Current equations for power mosfets

Discussion in 'General Electronics Chat' started by sp2821, Jan 10, 2014.

  1. sp2821

    Thread Starter New Member

    Jan 8, 2014

    I leanred that the MOSFETs have symmetrical structures and thus source and drain are interchangeable and have drain current equations
    @ saturation region
    @ linear region

    However, today I was told that power MOSFETs have different manufacturing process and have non-symmetrical shape which makes me to think that the source and drain are not inter-changeable.

    Then, if an nFET is used as a low side switch of a buck converter, does same linear region equation apply?

  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    YES the equation also hold. But we don't use this equation in real life. Why?
    Because we don't know W , L and Cox and un, and you don't find this information in data sheet. Instead of this MOSFET vendors give as Rds(on). And we use this Rds(on) in application when MOSFET work as a switch.
  3. crutschow


    Mar 14, 2008
    The main reason they are not interchangeable is because the transistor body in most power MOSFETs is connected to the source pin. Thus if you reverse the drain-source voltage, the drain body diode will be forward biased and the drain source connection will look like a forward biased diode instead of a transistor.

    One way around this, if you need to block voltages in both directions, is to use two MOSFETS in series with their sources (or their drains) tied back-to-back. That way one of the body diodes is always reversed biased.
  4. sp2821

    Thread Starter New Member

    Jan 8, 2014
    Thanks Jony130,

    I have a follow up question regarding your reply.
    So if the same drain current equation can apply even when the current is flowing in reverse direction, does that imply that the drain current equation is independent of source and drain's shape or asymmetry?

  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    The typical MOSFET isn't quite symmetrical - they usually have a body diode between S & D, with S & D correctly connected, this diode is usually reverse biased and for most practical purposes can be ignored.