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Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
hen918 wrote ...
nzoomed wrote ...
....
I have not pulled it apart yet, but as i speak, ive put my multimeter across the DC bus and it measures around 100 ohms, not sure what this should normally read, but its not a dead short.
If there was a dead short something would have made a really big bang! When they fail quietly they don't tend to go to 0 ohms. IGBTs should read many orders of magnitude higher than that, normally.
OK, I have removed the caps from the DC bus, and the resistance on the IGBT's is off the scale on my multimeter on one of them, and the other is way up in the megohms range.
Seems it must be the caps, but im going to need to test these on a proper PSU now...
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
nzoomed wrote ...
OK, I have removed the caps from the DC bus, and the resistance on the IGBT's is off the scale on my multimeter on one of them, and the other is way up in the megohms range.
Seems it must be the caps, but im going to need to test these on a proper PSU now...
I have seen this before plenty of times. It goes like this: Upper or lower transistor goes short for whatever reason, then other transistor turns on which results dead short over your DC bus. Energy stored in your bus capacitors can and will blow up bonding wires inside your igbts and any shorts will be cleared.
IGBT brick an be totally destroyed while collector-emitter measurements appear to be within normal limits (=no shorts, antiparallel diode is still fine). Check gate connections for resistance capacitance. Healthy gate should read resistance of 100s of kOhms to Mohms and capacitance of few 10s of nF. If not the brick is most likely destroyed like this one:
This CM300 had IGBT chips completely destroyed while antiparallel diodes were fine. Diode test over C-E was normal but gate connections did not show normal capacitance (gate connection was open circuit and less than 1n of capacitance..) so i decided to cut the brick open.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
You can check IGBTs (or at least those without short circuit protection circuitry) with a normal multimeter.
Set to diode mode, measure C (positive probe)-E (negative), keep probe connected to emitter and touch the gate with the positive probe then quickly switch it back to emitter.
If the multimeter uses a high enough voltage for diode test you should see the measurement go from out-of-range before touching the gate to something on the order of ~1-1.5V, rising as the gate capacitance discharges.
This may not work so well with large (brick) IGBTs, and almost certainly not work with anything with the "RTC Protection" circuit, but I have used with with success with smaller TO-247 IGBTs. It will also depend on the multimeter - some will not be suitable.
The other thing to test is with a current limited bench supply. Set to 10-15V at maybe 100mA or so, negative connected to emitter, repeat the procedure above with the positive lead. Should not flow current until you touch the gate, then should conduct until the gate discharges.
Edit: if it fails the multimeter test don't assume that it's dead unless others of the same type give different results. If it passes though there is a good chance that it's OK.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
run a signal generator into the feedback CT input, run the bus off a 12V PSU and you should be able to get the bridge to switch like normal (freq determined by sig gen - use something like 50-100kHz) if the IGBTs are fine
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Hydron wrote ...
run a signal generator into the feedback CT input, run the bus off a 12V PSU and you should be able to get the bridge to switch like normal (freq determined by sig gen - use something like 50-100kHz) if the IGBTs are fine
I didnt even have to do that! Ive just confirmed that one of the IGBT's has died, and now ive got to ask a few questions! I tested both IGBT's this way here:
After testing both, i discovered one has indeed failed, but im pretty sure the other one tests OK according to these tests, but i just hope it has not suffered any damage, is it better to replace both if one has failed?
I also think i may have discovered the cause of failure, but would like your thoughts on this. When i went to disconnect the GDT from the gates to test the IGBT's, I discovered one of the gate wires had broken off the spade connector (on the failed IGBT)
Measuring across E2 and G2 on this brick was 0 Ohms and across E1 and G1 was about 2.5 Ohms, so I knew straight away this was dead.
It does not appear to have melted off or anything, so I can only assume this bought on the failure perhaps? I know you have to be careful when stripping this solid core cat5 cable, and i thought I had when i was assembling the inverter.
Both TVS clamps pictured here on the failed IGBT were also giving me a reading on my multimeter, when the ones on the "good" IGBT are perfectly normal and measure no conductance with my multimeter.
Anyway, ive also attached a photo of the failed IGBT brick, this was a nightmare to open, but it confirms that it has failed. (how on earth are supposed to open these units for modification, ill never know? lol I had to destroy this to open it!)
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
nzoomed wrote ...
Anyway, ive also attached a photo of the failed IGBT brick, this was a nightmare to open, but it confirms that it has failed. (how on earth are supposed to open these units for modification, ill never know? lol I had to destroy this to open it!)
Mod edit: cleaned up your double posts :)
Now, we have some evidence right here!
Larger rectangular chips are your actual IGBT transistors. All of them have failed as you can see. But: more interesting thing is that you also have at least one diode chip that is failed in catastrophic manner. In DRSSTC there is only one reason why the antiparallel diode failes like that, and it is overvoltage.
My guess is that there was an overvoltage event that caused at least one diode short out which made opposing igbt chip to die to overcurernt and so on.
So, what may have caused the overvoltage then? Bus layout, insufficient decoupling and/or snubbering, switching timing and similar things. I had very similar CM300 failure (Diode and igbt chip damage) in past due to the 20µ plastic film decoupling capacitor failed open circuit during TC run :D
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Kizmo wrote ...
nzoomed wrote ...
Anyway, ive also attached a photo of the failed IGBT brick, this was a nightmare to open, but it confirms that it has failed. (how on earth are supposed to open these units for modification, ill never know? lol I had to destroy this to open it!)
Mod edit: cleaned up your double posts :)
Now, we have some evidence right here!
Larger rectangular chips are your actual IGBT transistors. All of them have failed as you can see. But: more interesting thing is that you also have at least one diode chip that is failed in catastrophic manner. In DRSSTC there is only one reason why the antiparallel diode failes like that, and it is overvoltage.
My guess is that there was an overvoltage event that caused at least one diode short out which made opposing igbt chip to die to overcurernt and so on.
So, what may have caused the overvoltage then? Bus layout, insufficient decoupling and/or snubbering, switching timing and similar things. I had very similar CM300 failure (Diode and igbt chip damage) in past due to the 20µ plastic film decoupling capacitor failed open circuit during TC run :D
Overvoltage, now thats interesting! I wouldnt have thought that overvoltage would be possible, since these are rated at 1200V, and my maximum bus voltage would not be any more than 700VDC.
As i mentioned, I discovered one of the GDT wires from the cat5 cable was broken off the spade terminal to the gate on that very IGBT, could this have caused the failure?
Do you think the other IGBT will be OK? (it appears to test OK)
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
nzoomed wrote ...
As i mentioned, I discovered one of the GDT wires from the cat5 cable was broken off the spade terminal to the gate on that very IGBT, could this have caused the failure?
Do you think the other IGBT will be OK? (it appears to test OK)
i had the same problem before (one disconnected gate) and the IGBT's failed even at a lower buss voltage i had this problem for a half bridge with discrete IGBT's but i guess but i guess your other IGBT brick has survived
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Have my bus caps died?
