Help understanding MOSFETs

Discussion in 'General Electronics Chat' started by whitehaired novice, Sep 14, 2018.

  1. whitehaired novice

    Thread Starter Member

    Jul 15, 2017
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    I am trying to understand MOSFETs. A tutorial I read seemed clear except it pretty implied that there was little or no current flowing through the gate—that it was essentially voltage driven.

    Yet when I read threads about MOSFETs I see “beefy gate driver” and “is there sufficient gate current?”

    Was the tutorial (not yours—yours has not been finished yet) oversimplifying?

    Related to this, though I don't know how, is the fact that the data sheets I have seen seem to be using a duty cycle of 0.5% !! That seems to me to be unusably low.

    Help is appreciated.
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    The gate looks like a capacitor to any driving circuit. For fast switching, you want to be able to charge and discharge that capacitance quickly.
     
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  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Take a look at this thread, it's got some interesting info in it.
     
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  4. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Check the Gate capacitance values for various MOSFET, some have a highish cap value, so the Gate driver needs the ability to charge the cap quickly in order to turn the MOSFET On/Off.
    E
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    Do you have a links for the tutorial?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    The missing factor that explains the discrepancy is the frequency of switching. As you've seen in the other answers, the gate is essentially a capacitive load. If your switching is slow, the (time average) current requirement is vanishingly low although you still want a good peak current capacity to achieve fast switching. A slow transition puts the MOSFET into linear mode and creates heat. You want it to be either on or off as quickly as possible and not linger in between. If you want to discharge and charge that gate capacitance at high frequency, you need the average current to approach the peak current. My personal rule of thumb is that any frequency below ~10-100kHz is 'slow' and anything over 1MHz is fast enough to require special attention.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Some of the tests with high current (such as on-resistance) are done at a low duty-cycle so the device doesn't heat up during the test (which will affect the results).

    Regarding the required gate current, suppose you want to switch a MOSFET in 100ns with a gate charge of 100nC (a value that a low on-resistance MOSFET gate capacitance might require).
    This requires a peak gate current of 100nC/100ns = 1A
     
    Last edited: Sep 16, 2018 at 2:29 PM
  8. whitehaired novice

    Thread Starter Member

    Jul 15, 2017
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    My thanks to all of you--you certainly improved on the info in the tutorial. (Scott, I just googled and found one. Thanks to all of you I don't need to re-visit it. It wasn't wrong, just oversimplified.)
     
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