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I'm having difficulties in understanding this statement from "The Art of Electronics 3rd Edition".
I don't understend how they estimated the MOSFET's switching time using the gate current and the feedback capacitance.

Here is the quote:

As another example, imagine switching a 5 amp highvoltage load with a power MOSFET (there aren’t any highpower JFETs), in the style of Figure 3.50. One might naively assume that the gate could be driven from a digital logic output with low current-sourcing capability, for example the so-called 4000-series CMOS logic, which can supply an output current of the order of 1mA with a swing from ground to +10V. In fact, such a circuit would be a disaster, because 1mA of gate drive into the 200 pF average feedback capacitance of the IRF740 would stretch the output switching speed to a leisurely 50μs.

Figure 3.50:
Figure 3.50
Thanks.

povo
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    I'm not sure why the term "feedback capacitance" was used, rather than "gate capacitance", but whatever. The point is that it takes a significant _charge_ to get the gate to it's turn on voltage. Charge is current x time. So the lower the current, the longer it will take. – Math Keeps Me Busy Jan 18 '21 at 23:21
  • The feedback capacitance is very important because it determines most of the switching time and switching losses. Check this question: https://electronics.stackexchange.com/questions/74867/relationship-between-the-gate-charge-graph-and-the-vgs-waveform – bobflux Jan 18 '21 at 23:41

1 Answers1

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Hmmm. My math doesn't agree with AoE. Assume that the MOSFET gate is a lumped, constant capacitance of 200pF. For a capacitor,

\$i = C\dfrac{dV}{dt}\$

so

\$\dfrac{dV}{dt} = \dfrac{i}{C}\$

If the current is 1mA then \$\dfrac{dV}{dt}\$ is \$5 \text{V}/\mu\text{s}\$, so it would take \$2 \mu\text{s}\$ to switch the gate voltage by 10V.

Maybe I am missing some context?

Elliot Alderson
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  • It's just that you need the charge for changing feedback capacitance voltage by approx 50V, see https://electronics.stackexchange.com/questions/401774/mosfet-miller-effect-length-of-the-gate-voltage-flat-area/402468#402468 for details – carloc Jan 18 '21 at 23:59
  • https://en.wikipedia.org/wiki/Miller_effect – Bruce Abbott Jan 19 '21 at 03:46