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When to consider MOSFET capacitance?

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magic:

--- Quote from: David Hess on July 13, 2019, 03:46:54 am ---In contrast to linear operation, a saturated switching application likely requires no protection at all because so little time is spent in the linear part of the transfer curve.  Then the gate series resistor is used only to limit the current from the gate driver or to slightly soften the hard switching of the transistor.

Even in switching applications, I have gotten into the habit of including a gate series resistor because it makes layout easier and makes for a convenient spot to measure the gate current and voltage if necessary.

--- End quote ---
Funnily enough, I once replaced a MOSFET which simply switched some device's voltage rail ON/OFF with a different type and it oscillated during the transition through linear region, driving the control circuit bonkers and turning the rail OFF again. There was no resistor directly at the gate and the PCB trace was long and meandrous. And the controller was open drain with pullup, which probably was why the MOSFET had enough time to develop oscillation.

T3sl4co1l:
Doesn't take much time to develop oscillations -- high voltage SuperJunction FETs will happily oscillate at ~300MHz, in a burst during the tens of nanoseconds spent switching, whether under 100kHz or over 2MHz. :)  Modern, lower voltage FETs offer comparable performance, too.

Tim

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