

https://drive.google.com/file/d/1iQYbKvam7idLdfpl3MY4IfTEXZ9FejbX/view?usp=sharing (https://drive.google.com/file/d/1iQYbKvam7idLdfpl3MY4IfTEXZ9FejbX/view?usp=sharing)

I also get weird traces like this:

(https://i.imgur.com/qA1elY6.jpg)

1 div = 200A
This doesn't make sense because there shouldn't be any more gate drive pulses after the current starts to go down, so there's probably something wrong with my setup.

Here's a picture of my entire setup:
(https://i.imgur.com/ECFcPQ4.jpg)

I felt all the parts afterwards and all of them were pretty cold, but I have a cooling system for the IGBTs

(Also somewhat unrelated, but I guess if I finish this project I might build a larger version using more IGBTs. I can get more of these 75N60s for dirt cheap on chinese websites. You can buy them here (https://item.taobao.com/item.htm?spm=a230r.1.14.19.1dff278a9sseTP&id=576405019792&ns=1&abbucket=16#detail) for roughly 20 cents each)
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 02, 2020, 12:30:48 AM
The part number listed on that web page is "IGW75N60T", which looks like an Infineon part that does not include an internal emitter-collector diode.  The parts you have presumably include such a diode since they haven't all fried yet.  It would be worth a quick test with a meter to see if there's a diode internally that conducts when the collector is more negative than the emitter.  If not, that may explain your failures. Perhaps they don't fail instantly with excess reverse voltage.  (The shorter "75N60" part number includes both parts with and without internal diodes.)

I'm guessing that the $0.20 parts are counterfeit.  I've had that experience with a number of FET parts from China.  Some factory is making similar parts and marking them with a more reputable companies label and selling them as genuine items.  They may work reasonably, but often fail to meet all the specifications.  It is possible that they are genuine, and someone has a large stock to dispose of.  That would be more likely if they don't include internal diodes, as most designs don't use such parts any more.

A single turn on the gate-drive transformer makes a relatively-low voltage signal, so I wouldn't be too concerned about the noise level on it.  Turning on the scope's 50-ohm termination on that channel will likely help reduce the noise.  (Of course, don't use 50-ohm termination on higher-voltage signals, to avoid frying the scope's internal termination resistors.)

The second scope image in your last post is quite helpful.  The current is dropping because the controller isn't locking to the current waveform.  It's driving the gates at a fixed frequency that is a bit lower than your resonant frequency.  After a few cycles, the drive is out-of-phase with the resonant current, so forces the current back down.  So, there's some issue with current feedback.  You could try reversing current-sense polarity, although that may not matter for this driver.  Looking at the driver image, it appears that the current-sense feedback may be rectified even for the primary feedback, not just for the over-current.  The FPGA may be auto-detecting the phase, and monitoring only zero-crossings.  It also appears that this driver is self-oscillating.  I consider self-oscillating a good feature, but it does need to lock to current sense after a cycle or two.  For some reason yours isn't.  Perhaps scoping the current feedback voltage at the driver input terminals would provide a clue to the issue.

The scope images in your video are due to expanding the horizontal scale (faster sweep).  The scope is triggering a second time on each burst, so showing two overlapping time windows.  That's what "trigger hold-off" is there to solve.  Increase the hold-off (lower right knob) so the scope waits too long to catch another trace within the same burst.  (The other option is to go to a slower sweep, then use the 10x horizontal magnification feature to view a section of the longer sweep.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 02, 2020, 12:43:52 AM
I'm currently using the transistors I bought from the seller listed above. The reason why they're so cheap is because they're pulled from old parts (that's also how I was able to get the capacitors for a couple cents each). I've taken a few of them apart and they do seem to have the correct die. Plus, I've tested them at 200A a while ago and they didn't fail.

Also, I'm pretty sure the polarity matters for the feedback. One of the pins is shorted to ground. The driver also seems to be working at the correct frequency, if you look at the second scope shot, the falling edge seems to always occur at the top of the sine wave. I think it's because I didn't set the trigger holdoff high enough, so the two traces got misaligned. Anyways, I'll test the setup later.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 02, 2020, 05:07:37 AM
I'm not seeing the same thing you are in the second scope plot.  The first falling gate-drive edge at the left of the trace occurs just before the positive current peak.  By the sixth falling gate-drive edge, it's at the current zero-crossing after the positive peak.  It's shifted about 120 degrees in 5 cycles.

If the IGBTs are genuine Infineon IGW75N60T parts, then you do have an issue with missing free-wheeling diodes.  Definitely check for that.  I think many of the IGBTs with diodes use two separate die within one package.  I've never dissected any.  A few data sheets specifically mention two die.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 02, 2020, 06:43:18 AM
I'll try flipping the FB transformer and trying again.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 05, 2020, 06:39:45 AM
I found out more information about the driver.

When I flipped the FB transformer, the driver did not oscillate. It only oscillated when the FB transformer was the right way around.

Also, I did a low voltage test with the trigger holdoff set to max, and I got a different waveform than shown in the second picture. My waveform showed the current increasing when the driver was on, and when the driver turned off, the current went down quickly. I'll send a picture when I get more time to run tests at higher voltages.
Title: Re: GDT keeps on killing IGBTs
Post by: John123 on March 05, 2020, 09:59:16 PM
For what it's worth, I've had really bad luck using ethernet wire for gate drive transformers. I had much better results using single strand/core bell wire, not sure what its called in other countries but here in the UK its what the primary telephone/broadband network people use to wire up people's houses (full fiber and coax to the house isn't in most places yet).

It's like ethernet wire and the same thickness but the conductor is just one single core of copper. When wound onto a core it "hugs" the core much better and the insulation doesn't fight back and try to straighten itself out like the ethernet stuff, this allows the internal conductor to couple better.

Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 06, 2020, 05:41:07 AM
A bit more testing today:

Looks like I need to figure out how to add phase lead on this thing:
(https://i.imgur.com/A1kT4h0.jpg)
(250V on bus, 2us/div, 100A/div)

Same thing but at 5us/div
(https://i.imgur.com/ScU5tid.jpg)

My controller has this interesting thing called "pulse skipping." This is what happens (170V on bus, 0.2ms/div, 200A/div):
(https://i.imgur.com/xw6QKGP.jpg)

The frequency seems to be right (you can see that there's about 1.5 small divisions = 600ns between the time the GDT switches and when the current switches). There seems to be phase lead, but it's the wrong way. It's probably hardcoded into the FPGA.
(https://i.imgur.com/iuKuSUx.jpg)

Maybe I'll have to flip the GDT connections?



Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob on March 06, 2020, 12:56:50 PM
Download this guide and go to page 10, be sure to check for skew between your voltage and current measurements, before looking for problems in your inverter: https://highvoltageforum.net/index.php?topic=111.0
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 06, 2020, 11:23:29 PM
Both of my traces are produced using transformers, the square wave is made by looping wire through the GDT and the sine wave is made with a 1:1000 current transformer. Is there anything that could cause delay using my current setup?
Title: Re: GDT keeps on killing IGBTs
Post by: Hydron on March 07, 2020, 12:39:00 AM
The gate drive waveform isn't as important as the actual output waveform when it comes to worrying about phase lead. There will be delays in the IGBT switching that you're not seeing. Aim for having it switch just a tiny bit before the zero crossing of the current - it is better to turn off a little early than late (avoids any diode recovery issues).
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 07, 2020, 01:43:03 AM
How would I measure the voltage safely wihtout a differential probe? Could I connect 9 1M resistors to drop the voltage?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 07, 2020, 06:08:08 AM
Since I understand that you don't have any 10x scope probes, a capacitive divider would work better than resistive for 135kHz signals.  Run coax from your scope to a small piece of foil around one of the H-Bridge output wires (around an insulated part).  The foil to wire will make a small capacitor, combined with capacitance of the coax and scope input.  Not calibrated, but plenty useful for measuring timing and rough waveform shape.

Yes, it does appear to need significant lead compensation.  The lag you have now is actually a bit worse than it looks due to IGBT gate-to-output delay as Mads mentioned.  At least it does appear to be locking to your current feedback in your recent scope images.

Good luck figuring out how to adjust phase lead on that driver!
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 07, 2020, 06:51:49 PM
Found this datasheet for my driver that I ran through Google Translate
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 07, 2020, 11:13:28 PM
I tried it by tying a wire around one of my primary wires. Here's some waveforms with 250V on bus:
(https://i.imgur.com/CgcrBo8.jpg)
(5us/div)

(https://i.imgur.com/s7GJ2RI.jpg)

(https://i.imgur.com/HaM9xpN.jpg)
(2us/div)

Edit:
Think I found the problem:

I threaded a wire through the feedback transformer that goes to the controller, and I got this triangular waveform:
(https://i.imgur.com/iyWHc0u.jpg)
(https://i.imgur.com/Qba39Ip.jpg)
(https://i.imgur.com/1drnZvB.jpg)
(https://i.imgur.com/QMTLncb.jpg)

Looks like the green toroids are causing the problem.
Title: Re: GDT keeps on killing IGBTs
Post by: Hydron on March 08, 2020, 12:25:22 AM
Sorry to be negative, but I think you're going to continue to struggle until:
- you get or build a driver that actually has real, open information about it's operation that you understand, and which also allows for a phase lead to be set. A random aliexpress driver might be OK if you can follow the design discussion etc on chinese forums, but if not then stick to something that's open and documented in an accessible way.
- you get some proper probes for looking at the bridge output (even a cheap ebay 100x passive probe will work if you reference it to mains earth)
- make sure that best practices are followed regarding the critical parts of the circuit. These are mainly the loop area of the IGBT connections to eachother, the output and their decoupling capacitors, GDT drive stray inductance and the IGBT gate drive network.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 08, 2020, 04:41:45 AM
Ok, I'll buy a cheap probe online.

In the meantime, I'm going to make the following changes:
1. Reduce GDT from 16 turns to 12 turns to reduce leakage inductance
2. Rewind feedback transformer (think the core is saturating)

My guess is that the feedback transformer is doing some phase shifting on the feedback signal. What type of core should I use for the feedback?'

Also, it seems like I just need an inductor and a resistor to adjust the phase lead.

(https://i.imgur.com/VCdCxci.png)

Edit:
Someone's already made a version with phase lead (https://www.aliexpress.com/i/4000220850472.html)
Title: Re: GDT keeps on killing IGBTs
Post by: johnf on March 08, 2020, 05:18:11 AM
Leakage inductance has very little to do with turns
It is a measure of winding coupling (k)
This has more to do with core shape, winding window utilisation, and winding order.

In fact reducung turns tends to raise leakage inductance percentage wise
One thing you can do is reduce your in and out wire length of the transformer as this directly adds to leakage inductance, you need to aim for lead lengths of a FEW cm's max
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 08, 2020, 10:06:23 PM
A real scope probe would be helpful, although your H-Bridge output waveform looks fine with the simple wrapped-wire probe.  (Fine except for the 60 degree phase lag instead of ~10 degree lead that would be ideal.)

Scoping the actual feedback voltage at the driver input would be more helpful than a loop through the transformer.  It should be fine to ground the driver board to your scope.  I think the spec you found said mounting holes are driver ground.  I'm guessing that the waveform you are seeing now isn't saturation, but rather due to the +-3V clamp load that the driver board applies to the current-sense signal.

[Deleted incorrect statement about 90 degree phase shift.]  Scoping the feedback input directly will help determine what's actually going on.

Current-sense issues could be due to low inductance rather than saturation.  There's lots of info on this forum about core requirements and options.  (If your scoping current transformer is low-inductance, then your phase lag issue may not be as bad as it looks.  A way to double-check your current waveform is to scope the high-voltage side of the MMC, where it wires to the primary coil.  Use the same technique of small capacitor coupling to your scope, but the capacitor needs to handle10-20kV.  Insert some wire to the inside bottom of a glass bottle or jar, with the outside bottom placed on the MMC-to-primary connection should be safe enough.  Voltage there will be 90 degrees out-of-phase with current, plus small voltage steps at the H-Bridge output switching times.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 10, 2020, 05:41:16 AM
I'm not seeing the 90 degrees phase shift...

(https://i.imgur.com/mpW07We.png)

I'll try using larger cores to see if they make a difference.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 11, 2020, 02:43:26 AM
To make any useful comments, I'd need to know the inductances of each wniding and the coupling factors used in your simulation.  I suspect some value(s) don't match normal DRSSTC implementations.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 11, 2020, 06:03:37 AM
I changed the cores:

(https://i.imgur.com/yWLmIV5.jpg)

Here's what the feedback signal actually looks like. The clipped sine wave is the FB signal and the square wave is output.

(https://i.imgur.com/vOLtb28.jpg)

(https://i.imgur.com/ir9zNLW.jpg)

(1st is 1us, 2nd is 2us)
Title: Re: GDT keeps on killing IGBTs
Post by: johnf on March 11, 2020, 06:35:52 AM
Why guess when you can measure!!!!!!
Measure your gate drive transformer inductance preferably at a frequency near to what you are using then short the other windings and measure inductance again shorted over open circuit value x 100 = coupling factor. the shorted inductance figure is your leakage inductance. Most simple LCR meters have a series of fixed measurement freqs. This is a bit of kit you must have long before an oscilloscope. without an LCR meter it you may as well put all your components in a box and shake it for a random solution
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 11, 2020, 08:01:17 PM
I was mixed up about the phase-shift implications of the current-sense transformer feeding a diode-clamp load instead of a resistor load, so deleted that bit of my earlier message.  However, that leaves a puzzle as to how this driver board would ever include phase-lead.  Either it just doens't support phase-lead, or the FPGA code is supposed to be implementing some sort of PLL internally to adjust phase.  If the latter, it doesn't appear to be working correctly.

Concerning inductance measurement, you can get that by scoping the ring-down waveform when a charged capacitor is discharged into the coil.  With enough voltage on the capacitor, saturation current can be measured too.  That's what I do at home, as I don't have an LRC meter either.  (Have a very nice one at work, so I do measure parts there.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 12, 2020, 12:05:34 AM
Looks like my driver simply doesn't have phase lead.

After upgrading the FB transformer, I get much better waveforms. It's switching the transistors 500ns too late instead of 1.2us, like last time. 500ns seems more reasonable, considering my IGBTs have switching times around 400ns. However, the driver is still switching the IGBTS too late. I'll probalby have to manually add a phase lead circuit somehow to solve this problem.

Here's some scope pictures.

(https://i.imgur.com/qAXcNpJ.jpg)

(https://i.imgur.com/LCJcwdV.jpg)

1us/div on top, 2us on bottom
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 12, 2020, 04:11:02 AM
If the current-transformer used for scoping has low inductance, you may be even closer to good phasing.  Low-inductance scope feedback will make the current waveform earlier than it really is, so make the H-Bridge output look later relative to current.  That's one advantage of scoping (through a small HV capacitor as I've mentioned before) the MMC output.  That waveform will be 90-degrees from current, and include small (relatively) steps at the H-Bridge switching points.  Phasing can be measured with that single waveform.

Do you know if your cores are all ferrite and not powdered iron? Powdered iron cores have much lower inductance per turn-squared.  They are intended for inductors, not for transformers.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 12, 2020, 04:28:47 AM
All the cores I'm using are ferrite.

I tried your idea, and it seems like the driver is still switching too late. It's about 400ns off.
(https://i.imgur.com/pFEXJfa.jpg)

(https://i.imgur.com/qE6TlMi.jpg)

1us top, 2us bottom

I'll try using a couple resistors and a variable inductor to try to change the phase lead.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 13, 2020, 01:00:22 AM
Thank you for the scope traces.  Looks to me that about 800ns of phase lead is needed, to get switching just slightly before the zero-current points.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 14, 2020, 02:20:21 AM
Did a small test using 2 100ohm resistors and 1 inductor (don't know the inductance, will have to measure)

(https://i.imgur.com/nKTT0HJ.jpg)

(https://i.imgur.com/kTGP6GZ.jpg)

Edit: the inductance is 0.38mH
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 14, 2020, 03:49:44 AM
Looks like you are getting reasonable phase lead with that setup.  However, I'm quite confused by the image.  It looks like the current transformer secondary is wired between the shields of two different coax cables, with the coax center wires just joined together.  I would have expected a single coax cable with the current transformer wired between the center and shield wires.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 14, 2020, 04:36:56 AM
That's so I could look at the FB input on my scope.

What's the ideal amount of lead?

Edit: Here's the best result I have currently:

(https://i.imgur.com/H19VDEw.jpg)

Seems to have gotten rid of all the ringing (will have to verify once I get my 10x probe)
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 14, 2020, 06:15:15 PM
Reduced ringing is one way to tell that the phase-lead is sufficient.  When the phase is lagging, the current reverses before the voltage.  In that case, current through conducting IGBTs has transferred to their internal anti-parallel diodes.  When the opposite IGBTs turn on, that causes a huge current spike due to the reverse-recovery charge of those diodes that must turn-off rapidly.

With phase lead, the conducting IGBTs turn off while still conducting a small amount of current in their normal forward direction.  That small current causes the voltage to switch softly just before the opposite IGBTs turn on.  (The reason for diodes in the gate-drive is to make sure the turn-off happens before the opposing turn-on.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 21, 2020, 06:46:06 AM
Here's what my coil looks like so far:
(https://i.imgur.com/wCPHAgk.jpg)
(I didn't have polyurethane varnish so I used acrylic paint instead)


(https://i.imgur.com/qDdVJUz.jpg)
Here's my topload, which I made from the limited resources I have.

The resonant frequency is around 195kHz (with a 3ft wire attached). Currently, I have the OCD set to 500A. I just need to 3D print some parts (to connect the secondary to the topload), then I can test.

Primary is about 4 and a third turns of litz wire.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 21, 2020, 06:45:15 PM
Are those steel screws holding down the yellow secondary holder?  Steel parts in or near the primary coil will be induction-heated, perhaps enough to become a problem (melt or burn parts).

Is the bottom of the secondary winding roughly at the top of the yellow holder?  If so, having your secondary bottom above the primary like that will reduce coupling factor, which may reduce performance.  It's safer, however, in avoiding arcs between secondary and primary, which would damage the secondary.  So, perhaps it's a good way to start.  (BTW, I started with my secondary bottom 50mm below my primary.  That resulted in arcs to my grounded guard ring, requiring patching holes in secondary coating.  Raised the secondary to 25mm below primary and added many more layers of polyurethane to the secondary to fix things.)

Was 195kHz with the top-load and 3' wire, or just the wire?

Good luck!
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 22, 2020, 12:40:31 AM
Tried it at half voltage so far, got a lot of buzzing but no arcs.

I think I'll have to tune it. Frequency is off by around 20kHz

I used about 900-1000 turns of 26AWG on my secondary, which seems a bit low. Will this matter?
Title: Re: GDT keeps on killing IGBTs
Post by: Zipdox on March 22, 2020, 01:09:17 AM
I saw this vid yesterday and though of it now
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 22, 2020, 04:51:47 AM
Well, it looks like it's making sparks but very small.

Definitely needs more tuning.

Tests were done at half voltage (170V)


Edit: Made primary 3.75 turns instead. Now, frequency is about 190kHz. Think the extra wiring conencting the primary to the bridge is causing a bit extra inductance. Will try to reduce tomorrow.

Here's a test on pulse skip mode, which seems to make longer arcs

Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 22, 2020, 09:28:04 PM
Yes, with a coupling factor on the low side, accurate tuning is more critical.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 22, 2020, 11:23:17 PM
I'll try moving the secondary down a bit too.

Could anyone give me a few tips on tuning the system? I'm going to get my oscilloscope probes sometime soon.

I think I did the same thing as you (davekni), like I'm using a spiral primary made out of litz wire too. Could you tell me how you tuned yours?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 23, 2020, 04:36:23 AM
First measure the secondary resonance frequency.  Then run your primary the way you had been without the secondary, and adjust its frequency slightly below the secondary frequency.  How much below depends on coupling factor and how long arcs will be (how much the arc will add to top-load capacitance and lower secondary frequency).  I'd start with the primary frequency only slightly below the secondary frequency.

The easiest way I know to measure secondary frequency is with a signal generator.  Do you have one?  Wire it through a resistor to a scope input, say 1k to start.   Connect the scope ground to line ground and/or your counterpoise or whatever you normally connect the bottom of the secondary to.  Connect the secondary bottom lead to the scope probe and 1k resistor.  Make sure the top load is in place and connected with whatever breakout point you are using.  Then adjust the signal-generator frequency until the voltage drops.  The frequency where the voltage drops the lowest is the secondary resonant frequency.  For increased accuracy, after you've found the frequency roughly, you can increase the 1k resistor to 10k.  That will make a sharper amplitude dip at the resonant frequency.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 23, 2020, 04:43:23 AM
My signal generator isn't really good (it's one of those cheap analog ones)

I'll probably get one of those AD9850 boards to get a better frequency generator.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 23, 2020, 05:27:33 PM
As long is you can adjust frequency reasonably smoothly, it doesn't matter what the quality of the waveform is.  Sine is best, but square or triangle can work.  The amplitude notching will look a bit strange, as only the fundamental harmonic is notched, while the higher harmonics will remain.  It's still easy to find the notch minimum in spite of the remaining harmonics.

An alternative secondary frequency measurement technique is ring-down.  Place a ~1nF capacitor across a scope channel input.  Then take the secondary ground lead, touch it to a somewhat high voltage (ie. 170V) source, then touch it to the scope input.  The scope should see a ring-down signal of a few volts.  That ring-down will be at the secondary resonant frequency.  You will need to trigger on the ring-down signal itself, and have the scope persistence set long enough to view a single-shot trace.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 24, 2020, 02:26:38 AM
Using your method, I got a resonance of 171kHz when measuring without a wire attached. With a roughly 5 foot wire attached, it dropped to 150kHz.

Here's another method I tried:
(skip to 5:58)

I got 224kHz and 208kHz using this method. Is there something wrong with this one. The guy in the video basically uses the secondary and his scope probe as antenna, and he measures the frequency where the scope picks up the most signal.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 24, 2020, 04:16:50 AM
That video method is very similar to the first method I suggested using a signal generator.  The only difference is that the video uses the peak of the top-load field rather than the notch in the drive voltage.  Any of these methods should work fine.  The signal-generator methods (both mine and the video's) are simpler to run.

The difference you are seeing between signal-generator and ring-down is to large to be just random measurement error.  One point comes to mind:  The video makes an implicit assumption that both the scope and signal generator are grounded to line-ground.  If either one is floating, the frequency will read higher than in actual operation.  To be certain, you could explicitly wire the scope ground lead to the signal-generator ground (to the other side of the generator output that the video says to leave floating) and then the two together to line ground.  Depending on how close other grounds are to your top load, connecting the primary winding to ground may make a difference too.  Grounding the primary coil will further improve accuracy, as it will be close to ground potential compared to the top-load's high voltage.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 24, 2020, 06:10:19 AM
You're right, tried it again with the signal generator grounded and got 171kHz and 150kHz.

Looks like I need 5 and a third turns. Will try tomorrow.

I tuned the primary to 156kHz. When I try pulse skip mode at 25% of the max voltage, I get a pretty big spark. When I try the other mode, which runs it normally with small pulse width, I get much smaller arcs, even at 50%. What pulse width should I use for the interrupter?

Also, do I need to increase the coupling?
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 27, 2020, 02:53:09 AM
Looks like I'm pretty close to being done


Here's a test at roughly 50-60% max input voltage. I can't go higher because don't have enough space. I'm running this on pulse skip mode, and I lowered the secondary to around 1 inch below the primary to increase coupling.

Edit: Test at 75%

Looks like I'm not getting big arcs because it's arcing to the bushes in the back.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 27, 2020, 03:00:52 AM
Nice to see!  All the frustrations of the process fade when things work in the end.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 27, 2020, 03:55:31 AM
I ended up blowing four of the IGBTs when I ran it at 90% input voltage. An arc formed between the secondary and the ground rail, and that seems to have caused the IGBT to blow.

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 think I need to lower the strike rail too.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 27, 2020, 05:32:32 AM
That's disappointing.  Hopefully the fried IGBTs didn't take out anything else.

Was the arc from the top-load to the ground rail, or from some point part way up the secondary?  The latter is the issue I had when my secondary started 50mm below the primary.  Raised the secondary to only 25mm below and added more polyurethane coats to the secondary.  Yes, lowering the grounded strike rail can help, but keep it slightly above the primary coil.

Two possibilities come to mind.  First is that the high-frequency noise generated by a ground strike coupled into circuitry, perhaps confusing the driver, making gate voltage transitions away from the current zero-crossing points.  Besides generally keeping wires short etc., make sure the ground connection wire for the strike rail doesn't pass close to drive circuitry.

The other is what I mentioned way back in reply 25 on March 1st:  Last I saw, you have relatively long and separated wires from your bulk caps to the H-Bridge.  That inductance can cause problems at the end of an enable pulse, when the H-Bridge changes from pulling current from the bulk caps to pushing it back as it removes energy from the primary tank circuit.  That issue forced me to redo my bulk-to-bridge wiring to reduce inductance.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 27, 2020, 05:46:41 AM
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.

I'm also using pulse skip mode, so the driver tries to keep the current at 500A. Maybe the current increased too much when it shorted.

The arc went from topload to strike rail. At 75% I was noticing corona discharge from ground strike rail.

For your comments, I don't think the wire connecting the strike rail to ground passes close to the driver. The driver is also in a grounded cage. I'm using the mains ground, will that cause problems?

Putting the capacitors closer might be difficult because I'm using a voltage doubler and I have to try to share the current equally between the capacitors. Maybe I could add 1000 or 2000uF more capacitance close to the IGBTs to solve this issue.

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 (https://www.mouser.com/datasheet/2/240/Littelfuse_Discrete_IGBTs_PT_IXG_120N60B3_Datashee-1591813.pdf)
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 27, 2020, 09:17:37 PM
Overcurrent is a possibility.  Some designs check only positive half-cycles, so it might be the end of the second half-cycle above 500A before shutdown.  Pulse-skipping could make that worse, as you're then hitting that current limit every few cycles, rather than once per enable pulse.

Pulse-skipping may be risky for other reasons too.  Depending on how it is implemented, it can make IGBT power sharing uneaven.  It also will make lower-frequency components to the gate-drive signal, which may require higher inductance and volt-second rating of the gate-drive transformer.  Or, if it puts some gates into the off 0V state, then there's extra half-voltage gate transitions, which are slower turn-off and may have some momentary weak turn-on overshoot pulses.

In short, I'd disable pulse-skipping mode until you have a chance to examine it closely with your scope at lower bus voltage and lower current limit settings.

The mains ground is probably fine.  I prefer to use both the mains ground and an aluminum foil (or screen or plate) ground plane under the coil against the floor or ground.

Additional 1000uF near the H-Bridge would be good.  Or, use lower inductance wiring from your existing bulk capacitors.  Multiple twisted pairs is good - that's what I needed to do even for a fairly short run.  Each pair has one wire VBus+ and one wire VBus-.  Even if you go with an additional 1000uF closer, I'd recommend twisted pairs between it and the H-Bridge.

The IGBT part you listed doesn't have an internal anti-parallel diode, so isn't a good choice for H-Bridge use.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 27, 2020, 11:05:05 PM
The left side of the half bridge didn't suffer any damage, but 3/4 IGBTs on the right side blew. I replaced all 4 just to be safe. None of the other pars seem to have failed, when I replaced the IGBTs, everything worked again up to 75% input voltage.

I lowered the strike rail, so I haven't gotten any strikes yet. I shorted the secondary and ran it at 50% input voltage, and I used your method to look at the voltage at the capacitor. It looked like a sine wave but it was considerably more fuzzy.

Also, during a ground strike, wouldn't the inductance of the primary drop, making the resonant frequency higher? Could that be a problem (like could it cause the IGBTs to switch at the wrong times?) I think the amount of phase lead produced by my circuit varies with frequency, and my phase lead circuit seems to work better at lower frequencies.

The IGBTs I'm currently using also don't have antiparallel diodes, so I'm currently using 4 fast diodes in place of them. Could this be a problem?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on March 28, 2020, 04:28:15 AM
Are your fast diodes on heat sinks along with the IGBTs?  If not, are you sure the diodes are all still good?  I'm wondering if the diodes got hot enough to either fry or have extreme leakage current and slow switching to the point that it fried IGBTs without the diodes themselves failing.  Skip-pulse mode puts more heat into the diodes, as does secondary arcs.

Yes, DRSSTCs usually run at the lower of the dual resonance peaks of the coupled resonators, so a shorted secondary will raise the frequency.  The frequency change generally isn't significant enough to cause problems, but might be if your phase-shift circuit is particularly sensitive to frequency.  Usually the more significant effect is the increased Q of the primary, since the secondary no longer uses much power.  That causes the primary current to ramp up relatively quickly and ramp down slowly.  The pulse-skip mode will then need to skip lots of pulses, moving more power to the diodes, and likely causing more issues with gate-drive low-frequency components.  Even without pulse-skip, there will be more power for the diodes during the decay after the enable pulse ends.  (I'm working on a circuit to detect ground arcs and terminate the enable pulse before current ramps up too much.  My IGBTs have internal diodes, but their power dissipation capability is much lower than the IGBTs themselves.)

The other possible issue with external diodes is inductive voltage drop between the IGBTs and diodes.  I don't have any personal experience with external IGBT diodes.  I think IGBTs are usually rated for ~20V reverse Vce.  I don't know how sensitive they may be to spikes beyond 20V.

The "fuzzy" sine wave was likely multiple captures as the primary current ramps up.  The key reason to look there is for accurate phase adjustment.  Typically you can see the sine wave along with small steps at the top and bottom when the H-Bridge switches.  Those steps should be just barely before the top and bottom crests of the sine wave.  Try triggering on the enable pulse to get a stable waveform, or set the scope's trigger hold-off longer to prevent multiple triggers per enable pulse.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 28, 2020, 04:35:58 AM
I successfully got my coil to work at full voltage without blowing up:


I checked and the overcurrent LED never lit up. I'll have to measure the pulse width, but it's set to 75% of the max pulse width the controller lets me set.

Edit: tried it at the highest pulse width, still no overcurrent. I think my transistors blew up because of overcurrent. Could also be faulty OCP?

Actually, I think I need a higher capacitance. Right now I'm using 100nF, I should probably use 200nF or something.

150nF would get me 4.5 turns and reduce the surge impedance to 7 ohms (from 10)

200nF would get me 3.75 turns, surge impedance = 5 ohms
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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.

(https://i.imgur.com/xyeSFAr.jpg)

(https://i.imgur.com/gCZGsk2.jpg)

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?
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob 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 (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!
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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...
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob 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?

Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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)
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob 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/
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on March 31, 2020, 01:33:16 AM
Some pictures:

(https://i.imgur.com/yOlDOZT.jpg)
^GDT waveform vs voltage at capacitor

(https://i.imgur.com/w7FrRDn.jpg)
^Same thing, but at 10us/div

(https://i.imgur.com/YpR1XFI.jpg)
Left side gate drive and right side gate drive

(https://i.imgur.com/iCfVq66.jpg)
Bridge output (looks like I need to increase phase lead)

(https://i.imgur.com/sZcu1Zf.jpg)
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.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 01, 2020, 01:51:05 AM
I fiddled around with the phase lead a bit:
(https://i.imgur.com/hYrkF3c.jpg)
I got it so that it switches before the peak.

(https://i.imgur.com/kAuzLSK.jpg)
Looks like there's still quite a bit of ringing, but I'm testing this at low voltage.

(https://i.imgur.com/Dto2cRk.jpg)
Here's a scope shot of the bridge at 5us/div instead of 2us.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 01, 2020, 04:15:41 AM
I'm still trying to figure out why the overcurrent isn't working as intended.

(https://i.imgur.com/iuGtqQE.jpg)

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:

(https://i.imgur.com/lT0oYd4.jpg)

(https://i.imgur.com/txDvnDO.jpg)

Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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:
(https://i.imgur.com/UTU9uD8.jpg)
Could the feedback be stealing current from the OCP?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience 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:
(https://i.imgur.com/W47OkxH.png)

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.
(https://i.imgur.com/CTOMCGi.jpg)

(https://i.imgur.com/qkwbps5.jpg)
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.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni 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.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 03, 2020, 04:27:08 AM
Looks like I found the answer

https://www.microsemi.com/document-portal/doc_download/14696-igbt-tutorial (https://www.microsemi.com/document-portal/doc_download/14696-igbt-tutorial)

Pages 2-3 say that high collector current could cause the IGBT to latch on.

I'm wondering if I should go back to using 100nf for primary capacitor since the primary current rises really fast
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 04, 2020, 05:37:11 AM
Got it to work at full voltage without blowing up. Set overcurrent to 500A.



I'm only getting 1 foot arcs, which means that my coil is probably a bit out of tune.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 04, 2020, 08:01:11 PM
Yes, for 500A and ~150kHz, 100nF MMC is probably better.  Hopefully that will increase your arc length.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 06, 2020, 05:09:29 AM
Looks like you're right.

Tested at half voltage, already working really well. Will have to go somewhere bigger to test.

Edit: This one's probably better. Moved the breakout point up a bit so it doesn't get cut off by the door.


Thanks for your help davekni!
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 06, 2020, 06:23:27 PM
Great to see success!  When you can test in a larger space, you'll find that higher pulse repetition rate extends the streamer length well beyond that of single pulses.  Hopefully everything is robust enough to handle higher rates for an impressive show.

You are certainly welcome.  I'm glad to have been of help.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 07, 2020, 10:24:16 PM
Just out of curiosity, is there a reason why 100nF works better than 200nF?
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 07, 2020, 10:54:57 PM
Yes.  For your fixed frequency and fixed 500A current limit, 100nF has twice the impedance, so twice the primary voltage and twice the primary energy.  It also takes longer to ramp up to the 500A limit.  The short ramp at 200nF may not be long enough for the primary energy to transfer to the secondary, besides having less primary energy available to transfer.

Are you still hitting your 500A limit on most pulses?  If so, even lower than 100nF might be better.  Of course, going lower requires your primary to handle higher voltage.  An arc across the primary coil will destroy IGBTs when the MMC suddenly discharges into the H-bridge.  Also be careful of running longer pulses, as the IGBTs may not handle just under 500A for the longer times.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 08, 2020, 01:05:28 AM
I'm already tripping the 500A limit at around half voltage. I'll try reducing the tank capacitor to 50nF.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 08, 2020, 04:15:12 AM
50nF sounds good, but do be careful to have sufficient voltage capability.  At 150kHz, 50nF has 21.2 ohms impedance, so will have 10.6kV at 500A.  Plan for at least 12kV to have margin for any delay in over-current shutdown.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 10, 2020, 05:19:24 AM
I changed it to 50nF and it seems to work better. Haven't tested it at half voltage yet.

Some pictures:

(https://i.imgur.com/0dWokDE.jpg)

(https://i.imgur.com/8Xf8f8u.jpg)

(https://i.imgur.com/YwCYzNs.jpg)

I guess one of the things I could change is moving the FB and CT coils onto the lower voltage side of the MMC. However that means my leads have to be longer.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 10, 2020, 09:15:17 PM
Yes, the current transformers should definitely be on one of the H-Bridge outputs, not on the MMC-to-primary connection!  Don't want to rely on CT insulation holding up to 10-12kV!  CT lead length isn't as critical as for gate-drive, but I'd  still suggest seeing if you can rearrange your layout to keep leads short.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 18, 2020, 05:51:48 AM
It seems like there's a few problems with my phase lead circuit. First of all, at low currents(~40A), the duty cycle is a few percent less than 50 because one of the diodes (the one connected to 0V) clamps more often than the other. Also, my circuit seems to also be current dependent (higher current results in smaller phase lead). I'll probably have to adjust it while testing at higher current.

I can't use the other circuit for phase lead because the rising and falling edges are not really steep, and the extra time it takes to reach the HIGH threshold basically offsets the phase lead.

edit: tried measuring, ended up having a spark form between primary and oscilloscope probe. Nothing broke (pretty sure there's a MOV inside the oscilloscope that saved it)
Title: Re: GDT keeps on killing IGBTs
Post by: Mads Barnkob on April 22, 2020, 09:19:36 AM
Yes, the current transformers should definitely be on one of the H-Bridge outputs, not on the MMC-to-primary connection!  Don't want to rely on CT insulation holding up to 10-12kV!  CT lead length isn't as critical as for gate-drive, but I'd  still suggest seeing if you can rearrange your layout to keep leads short.

The current transformers (FB/OCD) should be on the primary lead from inverter to primary coil. As Dave mentions, between primary coil and MMC there is a very high voltage present due to resonant LC voltage ring up. But even on the inverter to MMC connection you have some higher voltages than on the other leg.

It seems like there's a few problems with my phase lead circuit. First of all, at low currents(~40A), the duty cycle is a few percent less than 50 because one of the diodes (the one connected to 0V) clamps more often than the other. Also, my circuit seems to also be current dependent (higher current results in smaller phase lead). I'll probably have to adjust it while testing at higher current.

I can't use the other circuit for phase lead because the rising and falling edges are not really steep, and the extra time it takes to reach the HIGH threshold basically offsets the phase lead.

edit: tried measuring, ended up having a spark form between primary and oscilloscope probe. Nothing broke (pretty sure there's a MOV inside the oscilloscope that saved it)

40A or low test voltages are not sufficient for testing a Tesla coil. You need to have the system in a state of consistent energy transfer where different components minimum values for "linear" behavior is met. Like low voltages across a IGBT gives you a much higher output impedance or feedback signal is too weak for the driver to properly lock on in the heavy EMI.

Some 100-200 VDC bus and at least 100-200A in the primary circuit to even begin to think about looking at phase lead, you just risk adjusting the phase lead for the low voltage test conditions that are far from what you see at full power. Often you have your primary circuit detuned lower for longest sparks, as the secondary circuit goes down in frequency as the sparks grow longer (presenting a capacitive load).

You need to use a differential probe when measuring on inverter outputs to avoid damaging your oscilloscope. You risk high voltage transients or grounding the power circuit through your probe.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 23, 2020, 12:38:24 AM
Mads,
When you said "But even on the inverter to MMC connection you have some higher voltages than on the other leg.", are you referring to the other H-Bridge leg?  Hopefully the two H-Bridge outputs have the same voltage (180 degrees out-of-phase) or some electronics has fried.  The only difference that comes to mind is parasitic wiring inductance and the MMC's parasitic capacitance to ground, which wouldn't matter if the CT is close to the H-Bridge.

Separately, ritaismyconscience, have you measured your scope probe input impedance?  I had a probe fry in a way that it still functioned, except that it had about 600 ohms from tip to ground.  Took a while and some confusion on my part to figure out the probe issue.  Was initially using it probing a low-impedance signal source, so didn't notice any problem.  Moved it to a higher-impedance node, and results got confusing for a while.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 23, 2020, 06:08:02 AM
Checked and no change in impedance. Think I got lucky becuase I accidentally left it on 50 ohms when it hit. Anyways my scope should have input protection (it's fairly old though).

I measured the voltage by insulating a section with tape and putting a wire next to it (basically I made a capacitive dropper). It ended up arcing over an inch around the tape and onto the probe, but nothing bad happened. Also, I'm having a bit of trouble determining the phase lead with the current setup because the current ramps up in a few cycles and just stays there, and during the part where the current is constant, I can't really tell when the IGBTs are switching and when they aren't. It just looks like a normal sine wave.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 23, 2020, 10:28:17 PM
That inch arc explains why current-sense transformers should be on one of the H-Bridge outputs and not on the MMC-primary junction. :)

For phasing, if one channel is measuring current, you need another measuring H-Bridge output voltage.  Using a single probe works when measuring the MMC-primary junction (capacitively since you don't have a HV probe).  Perhaps you should put a small glass jar (or test tube or ...) over the end of the probe for better insulation.  That capacitive-coupling probe method is for high-impedance scope input.  With 50 ohm scope input, you'd get the derivative of the voltage (high-pass-filter).
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 25, 2020, 07:26:37 AM
(https://i.imgur.com/tV3FhJJ.jpg)

Think I got phase lead set correctly

(https://i.imgur.com/KHKg2FM.jpg)

Here's my setup. I probed the H-bridge to Primary wire.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 25, 2020, 07:51:57 PM
Is that a current or voltage waveform?  If current, the only slight glitch that might indicate switching time is at peak current rather than zero.  If a voltage waveform, the H-Bridge output should be a square-wave, not a sine wave.  The sine-wave voltage I'd suggested probing for phasing is at the MMC-to-primary node, probed through good insulation (ie. small glass jar).
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 25, 2020, 08:30:43 PM
I'm measuring voltage, not current. I think I probed the two H bridge outputs with the method you described, and one of them looks like the sum of a square and sine wave, and the other just looks like a sine wave.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 26, 2020, 02:04:23 AM
Both H-Bridge outputs should be square waves, except for the vary tail end of ring-down after a burst ends.  The sine + square is the junction between MMC and primary coil.  That's where the voltage is high and you can get the 1" arc, so why good insulation is necessary.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 26, 2020, 05:48:25 AM
Looks like I didn't set it correctly.

Here's a simulation. I added a bit of inductance and resistance between the square wave output and the inductor to simulate the parasitic inductance of the wire:
(https://i.imgur.com/GwkW2Mr.png)

Seems to have some phase lead.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 26, 2020, 06:43:27 AM
Probing there makes it difficult to know phase accurately, as the scope signal depends mostly on parasitic resistance and inductance (20m and 200nH).  Probing at the high-voltage node, between the 22uH and 100m-ohm, will give you a more accurate measure of mmc voltage, which will be more accurately 90 degrees from current.
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 26, 2020, 09:39:09 PM
I can't seem to get it work with your method. The controller does a really good job keeping the current constant, so waveform just looks like sine wave:

(https://i.imgur.com/lRDcr4z.jpg)

I'm measuring it at the highest voltage, which should also correspond to the highest current.

Edit: Here's a zoomed in version of the previous picture. There does seem to be a small bump right before the peak. Not sure if this is where the transistors are switching.
(https://i.imgur.com/zYI2hUo.png)

Edit 2: Tried a new way. I took the sine wave and subtracted the H bridge output, and it looks like this:
(https://i.imgur.com/ngyzzEG.jpg)

I'm only testing it at low voltage, but it's enough to trip the overcurrent.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 26, 2020, 10:10:36 PM
You could probe the H-Bridge-to-MMC connection to verify that there's still a square-wave there.  Just in case one side of the H-Bridge fried, or the bus voltage is collapsing during the burst, then there wouldn't be the step on the MMC-to-primary connection.  If the MMC side of the H-Bridge has a good square-wave signal, then those same steps will show up on the high-voltage (primary winding) end of the MMC on top of it's higher-voltage sine-wave component.  (Check in your simulation too, probe the 50nF to 22uH node.  I think your simulation likely has less loss than real life, so you may see over 12kV, but there will still be the step on top.)
Title: Re: GDT keeps on killing IGBTs
Post by: ritaismyconscience on April 26, 2020, 10:13:12 PM
I tried probing it and there is a square wave. I think the capacitor method doesn't work very well for the H bridge for some reason. I also tried measuring through a 10M resistor, and I got the same waveform (a sine wave with some ridges). I onl y got a square wave when I connected the probe directly to the output.
Title: Re: GDT keeps on killing IGBTs
Post by: davekni on April 26, 2020, 10:38:04 PM
At low bus voltage, the H-Bridge output square-wave will be low amplitude.  With bursts long enough to ramp current to your 500A limit, then the MMC-to-primary node will still have max voltage.  Perhaps the H-Bridge output steps are just too small to see on the full MMC voltage.
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