Bridge output

[Felix M.]

I changed the slow brick 1 for a new one with no resoults. Even a second change with a second unused brick wouldn't make a difference. After then putting the gate board of brick 1 on brick 2 and the other way around for the secon board, it yet again didn't change anything. Allways the brick with its output connected to the MMC looked wobbly so I also changed that, no results. I then changed brick 2 (the faster one) and captured those shots:

Gates on brick 1:


Gates and output on brick 1:


Gates, output and current on brick 1:


Rise and fall times on brick 1:



Gates on brick 2:


Gates and output on brick 2:


Gates, output and current on brick 2:


Rise and fall times on brick 2:



Could my low busvoltage cause any wrong measurements in terms of IGBT characteristics? I was told I should go in the 100V+ region (I'm doing test at 64V). And about the brick change: could it be possible that brick 1 was actually ok the whole time and the faster brick 2 was weird for some reason?

[davekni]

Could my low busvoltage cause any wrong measurements in terms of IGBT characteristics? I was told I should go in the 100V+ region (I'm doing test at 64V).

For delay testing as you are doing, current is often more important than voltage.  You could use a longer enable pulse to let current ramp up higher.  Removing or reducing any dummy load on primary coil (ie iron pan or whatever you may be using) also helps current ramp up higher.  Looks like you are already hitting 80A or so.  Not likely to be much different at 300A.

And about the brick change: could it be possible that brick 1 was actually ok the whole time and the fatser brick 2 was weirf for some reason?

Yes, a possibility.  However, that skm200gb128d spec lists typical Td(off) as 620ns at 125C die temperature.  You are much cooler than 125C for this low power testing, so delay should be a little lower, not over twice (1.4us as you show).  Makes me wonder if you have one genuine brick (or at least a faster counterfeit) and remaining bricks are slower counterfeits.

Either way, might as well use two matching bricks and continue with your build.  Will require large phase lead to compensate.  Worst outcome (probably) would be that the bricks fail at higher power.

May be better with larger gate resistors for these slow bricks.  There may be some cross-conduction.  IGBT turn-off is usually slower than turn-on, as is the case for spec speeds on this part.  The glitches in Vce scope traces just before each Vce transition may be due to some cross-conduction (also called shoot-through).

BTW, ignore my previous comment about excessive dead time.  That was based on your earlier differential H-bridge output plot and a now-false presumption that the pause was not due to mismatch of the two half-bridges.

[Felix M.]

I‘ll continue with the two brick I have build in right now and see how they do at higher voltage and power. They are going to be replaced with SKM400 bricks in some time the future. I could break one of the three slower bricks open but I’m not sure about this idea. They were bought from a private person who kept them cold and dry in storage for smth. like 2 years.

The max. current I ever scoped with the longest on time my interrupter can provide was 191A, OCD is set at 200.