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MOSFET: before or after PSU ?

Discussion in 'Power Electronics' started by johnny_cash, Jun 20, 2014.

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  1. johnny_cash


    Aug 13, 2013

    I plan to drive the PSU for an SSD drive (about 400mA) from a port line (0 -> 5v)

    I would like to do it with an N MOSFET.

    My problem is i don't understand if i should put my MOSFET between +VCC and SSD psu or between SSD gnd and my gnd (thus my gnd will be over 0v than Vgsth...

    MCU is powered with VCC and SSD with VCC (5v), i fear MOSFET Vgsth :)

    Last, does this is suitable, instead of a MOSFET : ?

    Or do you see a better solution ?

    Thank you !
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    An NMOS Fet should be placed between SSD and GND. If you place it between Vcc and the SSD,. you need a specialized gate driver to create the necessary gate voltage >> Vcc.
    See also this thread.

    What you selected is not a MOSFET but an optically isolated switch using and LED on the primary side to drive MOSFETs on the secondary side. this is also known as solid-state relay. You can put this component also into the Vcc line of the SSD, which may (in this special case) be the better design since the ground reference for all signals is then fixed. Note that this component is rated for 1A continuous current - check against the current consumption of your SSD whether this is enough.

    Note also that you will need a current limiting resistor to drive the built-in LED from your microcontroller.
  3. johnny_cash


    Aug 13, 2013
    Hello Harald,

    In fact, i ran in some much trouble with MOSFET based-design that i used this SSR in place of (i drive it with 390ohms for 10mA current in the built-in LED of the SSR). The SSD is given to peak at 700mA so i choose an 1A device, with normal operations at 400mA

    Thank you for the reference thread, this is exactly what i'd like to understand better.

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