CM400 Induction Heater

[markus]

Yes, I've also had fake mosfets from china with a way too small die, after which I bought the real stuff from Mouser. edit: I also remember some mentions of fake IGBT bricks, where the seller somehow replaced the casing and filled in opaque goo instead of the previously clear goo to hide the die. The people were impressed that all this effort was worth the money.

But with the current IGBTs I'm pretty sure that these are just some local guy stripping some inverters or something and making a bit of money. I got a bunch of random 100A, 150A and also some 75A and 200A modules from him that still had the old gate circuitry on them.
Also I've already run the same IGBTs for some (limited) amount of time with the old setup from my previous post (much of the setup including driver also being the same). At 360Apk and 7 turns on the matching transformer this gave me some 50Apk through the IGBTs.

[davekni]

But with the current IGBTs I'm pretty sure that these are just some local guy stripping some inverters or something and making a bit of money. I got a bunch of random 100A, 150A and also some 75A and 200A modules from him that still had the old gate circuitry on them.

Does sound likely to be genuine given old gate drive circuitry etc.  I have purchased counterfeit IGBTs of TO247 variety from EBay (and gotten refunds after proving data sheet spec's weren't met).  Your situation sounds different enough.

There should be no issues with the matching transformer - at 43.5kHz and 7 turns I get a flux density of 314mT, which still leaves me with a fair amount of head room given a maximum flux density of 380mT for the K4000 material. At the time of failure I used twice the amount of turns, so saturation should be pretty much impossible.

Probably not, but I don't know enough about UD+ to be certain.  Controlling by secondary current leaves the possibility of sub-harmonic component that builds up excess current due to resonance of primary coupling capacitor set with transformer inductance.  Might be safer to control by primary current.

What I noticed was that on the bus caps, one screw turned out to be loose (on the connection between the two series caps). There was still contact however, and I do not think that lacking a bus cap would lead to my bridge exploding randomly after a day of testing anyway.

Hard to say when some jarring or magnetically induced force from current may have made momentary disconnects.  I'd inspect that connection for any subtle signs of arcing - black residue or pitting.  Loose power connections are often problematic.

I did notice a strange kind of ringing however, both in normal operation and in freewheeling mode (tested with signal generator / scope). Where does that come from, and is it dangerous? The dip in the middle is not really that deep and looking at the data sheet, the IGBT only starts conducting at 6V so that amount of ringing should not pose much of an issue yet. Nevertheless I want to get rid of it of course, any tips?

I'm guessing this is Vge ringing.  Not specified.  If so, scope images are needed to make any useful recommendations.

As I was writing this I remembered that when I was checking gate wave forms sometime earlier, I managed to produce a ground loop from one emitter through my scope lead, popping the breaker (I forgot to remove the scope lead when switching from the isolation transformer to regular mains). I am pretty sure this happened to be the same IGBT that is now shorted, could that have damaged the gate somehow?

This would be first guess as to a cause.  Presuming scope ground was connected to smaller Kelvin emitter pin (pin paired with gate pin), resistance of that path might be high enough to fry something with momentary fault current.  (Second guess is above loose power connection.)
BTW, I made the almost-same mistake once too, forgetting to remove scope ground when switching from isolated to line power.  Wasn't on gate return, so didn't fry anything.  Was a bit surprised that scope and probe both survived too.

And last of all, the ebay seller I got these from did not take any measures against ESD and just packed them into refrigerating plastic bags, maybe I was unlucky and the gate got slightly damaged?

Poor packaging, but not likely the issue.  I have seen latent failures due to prior partial damage from ESD, but not specifically in IGBTs or power FETs.

Hope your repair and future work goes smoothly.

[markus]

Thanks for your input!

This would be first guess as to a cause.  Presuming scope ground was connected to smaller Kelvin emitter pin (pin paired with gate pin), resistance of that path might be high enough to fry something with momentary fault current.  (Second guess is above loose power connection.)

In that case I will assume that to be the failure cause and rebuild everything. Yes, the scope was connected to the small gate / emitter pins. So far it looks like the scope survived just fine, gotta see if any issues pop up later.

Controlling by secondary current leaves the possibility of sub-harmonic component that builds up excess current due to resonance of primary coupling capacitor set with transformer inductance.

I think that if the cause would have been overcurrent, the heatsink should have been hot. Still, I will check the primary current during the next run with my scope.
I calculated the resonant frequency of the coupling cap + transformer inductance beforehand, using an estimated inductance of 14,15uH from a similar project (value is for matching transformer with 8 turns and 25cm2 cross section). With 192uF capacity, this gave me a resonant frequency of 3kHz, which is about one order of magnitude away from the expected operating frequency. However while writing this, I noticed that the current matching transformer I put together for testing only has 8.6cm2 cross section, so might have a lower inductance than the estimate. I will check the actual values just to be sure.
I will think about controlling the primary current instead of the secondary. It would have the demerit that I have to readjust the current limit if I change the matching transformer turn ratio and risk exceeding the celem current rating if I forget to do so. But of course with controlling the secondary current, I might exceed the IGBT current rating instead if I am careless.

I'd inspect that connection for any subtle signs of arcing - black residue or pitting.  Loose power connections are often problematic.

I did check for that, everything looked fine.

I'm guessing this is Vge ringing.  Not specified.  If so, scope images are needed to make any useful recommendations.

Yes, the ringing is on Vge. I did attach scope shots, but I guess since they were not included in the post itself they were hard to see. Here they are again, the first with normal operation (signal generator) and the second with freewheeling:

[davekni]

Yes, the ringing is on Vge. I did attach scope shots, but I guess since they were not included in the post itself they were hard to see. Here they are again, the first with normal operation (signal generator) and the second with freewheeling:

Now I see those plots in your previous post.  For some reason only the top of your three images showed up the first time I read your earlier post.
Looks like ringing of UD+ output coupling capacitors with GDT parallel inductance.  Presuming UD+ output looks like that of UD2.7, there is a coupling capacitor paralleled by another capacitor with resistor in series.  Adjusting R/C values to be critically damped with your GDT inductance will make ringing die down faster.  Larger capacitors will reduce ring amplitude.
However, as you said, ring is small enough as is.  No fixing is necessary.

[Anders Mikkelsen]

Happy that you made nice use of the cap already.

For the heater, you definitely want the current sensing for overcurrent protection on the primary side of the coupling transformer. The capacitor will take minutes to overheat, while the IGBTs can fail in tens of microseconds if the current exceeds a safe value. The way you have it now, the magnetizing current, and its potential resonance with the coupling caps, draws current that is completely unmonitored. Current monitoring for the sake of control can still be kept on the secondary side, but keep in mind that the expected high currents in normal use will require a very substantial current sense transformer. If you have two 1:32 transformers, the current in the secondary of the first transformer will be over 30 A at the full 1 kA tank current. This is usually not a problem in DRSSTCs where the RMS current is in the 100 A ballpark, but in induction heaters when you have steady-state high current it can be problematic.

It also makes sense to measure Vce of the IGBTs in operation, to rule out overvoltage as a failure cause. The last problematic thing I can think of is potentially cross-conduction, and the resulting high current spikes and L*di/dt Vce overshoot, since the IGBTs take significantly longer to turn off than they take to turn on. Make sure you have sufficient dead-time to account for this. Does the controller implement dead-time, or is this only ensured by turn-on gate resistance vs turn-off gate resistance? The latter case is not ideal, as you would have to really slow down the turn-off to mitigate cross-conduction.