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High speed Mosfet

Discussion in 'General Electronics Discussion' started by Little Nickey, May 3, 2012.

  1. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    Hi there

    i need a bit of help I've got a circuit board that has a 34V 50A supply and a 5v2 3A supply.
    across the 34V supply there is a large capacitor to eliminate back EMF from the motors that it drives.

    The problem i have is if there is a problem on the 5v2 supply and for some reason it goes below 4.5V it blows the over voltage Mosfet that is on the board.

    Is there a way that i can build a Mosfet switch that will disconnect the 34V supply if the 5v2 supply goes below 4.8V, i have been told that i would need a very fast circuit to do this.

    any help would greatly be appreciated.

    thanks in advance.
     
  2. Wabajig

    Wabajig

    75
    0
    Apr 14, 2012
    I think your possibly blowing the mosfet because you are drawing to much current. So I suggest if you need a 5V supply with a lot current, use a computer power supply. Depending on the wattage of the computer power supply, you can draw upwards of 20A or so. Read the power supply specs usually written on outside of unit. Note: Share a common ground with +34V supply and if you find you are drawing alot (feel if the wire from computer power supply is getting warm) of current use more than one 5V wire soldered together. Or if you feel ambitious you can go inside and wire it with a single 12 gauge multistrand wire from each source (12V,5V,3.3V). That would make nice an neat. Also don't cut off the 20 or 24 pin plug from power supply, you will need this to turn on the power supply. If not sure how to trigger the power supply on, post a picture of your supply cable, and I'm sure someone will help you. Hope this helps.
     
  3. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Before looking to kludge to correct the problem caused by a drop in the 5V2 supply, shouldn't you try to find out why it is dropping out?

    Bob
     
  4. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    Hi Wabajig.

    unfortunately i cannot use a computer power supply.
    the unit i am working on is battery powered and on the connector it has its own 5v2 supply and 34V supply.
    the reason why the Mosfet is blowing is because the circuitry behind the over voltage Mosfet is powered by the 5v2 supply and when that voltage goes to low it turns on the mosfet which puts a short circuit across the capacitor which then in turn blows the mosfet.

    ive quickly drawn a layout of the circuit in paint to try and help with what im trying to do.

    the problem is not that there is to much current being drawn from the 5V2 its that if for some reason the 5v2 turns off it leaves the Mosfet drive pin floating and this causes it to turn on.
     

    Attached Files:

  5. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    Hi bobK.

    it looks like the 5v2 line has a sense line aswell to detect any faults, so if the battery detects any faults it will turn off the low voltage, but it doesn't turn off the 34V supply.
    so im pretty much just trying to turn off the 34V supply when the 5V2 gets turned off, but preferable just before this happens.
    there is some capacitance on the 5v2 supply so it takes a little while before it gets to low.
     
  6. Wabajig

    Wabajig

    75
    0
    Apr 14, 2012
    I'm an electrical engineer as well as technician, and it would be nice to see your schematic to see exactly how it's hooked up. Analyzing the circuit might give us the clue as to your transistor going bad. Can you post a hand drawn schematic or maybe the model # of unit or a link to a schematic. I'll be glad to help.
     
  7. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    A simple pull down or pullup resistor on the MOSFET gate (depending on which would turn it off) might do the trick if the problem is that the gate floats when the 5V2 turns off.

    Bob
     
  8. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    Here is a schematic of the OVP part of the board.
     

    Attached Files:

  9. Wabajig

    Wabajig

    75
    0
    Apr 14, 2012
    Is it Q3 that is shorting?
     
  10. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    Yea is it, sorry should have mentioned that.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    I would recommend that you provide an alternate 1.25V reference.

    When the 5V rail dies, Q3 gets turned hard on.

    I would suggest that either you generate a 1.25V reference from the 30V rail, or provide a path from your 30V rail to the input of the regulator (it will need to be reduced to about 5V and you'll need to decouple it -- the latter applies to both methods).

    What is the source of your 30V? Q3 looks like a shunt regulator. Is it a solar panel?
     
  12. Little Nickey

    Little Nickey

    6
    0
    May 3, 2012
    The source of the 30V is from a LiPo battery pack which also provides the 5V line through the same connector. ill have a look at both option's and see what i can come up with. thanks for the help.
     
  13. Wabajig

    Wabajig

    75
    0
    Apr 14, 2012
    What Steve said is fine, but you still need to address the 5V supply dropping so hard. Good Luck
     
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