Mosfet_STT7P2UH7_Switching Voltage_help voltage

Discussion in 'The Projects Forum' started by mishra87, Aug 17, 2016.

  1. mishra87

    Thread Starter Member

    Jan 17, 2016
    170
    2
    Could any body explain me the biasing voltage to switch ON/OFF the mosfet STT7P2UH7.
    output is taken from drain
    source is +ve rail - 4.2V- 3Vdc
    Gate controller signal - high/low_3V
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    According to this data sheet you should apply a Vgs of at least -1.5V to turn it ON and a Vgs of <0.4V to turn it OFF.
    If the MOSFET source is connected to a plus voltage of 3V to 4.2V then you need to add a NPN transistor buffer stage from the controller output to the MOSFET gate.
    Connections are:
    Controller to resistor to NPN base.
    NPN emitter to common.
    NPN collector to (second) resistor to +ve rail.
    MOSFET gate to NPN collector.

    The MOSFET will turn on when the controller output is 3V.

    How fast do you need to switch the MOSFET?
    That will determine the values of the resistors and the type of transistor.
     
    mishra87 likes this.
  3. mishra87

    Thread Starter Member

    Jan 17, 2016
    170
    2
    Actually i want to switch OFF the output of DC-DC converter i.e TPS61088EVM-677
    http://forum.allaboutcircuits.com/t...ble-shooting-help-needed.126609/#post-1030246

    The output of this DC DC CONVERTER is not completely cut OFF means we wont get 0V when i disable EN pin of IC..

    There out follow input with some losses. So i wanted to put a p channel Mosfet to completely disable the output of DC DC converter to avoid battery drainage.

    So if my source voltage operating range is 3.5-4.2Vdc and gate signal is from controller.
    how to implement this. !!
     
  4. mishra87

    Thread Starter Member

    Jan 17, 2016
    170
    2
    What could be the resistor value ???
    How to determine this ???
     
  5. crutschow

    Expert

    Mar 14, 2008
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    3,237
    The resistor values are determined by how fast you do to switch.
    If it's just for infrequent switching, then use anywhere from 10k-50kΩ for both resistors.
     
    Last edited: Aug 17, 2016
    mishra87 likes this.
  6. mishra87

    Thread Starter Member

    Jan 17, 2016
    170
    2
    I tried the attached schematic but it did not work.

    N channel MOSFET gate is pulled high/low and always connected to 3V and a switch is placed between Gate and 3V to change the state of NMOSFET Gate.

    Please anybody help me out to why it did not work.

    Which device I need to connect. Altimately I want to disconnect the 5V to load when it is not in use through controller.

    Could any body any alternate solution.

    Thanks in Advance !!!
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Your circuit doesn't work because the gate of the bottom (control) MOSFET is open when the switch is open.
    You can't leave the gate of a MOSFET open, as it will float to an undefined voltage (the gate looks like a capacitor).
    Connect a 10k-50kΩ resistor from the gate to ground.
     
    Last edited: Aug 17, 2016
    JWHassler likes this.
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