Vgs-th & Miller plateau

Discussion in 'General Electronics Chat' started by ankurtech, Feb 21, 2012.

  1. ankurtech

    Thread Starter New Member

    Jan 26, 2012
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    In all mosfets vgs-th is much smaller than miller plateau as in graph of gate charge. ? But I think for mosfet to conduct we have to apply voltage greater than miller plateau. Isn'i it. If not, what miller plateau signify.

    I have a mosfet whose Vgsth is 1V & miller plateau at 2V (suppose values only), now if I apply 1.5V Vgs will it allow to flow current or I have to cross miller plateau
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Applying 1.5V Vgs will indeed cause drain current to start flowing. But you don't reach the miller plateau unit Vds of the transistor is changing.

    The miller plateau occurs during switching when Vds starts to drop due to the load resistance as the drain current increases. This voltage decrease causes the charge current into the gate-drain capacitance to increase and doesn't stop until Vds reaches its minimum voltage and the transistor is fully turned on.

    Thus the miller plateau signifies the start of an increase in gate current that occurs when the transistor drain voltage starts to drop during the switch turn-on. It derives its name from the Miller Effect which is the increase in apparent input-output capacitance of an amplifier stage due to the amplified signal at the output when it is 180 degrees out-of-phase with the input.
     
  3. ankurtech

    Thread Starter New Member

    Jan 26, 2012
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    So that means miller plateau has nothing to do with Vgs. Even at Vgs-th fet starts to conduct but it will start when vds falls during miller plateau.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,988
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    The are related to the extend that the miller plateau is always above Vgs-th but the difference between them depends upon the transistor and the source voltage and load. Vgs-th is a static voltage that doesn't change with operating conditions while the miller plateau is affected by those conditions.
     
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