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Need help regarding IGBT failure

Discussion in 'General Electronics Discussion' started by Yamo Amekama, Mar 14, 2011.

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  1. Yamo Amekama

    Yamo Amekama

    3
    0
    Mar 14, 2011
    Hello,

    I am currently working on a project that involves controlled switching of a 400v 1000uF capacitor into a low resistance high inductance load (1.5R 1400 uH). The capacitor must be discharged into the load and then abruptly halted mid pulse. So I decided to use an IGBT due to its fast on/off state switching and high current/voltage capability.

    My setup is very simple in that the IGBT has 7-10 Volts put on its gate via voltage divider and 14VDC power supply and then abruptly removed via an optical sensor. The IGBT is connected in series between the capacitor and the load.

    Unfortunately, every time I reach about 200-250V on my capacitor and switch the IGBT on they break (C E shorted). I don't understand why because the maximum current my system can draw (400v on capacitor) is under 200 amps in under 10 milliseconds. I tried using a 20N60A4 IGBT which according to the datasheet can withstand a 280A surge in current. I have also tried a STGW35NB60 which according to its datasheet can withstand a 250 amp surge.

    The IGBT's work properly below 200 V with this particular load, but it seems that at 200 Volts or higher the IGBT simply cannot handle the pulse. I tried using two of the 20N60A4's in parallel and this helped me surge with a voltage of around 400V but still failed after a few trials. SCR's work very well in my application as they seem virtually indestructible to current pulses but I cannot use them due to their inability to be turned off mid-pulse.

    Does anyone have any information as to why my IGBT's keep failing or any possible alternative semiconductors in my setup?

    -THANKS
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Coilgun, eh? ;) What about the spike you get from the inductor at turn-off? Unchecked that would easily exceed any transistor voltage rating I know about.
     
  3. Yamo Amekama

    Yamo Amekama

    3
    0
    Mar 14, 2011
    Yep how'd you know! :p

    That is something that I haven't thought about before and it could be the problem.

    The stipulation is that I have run tests where I used a momentary switch to turn the IGBT on (not trying to use cutoff) and the IGBT STILL shorted. In this case the inductor had plenty of time to dissipate its magnetic field before the IGBT turned off. It could be possible that I released the switch to quickly but unlikely since the system current goes to zero in under 10 milliseconds.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Because you're discharging a capacitor into an inductor, the capacitor will discharge, then the energy stored in the inductor will charge the capacitor again -- in the reverse direction. Once the energy in the magnetic field has dissipated and the current falls to zero, the capacitor will again discharge into the inductor. This will continue forever in a series of diminishing oscillations until their magnitude is lost in the thermal noise.

    In your case (and indeed in many practical cases) there is a circuit element (your IGBT) that is sitting between the capacitor and the inductor.

    If your IGBT was one that does not have an inbuilt diode, then the reverse current could not pass through. This would cause the voltage across the IGBT to rise to a very high level until (most likely) it was forced into conduction in a very unhealthy mode.

    It appears to me from the datasheet that this device does not have an inbuilt diode. If that is true then it is reverse voltage that is almost certainly killing the device.

    Either place a reverse biased diode across the inductor, or across the IGBT (or indeed, both). The diode needs to be rated for the full operating voltage and current of the coil.
     
  5. Yamo Amekama

    Yamo Amekama

    3
    0
    Mar 14, 2011
    @Steve

    Yes that is definitely something I considered. I placed an anti parallel diode across the inductor and even went so far as to place a high power diode in series with the IGBT and inductor to prevent reverse current/voltage from hitting the IGBT. This did help to extend the amount of voltage/current I was able to dump into the inductor but unfortunately the IGBT still ended up breaking.

    @Resqueline
    After running a simulation I found that when cutting off the IGBT when the coil has near full current (81 amperes) there is in fact a voltage spike of 135,000 volts over the course of 2 microseconds. This would definitely seem high enough to destroy my IGBT. It seems that this particular setup is futile due to these problems.

    At this point i'm considering scrapping this setup unless someone has an alternate idea.

    Thanks for the input guys!
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Please note where I said to place the diodes. Your placement does not help (as you have found)
     
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