Fet as switch

Discussion in 'General Electronics Chat' started by achilez, May 31, 2010.

  1. achilez

    Thread Starter New Member

    Mar 23, 2010
    Forgive me for this ramble, but I just needed to air these thoughts, and get a confirmation that I'm not missing something (or better yet, get errors pointed out so I can correct them). I don't think I'm adding anything new here, but....

    When using a mosfet as a switch isn't it desirable to operate the device in its ohmic (linear) region, i.e., in the region where Vds < Vgs - Vth? I'm thinking the ideal point would be on the knee of the Id vs Vds curve -- at the "end" of the ohmic region or beginning of the saturation region.

    Or to put it another way; for a given Vds, to operate as efficient as possible (i.e. Pout/Pin is large) and thus, relatively, dissipate as little heat as possible in the mosfet, the gate voltage should be such that the drain current is just beginning to flatten out, somewhere around Vgs ~= Vds.

    Why does it get hot at Vgs = 4.5V? Because the Rds(on) is a function of Vgs and it drops exponentially with increasing Vgs, eventually converging towards the value usually given at the top of the datasheet. At 4.5V the channel resistance is still relatively high. My thoughts here is that it is actually the Rds(on) vs Vgs plot (missing from the linked datasheet in this thread?) that really sets the lower limit on the needed gate voltage. So to use the FET as a switch, the first (?) thing to look for in the datasheet is at what gate voltage does the device's Rds(on) reach it's final (lowest) value.

    Does this sound about right?
  2. SgtWookie


    Jul 17, 2007
    This is the plot I referred to in my last post.

    It should answer your question.

  3. beenthere

    Retired Moderator

    Apr 20, 2004
    This topic is different enough from the original - http://forum.allaboutcircuits.com/showthread.php?t=38967 - that it has its own thread now.

    If the device is to operate efficiently as possible, saving a bit of gate charge is not the way to accomplish it. The device should be driven into the fully conducting state as rapidly as may be. Otherwise, you waste power as heat in the FET because it is not actually switching - meaning to go from a state of non conduction to a state of full conduction. Partial conduction saves nothing.