how does N-channel level shifter simplifies driving from logic

Discussion in 'General Electronics Chat' started by sharmilla, Feb 2, 2009.

  1. sharmilla

    Thread Starter Member

    Nov 3, 2008
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    hie all ,

    How does using an N-channel level shifter simplifies driving from logic. why can't just drive the p-channel mosfet straight away. please advise

    Many thanks
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    1) Can your logic take the voltage?
    2) Can your logic provide/sink the current needed to turn the gate on quickly?

    John
     
  3. sharmilla

    Thread Starter Member

    Nov 3, 2008
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    so u mean by using n-channel mosfet in this way, it simplifies the gate drive of p-channel for a high voltage...and also to provide enough current to flow to the gate of p-channel.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I am not sure what you mean by simplify. Running the gate directly form the logic chip is simple, but simple may not work.

    What I listed are some considerations for you as the designer. To get more detailed than that, we would need more information on your circuit. Particularly, what is the voltage provided to the P-mosfet source? I assume it is considerably higher than the logic supply.

    The main issue, however, is probably the current capability of the logic circuit. Gates of big mosfets can have instantaneous turn-on/off currents of amperes.

    John
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    In addition to potential problems with incompatible voltages, the o/p will be inverted if we run the P-MOSFET directly from the logic. P-channel devices conduct when the gate goes low.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    That depends on what you want do do, doesn't it?

    I am running a small P-mosfet (STS5PF20V) directly from a PIC MCU (200 ohm gate resistor for current limiting and a pull-up resistor for startup). It has a logic-level gate and stays off because its gate is held high by the PIC. Of course, I couldn't do that so simply, if the mosfet source was at 20 volts. John

    John
     
    Last edited: Feb 3, 2009
  7. Skeebopstop

    Active Member

    Jan 9, 2009
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    I feel you could do it directly with the N-FET, so long as the N-FET were below the load.
     
  8. vansterritory

    New Member

    Aug 5, 2011
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    Hello,

    at last i found a forum discussing about level shifter for driving high side MOSFET..
    I wanna use this scheme for my buck converter. but im quite confused with the value of each component. how can i calculate each R and zener?

    thanks!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have a look in here:
    http://www.ti.com/lit/ml/slup169/slup169.pdf
    P-channel MOSFET drivers are discussed starting on page 18, but the whole document is recommended reading.

    You would really be better off using an N-ch MOSFET and a high-side driver. The selection of P-ch MOSFETs is much more limited than for N-ch, and the main reason for doing so is that for all other parameters being equivalent, P-channel MOSFETs have 2.5 times the total gate charge requirement than N-ch MOSFETs do.

    [eta]
    You might find the attached interesting; I'm using a common 555 timer IC as a high-side driver for an N-ch MOSFET and current regulator for a couple of LEDs. It's about 85% efficient in the configuration shown, which isn't bad at all.
     
    Last edited: Sep 22, 2011
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