voltage measurement across IGBT

Discussion in 'General Electronics Chat' started by khalid4145, Oct 28, 2008.

  1. khalid4145

    Thread Starter Member

    Jul 22, 2008
    16
    0
    I want to measure the voltage across the IGBT to moniter the voltage if there is short circuit or not and then feed it back to microcontroller? what is the best way of doing that. Do i have to use a differential amplifier, isolation amplifier or voltage transducer. i am attaching the circuit. if you have any circuit to measure the voltage please try to post. the maximum voltage when the device is on is equal to Vce (sat) is 1.8V according to the data sheet. the maximum voltage when device is off 120V.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    In an H bridge like that, the short circuit current should be enough to alert you that one of the FET's has failed short. You could simply sense the increased current and open a relay to isolate the H bridge more easily than devising a scheme to isolate each FET and check for failure individually. Even a 10 amp fuse would work.
     
  3. khalid4145

    Thread Starter Member

    Jul 22, 2008
    16
    0
    you are correct but in my project i have to sense the voltage across the IGBT and feed it back to uC which in turn will activate a relay to isolate the faulty IGBT.

    Actually my idea is to intrupt a fault in the circuit by giving the IGBT driver a dc signal of 5V from microcontroller (it is not a real fault), when it is on all the time , the voltage between the collector and emitter should be the saturation voltage Vce (sat) which is 1.8V. so if the microcontroller sees a continous of 1.8V then a short circuit occurs and start to activite a relay to disconnect the IGBt from the circuit. Does opto isolater do the work. if so, would you please provide me with data sheet suitable to my application?? do i have to add some circuits between the opto isolater and the uC? Any body have another ideas??
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You could place a resistor and the emitter of an optoisolator across the IGBT so the optoisolator got 30 or so mills current with the IGBT off. Monitor the output to see if there is no signal when the IGBT should be off (no signal meaning the device is shorted).
     
  5. Bailey45

    Member

    Oct 27, 2008
    18
    0
    You should use a Optoisolator with a lower input current requirement. (Fairchild
    H11B1) At 30mA you would require approx 4000 ohms in series to limit current and a 5-10 watt component to dissapate power. With 1 mA the value would be in the 100K range @ .25 - .5 watt. This would also reduce the leakage current to your load caused by your sensing circuit.
     
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