Turn on/off behavior of MOSFET

Discussion in 'General Electronics Chat' started by sp2821, May 28, 2014.

  1. sp2821

    Thread Starter New Member

    Jan 8, 2014
    Hello all,

    I am trying to find a switching power loss on MOSFETs

    This is what I tried initially.
    When Vgs is less than Vth, the drain current is 0 and Vds=VDD. No power is lost.
    When Vgs crosses the threshold, the FET starts to conduct a current and Vds starts to drop simultaneously. Power loss is a product of Id and Vds.
    When the FET enters a triode or linear region, Vds=0 (assuming an ideal FET) and power loss is=0

    I tried a simulation on LTspice with a random FET and it showed the same result.

    But I found Vishay's app note that shows different turn-on waveform in Fig. 4
    Their waveform shows Vds doesn't drop until the drain current reaches its final value.

    How is it possible for Vds to stay at VDD when Id≠0?
    What is holding Vds to VDD? and is my circuit different from Vishay's?

    Thank you
  2. nigelwright7557

    AAC Fanatic!

    May 10, 2008
    The mosfet has turn on and turn off times.
    Also the gate can take a while to charge/discharge.
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    I think they have a bad picture.
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    sp2821 likes this.