working of P channel MOSFET in series

Discussion in 'The Projects Forum' started by jom, Jan 16, 2013.

  1. jom

    Thread Starter New Member

    Jan 16, 2013
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    can i connect a p channel mosfet in series with both the source and gate tied?

    how does the biasing of the circuit happen ? the circuit is shown in the attachment.


    Thank you.
     
  2. jom

    Thread Starter New Member

    Jan 16, 2013
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    actually i had to disconnect the load when the battery goes low which is monitored by a comparator which sends a high signal to the CTL pin of 4412, but when this happens the mosfet F2 diode conducted so to overcome that i used F1 in series,

    can i use it like this ?? or can i just change the soucre and drain of F2 ??i have attached the schematic.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,042
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    You only need two MOSFETS in series if the load is being powered from an alternate source and you need to block current flow in both directions. (MOSFETS conduct in either direction when ON so that is why connecting them in inverse series works). But if you only have one source (battery) then you just need F1.
     
  4. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Something isn't right about that second schematic, but I can't quite figure it out. What are you trying to do? Nothing is connected to Vin and there's a floating resistor and floating control pin.

    http://cds.linear.com/docs/Datasheet/4412fa.pdf
    Im guessing you've already looked at this, but in case you haven't, I've provided a link to the datasheet.
     
  5. jom

    Thread Starter New Member

    Jan 16, 2013
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    the vin is connected to the battery and the resistor is connected to a 3.3v supply and control pin is connected to the out of comparator which i have not shown in that ..
     
  6. jom

    Thread Starter New Member

    Jan 16, 2013
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    i do have an auxilliary source when that is connected only that provides for the load,that part works fine.i had to cut the battery when its below certain voltage so i did connect one more in series, but what about the drain voltage with respect to the source for those mosfet s to be OFF ?

    Thanks
     
  7. crutschow

    Expert

    Mar 14, 2008
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    To turn both P-MOSFETS off you connect a resistor (say 10kΩ) from the common source connection to the common gate connection. You then leave the combined gate connection open to turn them OFF (the resistor will keep the gate-source voltage at 0V for the forward biased transistor). To turn them ON you tie the combined gate connection to ground.
     
  8. jom

    Thread Starter New Member

    Jan 16, 2013
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    will the mosfet above ie the mosfet F1 will it turn ON when the lower mosfet is ON ??? the gate is controlled by the LTC4412 chip .
     
  9. crutschow

    Expert

    Mar 14, 2008
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    If the output to the MOSFET gates is high or floating (with the gate-source resistor) then both transistors will be off. When the gates are connected to 0V (ground) then they will both be ON.
     
  10. jom

    Thread Starter New Member

    Jan 16, 2013
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    i did try using the online schematic editor and simulation using circuitlab.com ,when the battery supplied 9v and the gate s wer at 7v then only one mosfet on the left turned ON , the simulation result is shown on the left side of the attachment, the OUT node is zero even after the first mosfet is ON .In my design i use a p channel mosfet FDN306P, the symbol used in that editor is for P channel only.

    Thanks
     
  11. jom

    Thread Starter New Member

    Jan 16, 2013
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    as i lowered the gate voltage it conducted well ... :) here is what i did .
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Of course the OUT node voltage is zero. You are measuring the ground node so it would be zero independent of whether the transistors are on.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    3,244
    Why such a small (high resistance) load?
     
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