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IGBT max ratings question

  1. Dec 22, 2016 #1
    I’ve been wondering about the capabilities of IGBT and Mosfets in general , I recently had a few problems with an smps and here is what I wonder , for example the IGBT used in the supply has an absolute maximum rating of 600v and 120 amps at room temp, I can’t recall the exact IGBT in question but I have seen this with other ones too so let me just put for example FGH60N60SF or FGY75N60SMD or other devices , now I have seen they blow whne there is a sudden very large load on the secondary of the smps transformer and in other cases , but let’s take the secondary shorted case as our example.

    My mains voltage is 230v and the socket delivers about 16-20 amps max then the voltage would probably start to sag. So simple maths gives us about 325v DC rectified. It’s a half bridge topology , prior to the switching transistors there are 4x1000uF capacitor bank , now if there manages to be a short at the secondary side the transistors blow up or short out.

    My question is why is this so since their ratings are so high , 600v max voltage and 100 to 150 amps of collector current , in such case I was hoping for my 5amp mains fuses to simply blow out and the whole thing would settle easily since the secondary current translates back to the primary side but instead the transistor blow.

    Could it really be that the 4x1000uF cap bank stores enough energy to be capable of giving more than 100 amps of current for a long enough moment to exceed the quite large rating of the transistors , or is this because the rather small to 247 and other devices simply get too hot too fast under such load and fail primarily due to heat?
     
  2. jcsd
  3. Dec 22, 2016 #2

    Hesch

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    Are you sure that the VGE voltage is a completely clean square wave witout any noise and ≥ 12Vdc ( on ) ?

    ( even when the transformer is short circuited ).
     
  4. Dec 22, 2016 #3

    jim hardy

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    yes to both questions.

    Datasheets are your friend.

    https://www.fairchildsemi.com/datasheets/FG/FGH60N60SF.pdf


    FGH60N60SF _datasheet.jpg


    If 4000 microfarads are delivering 180 amps, what is the rate of voltage decrease ? dv/dt = I / C
    Now 180 amps / 4000 microfarads = 45 volts per millisecond ,
    so if starting from 300 volts your capacitors could deliver a 180 amp pulse that persists for several milliseconds.
    They might well deliver more than 180 amps, the max for that device.

    Now to Hesch's question,
    FGH60N60SF _datasheet2.jpg
    Vce goes way up and that drives power dissipated in the device sky high

    Now to your second question, heating of that package: (see datasheet clip above)
    At 40 degC per watt junction to ambient and maybe ambient of 30C, with no heatsink your device can only handle three watts continuous and still hold junction below 150C.
    If you put it on a heatsink that can hold the case to 30C, it should handle 120/.33 = 363 watts. Do you see how they got 378 with case at 25?

    Are yours on big heatsinks?

    old jim
     
  5. Dec 31, 2016 #4
    Also - standard fuses are not fast enough to protect semiconductors - so you can definatly send way too big of a current pulse with this set up. The IGBT is the "faster" fuse. Also - how is the IGBT being driven? If the driver circuit also see the sag, it may drop the gate voltage, increasing the Vf in the IGBT - sending the losses through the roof. Does the the driver have DeSat and some form of Over-current protection?
     
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