Y my mosfets are blowing up????

Discussion in 'The Projects Forum' started by onlyvinod56, Jun 15, 2009.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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    Hello,

    i have posted few threads earlier regarding my project.
    Its an AC ~ AC converter. I need a variable frequency 3phase voltage.
    Iam expecting 10 - 100Hz range.

    For this i took HEF4752.
    I found only two articles based on this IC and the datasheet is also not satisfactory.
    Any ways. i have designed the circuit.
    I observed the six pulse trains in three dual trace oscilloscope at a time.
    Im veryu much satisfied with this. see the schematic attached.

    But the problem comes here. Whenever i connected the pulses to the 6 mosfet bridge, all the mosfets are blowing up......:confused:

    First i used STW7NA100 mosfets..... later i tried with IRFZ44....

    What should i do......

    I have one doubt. What is the imortance of a mosfet driver?
    Is it that much imortant? I have used six 4N35 optocouplers. Is it ok?
    Help me plaese
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Where is the circuit?
     
  3. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Onlyvinod56,
    Your circuit did not attach to your post.
    If possible, please use the .png format for your images; it's compact and not a "lossy" format like .jpg is.

    Just by your reported symptoms, it sounds like you may have both the high side and low side MOSFETs on simultaneously, which effectively shorts out the supply (a condition known as "shoot-through").

    MOSFET drivers are important. They are designed to provide high current to/from the gates of the MOSFETs to switch them on and off quickly. Some drivers incorporate "dead time" to help prevent the "shoot-through" condition.

    4N35 optocouplers by themselves would not be able to source or sink much current; they would be woefully inadequate as large power MOSFET drivers. The MOSFETs would spend most of their time in the linear region, generating a great deal of heat, and burning up.

    You really need to post your circuit.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The link you posted doesn't contain a schematic, only block diagrams and some pin-outs of ICs. Unfortunately, the Chinese didn't translate very well to English either.
     
  6. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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  7. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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    ok here iam attaching the image.
     
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  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the attached schematic is nothing about the mosfets.
    How are the mosfets connected?

    Greetings,
    Bertus
     
  9. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    What is that table 2.
    Im not getting it.

    It is obvious that the two mosfets in a same leg can never be conducted simultaneously... right

    today i checked with a trial circuit. see the attachment.
    I have faced the same problem with this circuit too.
     
  10. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    see the pdf file attached for mosfets connection and also the image
     
  11. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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    Can you suggest me a good driver
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    In the PDF there are no mosfets used.
    There are highvoltage transistors used.

    [​IMG]

    Greetings,
    Bertus
     
  13. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1

    ok. But from that pic, you can get the idea of pulse sequence pins like upper leg & lower leg of RYB phases.

    ORM1 = firing pulse for the top mosfet of one leg
    ORM2 = firing pulse for the bottom mosfet of the same leg
     
  14. franzschluter

    Active Member

    Jun 1, 2009
    95
    0
    You need a capacitor to charge your gate dude. MOSFET is voltage controlled device. And the ON voltage required to turn it on must be much greater than Drain to Source typically it is computed as "Voltage output of your optocoupler+Voltage on Drain and Source. This is why a capacitor is needed to switch on in the first place. If it is only half on then it will also start smelling dude.

    Cheers
    Franz
     
  15. bertus

    Administrator

    Apr 5, 2008
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  16. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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    but how did they get result with the above one.....
    there z no driver for that circuit..right.
    whats dat 47 ohm & 0.1 micro farad ..... is it a snubber?
     
  17. franzschluter

    Active Member

    Jun 1, 2009
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    That looks like a protection circuit. Diode can be used to protect from high load when emf goes off... This circuit is double protected with Cap and Resistor in series. But if you intend to use MOSFET on that circuit it will not work. You need a charge pump capacitor with a MOSFET driver so you have VCC+Vload in your gate to switch on since gate drive is high impedance. However for TIP or Power Transistors this circuit can be used. You might have to adjust a couple of resistor to match your transistor.
     
  18. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    i have seen a mosfet driver datasheet now.
    Im confused about the rating of it.

    3A Dual High Speed Power Mosfet Driver TC4423/24/25
    http://ww1.microchip.com/downloads/en/devicedoc/21421d.pdf

    What is that 3A?

    In my project, i have to connect a 3 phase slip ring induction motor whose current rating is 4.9A.

    Is there anything to worry about these 3A driver with 4.9 load?
     
  19. franzschluter

    Active Member

    Jun 1, 2009
    95
    0
    Ah.. It just means the output of your driver can handle 3A. This is not in anyway related to the motor itself.

    You driver -> Connect to MOSFET -> Connect to motor

    That's the flow. Your MOSFET should be a power mosfet that can handle at least 6A Ids (drain to source current ok).

    Also please design a circuit with charge pump capacitor. It will not work without it. This capacitor is computed and must be considered. The capacitor will help turn on the mosfet and without it.. It will not turn on or do it only 80% on which will make your MOSFET burn. This capacitor is also in conjunction to Rb (The resistor that is in between output and mosfet gate). Depending on Rb and Capacitor you can make it switch properly or not. Just do some googling and you will find some typical equations to use.

    Look at typical application on how to use for reference..
    http://www.irf.com/product-info/datasheets/data/ir2117.pdf

    You can also use a regular transistor to turn on your mosfet. Your regular transistor must have same supply as supply from MOSFET. But the load will be carried by the MOSFET not by the transistor.

    See application...
    http://www.neufeld.newton.ks.us/files/electronics/mosfet-motor/MOSFET-motor-driver.png

    Cheers Franz
     
  20. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Ah.. It just means the output of your driver can handle 3A. This is not in anyway related to the motor itself.

    That's the flow. Your MOSFET should be a power mosfet that can handle at least 6A Ids (drain to source current ok).
    Also please design a circuit with charge pump capacitor. It will not work without it.


    You can also use a regular transistor to turn on your mosfet. Your regular transistor must have same supply as supply from MOSFET. But the load will be carried by the MOSFET not by the transistor.

     
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