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  1. #1
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    Power switch using nmos and pmos

    Hi everyone,

    My question seems to be posted frequently, but as far as I try, I can't find out my design error...
    The goal is to have a temperature driven circuit, using a LM35, a schmitt trigger and nmos+pmos, I would like to let the VCC go or block it.
    The simulation below simulates the LM35 using a sine wave, between 0 and 1V (0C and 100C) : it should change its state when temperature is above 70C, and get back to normal when temperature goes down 50C.
    nmos_pmos_power.png

    I am using LTSpice in order to help me validate the principles, and it works well, except the pmos usage.
    As you can see, the output of nmos is good (5V when temperature is above 70C, 0V when temp is less than 50C) but the output of pmos is not ok : it is always around 5V...

    What's wrong in my design ?

    Thanks a lot for your help !

    •   Alt15th April 2013, 16:07

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  2. #2
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    Re: Power switch using nmos and pmos

    For starters, your PMOS is upside down (rigged as a
    source follower but the polarity is backwards). In a
    real life setup you'd be conducting through the body
    diode no matter what, getting about 4.5V +/-. You
    also show no load resistance, so it would pull right
    up to the supply.



    •   Alt15th April 2013, 16:53

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  3. #3
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    Re: Power switch using nmos and pmos

    OK, I changed my PMOS, and know, added a load, and I always have 0V.
    I am quite sure this is obvious...nmos_pmos_power_2.png

    - - - Updated - - -

    OK, got it, I needed to specify a PMOS model.
    When using a IRF7210, and a 1ohm load, I get 5A, which is what I am looking for !
    nmos_pmos_power_3.png

    Is this circuit OK, or should I add some specific parts in order to have a good design ?
    I think about diodes, R, C, etc. ?



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