Can not understand why current is flowing from Source to Drain in Synchronous buck converter

Discussion in 'General Electronics Chat' started by nikee, Dec 15, 2014.

  1. nikee

    Thread Starter New Member

    Jan 17, 2014
    15
    1
    To my understanding the mosfet used in synchronous DC-DC converter is n-Channel and source is connected to ground but while conducting the current path is such that current flows from source to drain. To my understanding for a n-channel mosfet current should flow from drain to source and not from source to drain. I think I am just missing something very obvious but can't find it any where. Initially I thought that they are relying on body-diode to conduct but that's not the case, the losses are being calculated on RdsOn value. I may be sounding silly but I am just missing something very crucial. Any help will be highly appreciated.

    Regards
     
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  2. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    If you are talking about normal positive to negative current flow and not electron flow, then yes, the normal current flow is from drain to source for a N-MOSFET. But a MOSFET is bi-directional so, if Vgs is positive such that the MOSFET is biased ON, then current will also flow in the opposite direction, the same direction as the body diode (meaning the drain and source have now interchanged places).
    The only thing that determines which we call the source and which the drain is the direction of the body diode. The one called the drain node is the one that normally reverse biases this diode.
     
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  3. nikee

    Thread Starter New Member

    Jan 17, 2014
    15
    1
    Oh yes, I should have used the search key "bidirectional" but then I always thought MOSFET are bi-directional because of the body diode, It's been said that 'a little knowledge is a dangerous thing'...lmao. So as long as my Gate is at higher voltage than source (Vg > Vs+Vth) the current will flow, either from drain to source (if Vd>Vs) or from source to drain (if Vs>Vd). Really thanks for your quick and very simple explanation.
    Regards
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Yes. But for the MOSFET it be fully ON as a switch the Vgs must be significantly higher than Vth, typically Vgs = 10V for a standard MOSFET and 3-5V for a logic-level type, as shown in its data sheet under Von resistance.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The MOSFET has a parasitic "body diode" that in most usual situations is reverse biased and plays no part.

    The body diode is sometimes aligned in the forward direction in polarity protection circuits where Vf of a regular diode would be too high - the diode itself in this application still isn't used, even though the MOSFET channel is the wrong way round it will still conduct with the correct gate bias relative to the channel, since the channel is biased to conduct; its RDSon has a lower drop than the body diode Vf.
     
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