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Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Hi there, I have been doing some tests on my new DRSSTC that I'm working on but have had an exploded IGBT. However this explosion happened during burst mode operation where I suspect power dissipation wasn't too extreme as the heat sinks were not getting particularly hot at all. The details are as follows:
IRG4PC50UD full bridge
350V bus
over current protection at around 250A
7.5 turns on primary
176nF tank capacitor
4700uF bus capacitor
TVS protection on IGBT's
2 x 10uf polyprop capacitors as snubbers
Steve wards older driver
The fail has also blown the tracks into the collectors of both the IGBT's on one side of the H Bridge - I'm guessing that one failed short circuit and then when the other switched on the bus capacitor was essentially shorted through the IGBT's with all 4700uf dropped across them! Can anyone let me know if this makes sense please?
I'd also welcome any insight as to what may have caused the IGBT failure as I suspect 250A isn't excessive for the IGBT particularly as I'd only been running in burst mode and even then for 3 or 4 second runs.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
chris_inkubate wrote ...
The fail has also blown the tracks into the collectors of both the IGBT's on one side of the H Bridge - I'm guessing that one failed short circuit and then when the other switched on the bus capacitor was essentially shorted through the IGBT's with all 4700uf dropped across them! Can anyone let me know if this makes sense please?
I'd also welcome any insight as to what may have caused the IGBT failure as I suspect 250A isn't excessive for the IGBT particularly as I'd only been running in burst mode and even then for 3 or 4 second runs.
Look at the bottom graph at page 4 in the datasheet:
The thermal impedance raises fast with pulse duration for small devices like the To-247, they do not at all match the longer pulse durations that larger IGBT bricks do, as they still have quite low thermal impedances at those on-times and by long we are talking under 500uS.
My guess is that your junction temperature simply could not cool down fast enough between the bursts and suddenly it melts, short out the supply and the weakest point in your bridge construction is the tracks of the PCB that blows off.
250A peak current is close to the limit of TO-247 devices, I think I blew some at 350A from excessive peak current. You likely blew them from too long on-time.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
The failure that you are seeing were one IGBT fails and then kills the other is very common (also known as crowbar) and will happen almost every time an IGBTs fails unless you have a very fast OCD scheme that can shut off mid cycle. However most DR drivers will only shut off at the end of a zero crossing so it’s unlikely that you will be able to mitigate the crow-baring effect without changing the OCD scheme significantly.
The first IGBT most likely failed from one of two things the first being over current, which is likely considering you are running right over the SOA of those IGBTs.
The second is an overvoltage transient on your bus. TVS across a bus layout is rather useless, because they will not be able to absorb the kid of energy associated with an event like this, let alone react in time. Bus transients are usually caused by poor bus layout, or abnormal parasitic oscillation in the IGBT and bus copper. A good way to check is to scope your inverter output while running at lower power. If you have an abnormal amount of ringing at the voltage transitions, this is likely what caused the IGBT failure.
We may be able to help further if you post a photo of your bus layout and a scope shot of your inverter out voltage.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Thanks for the replies and its certainly interesting to learn about the thermal impedance vs. pulse duration. I was taking some liberty running those IGBT's over current but believe this isn't uncommon for this device in DRSSTC use, but I suppose it all depends on the specs of the coil they are used in!
I have attached an image of my bus of which I'm not sure how good/bad it is in real terms. Would I be correct in thinking that there could be fairly high voltages on the inverter output even at low say 24V in and low duty cycle? Just a little concerned that if I scope the output that I do it safely!
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Well, your bus layout is not really optimal. Your + - bus planes are far away from each other and not overlapping. I wouldn’t be surprised if you had excessive ringing on your inverter output. If you plan on simply replacing your IGBTs I would look at your output voltage on a scope and see how bad the issue really is. As per my previous statement, I’m now leaning to overvoltage as your failure condition. However, you still need to verify this before taking any corrective action.
Also in regards to the thermal impedance data in the datasheet, use that data carefully. That data only models the thermal impedance from die to case. Without incorporating your cooling solution into that model, that data is not very helpful.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I agree that the PCB layout is not optimal, but this is a single sided board and there is C3 and C4 snubbers across the DC bus, right at the switches, it is still a good idea to check your inverter output waveforms if you can, be sure to use a isolation transformer for the bridge supply or a differential probe to avoid damaging your oscilloscope..
I would get rid of all 90 degree corners and have it all 45 degree or round, you have so many places where you could just fill it out to a larger plane, get some more trace width in around those IGBTs, move some of the other components a bit away so there is room for power traces.
I do however disagree that the thermal impedance data is not helpful, there is so little mass and material in TO-247 packages to absorb and transfer heat that it is a issue not to take on lightly. Our very low on-times is the very reason we can run the IGBT switches over the rated peak current as the generated heat can dissipate in-between bursts. If it is mounted on a normal heat sink you can slide the X axis some, but it will still, roughly estimated, scale the same for that cooling method.
I attached a application note by Powerex on the thermal impedance issue
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
I'll certainly look into re-designing the PCB taking into account this feedback. I assume I can really just leave out the TVS's which will allow more room for power bus tracks.
I managed to get these scope captures of the output which hopefully may make some sense. I had to scope the OCD burden resistor as my scope is only a Chinese PC oscilloscope and kept locking up directly connected!
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I think that 350uS is way too long on-time for a DRSSTC using TO-247 switches, instead of using long on-times to get a good break out you should try to tune it more precisely and you will see you can get a better output at under 100uS.
Please someone correct me if im wrong about te following: The steps seen in the first shot is actually 4-5 cycles of power transfer to the secondary coil. Effectively is works the same as burst mode that it is just on for such a long time that more cycles find place.
The small ripple in your traces could be parasitic ringing, but as it oscillates at 1MHz and that is also the sample rate of your cheap Chinese scope, it could just be that it picks up some noise at every sample.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
I've now replaced the IGBT's with four ST STGW80V60DF parts and keeping the on time at 100us max and no failure as yet. Getting around 26" arcs so will work more on tuning and then onto optimising my PCB bus and see how much more I can get from it. Thanks for the feedback so far - I have some brick IGBT's but wanted to learn the ropes with cheap IGBT's that I don't mind as much destroying. Which is whats happened!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Chris,
I think your IGBTs are just failing from over-temp.
Using phase lead could lower the power dissipation in the IGBTs by maybe 10-30% compared with my "old driver", especially if the resonant frequency is high.
Its helpful to judge an IGBTs ruggedness by its thermal resistance from junction to case (Rjc). The power ratings of a TO-247 IGBT can range from maybe 100W to 1000W, and its purely related to how big the chip is inside. Bigger chips clearly can dissipate more heat. Your IRG4PC50UD has a Rjc of 0.64 *C/W, and the STGW80V60DF has Rjc = 0.32. The ST part likely has about 2X the silicon area compared to the IR part, and yet they have nearly the same peak collector current spec. The difference will be how quickly the IR part heats up during a "burst" vs the ST part, having twice the thermal capacity makes a big difference, hence it survives. If you shop around you can find TO-247 devices with Rjc < 0.2 or even .1 (though they dont have mounting holes by that point, you need to clamp them).
Also beware of the thermal resistance that those transistor isolating films/pads give. The highest performing, inexpensive option seems to be very thin 1 mil kapton film with just the right amount of thermal compound. I calculate that the 1mil film gives about .5 *C/W resistance from the case to the heatsink, which is significantly more than the junction-to-case resistance. Those silicone rubber types can vary from about twice that resistance to several times higher, making the thermal resistance easily 10X the junction-to-case resistance, basically crippling the parts. Mounting the parts directly to individual heatsinks can drastically imrpove an IGBTs performance.
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Quick IGBT failure in DRSSTC
