Possessed IGBT will not turn ON

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Swend

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Hi All!

I made a PCB for my power supply and assembled it, but now Q2 won't turn ON, and I swear it did when it was a ratsnest. This is the simple schematic:



I have tested Q1, and that one works fine, but Q2 just makes a little 1uS burst on pin2/q2 even though the gate pulse is 50uS. And if I connect pin3/u6 directly to the gate pin1/q2 it still only makes that 1uS burst on pin2/q2 regardless how long I keep the connection. (q1, q5 and q6 are off at the moment).

Either Q2 must be possessed or I'm missing something obvious, can you please help me out?


 

An obvious suggestion would be to very carefully re-check your circuit board/pcb for errors. And also for dry solder joints etc.
Though it's a great feeling to have a new build work right away, I am always suspicious of anything that works perfectly 1st time around.
 
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    asdf44

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You say "Pin2 Q2", but it's a differential probe. What's the reference node? Similarly, how are measuring Pin1 Q2?
 


Yes you are right, therefore I abandoned the PBC and started populating a new PCB from scratch making sure every component worked as I soldered. So now it will turn on, but I have no idea what went wrong in the first place.

- - - Updated - - -

FvM said:
You say "Pin2 Q2", but it's a differential probe. What's the reference node? Similarly, how are measuring Pin1 Q2?
Click to expand...

My scope is floating, so I'm just using one (and one only) regular probe as a differential probe. I have to make some differential probes when I get the time, so I can use more channels at the same time. The reference is the GND of the driver for GATE, and HV 0 (zero) for the other.
 

My scope is floating, so I'm just using one (and one only) regular probe as a differential probe.
Click to expand...
Let me rephrase the question, where do you connect the ground clip in both measurements?
 

Too many years ago, when I was into amateur robotics, I'd a home-brew motor-drive board which inspected & tested clean, yet would not work...

Much baffled searching later, I found a hairline fault across a broad track. This had appeared okay under wary examination, tests showed continuity, but it could not carry load, which promptly popped the tiny 'working' bridge...

Murphy's Law Rules !!!
 

FvM said:
If you are sure that the -600V is actually present, look for a PCB discontinuity.
Click to expand...

Yes I measured the -600V it's there. Actually I also had some noise problems with the power supply in my other open thread, could be related. Anyway I abandoned the PBC and started populating a new PCB from scratch making sure every component worked as I soldered. So now it will turn on, but I have no idea what went wrong in the first place, could be noise could be discontinuity as you say, or both - this build is a bit complicated for me with the mix of HV and LV.
 


I would urge you to double check the 'new' PCB routing as well to ensure it matches your ratsnest circuit. Stress on 'new pcb' since that is what you wrote in #1. There could be an error there as well.

All of us here waiting with baited breath to hear about the outcome. :smile:
 


"baited breath" - now I have performance anxiety.. In order to not disappoint anyone let me make it a two part cliff hanger, I have assembled the new PCB and all IGBT's can turn on/off, next step is to connect some HV on it and see what goes up in smoke....

You know, the day before yesterday was a humid day and my windows was open, nothing worked and noise flying everywhere, couldn't measure anything, this is really sensitive to humidity.
 

kripacharya said:
opcorn: it's going to work. Just connect it all up slowly, safely, securely, step by step.
Click to expand...

It seems your chanting helped ! Well not fully happy yet, but getting there.



The commutation is Q1-Q6-Q2-Q5 each one is 50uS. Oh and I didn't put in L1 and all the caps yet..
 

kripacharya said:
Well ? What happened? Did it work ??
Click to expand...

Yes of of course it did, summoning higher powers always helps although you don't always know why/how it helped. I have moved on now and trying to charge a capacitor with my power supply but find it very difficult, perhaps you could have a look at it? The thread is here **broken link removed**
 

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