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I am trying to build a MOSFET switch circuit. The MOSFET I am looking to use is FDN327N.

For MOSFET switches, I would add a resistor to limit the initial current rushing into the gate of MOSFET to protect whatever driving it. But for the FDN327N, gate charge is only 4.5nC and I am wondering if the gate resistor makes sense (in my mind it will only slow the switching and does nothing).

Is there any harm for switching the FET that has such a low gate charge directly from the microcontroller?

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    \$\begingroup\$ It really shouldn't be a big deal to use a 100 ohm series gate resistor. \$\endgroup\$
    – Andy aka
    Commented Oct 13, 2020 at 9:36
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    \$\begingroup\$ It depends what the load is and how fast you will be switching it. What is the load, how much current it needs, is it capacitive, inductive or resistive? What is the MCU GPIO output high voltage at the gate, is it 3.3V ? Do you plan to simply turn it on and off every now and then, or use PWM at tens of kHz to drive it? \$\endgroup\$
    – Justme
    Commented Oct 13, 2020 at 9:50
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    \$\begingroup\$ Just switching on and off say every second to give a pulse signal (duration around 2ms) to the load. The load is capacitive and 5V from the micro \$\endgroup\$
    – Moon
    Commented Oct 13, 2020 at 9:59
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    \$\begingroup\$ The safe option is to add a 100 ohm resistor. Why are you thinking/doubting so much about a 100 ohm resistor that costs 2 cents? Just do what most others do and that is to add a gate resistor. The exact reasons why/why not a gate resistor is a good thing aren't relevant when you're a beginner (and worry about everything) so just do what the experienced people do. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 13, 2020 at 10:15
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    \$\begingroup\$ But for FDN327N gate charge is only 4.5nC Geez, 4.5 nC, that's a lot! I work with MOSFETs that only need 1/10th of that. Size is always relative so never say that a charge "is only" 4.5nC, you cannot draw conclusions like that. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 13, 2020 at 10:18

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There are several reasons for using a gate resistor, like:

  • Damping ringing between wire inductance and gate capacitance;
  • Slowing down the switching transition and preventing spikes (will also help with EMC);
  • Reducing the gate drive current spike to a level the microcontroller can safely handle.

If any of these apply for your application, you should use a gate resistor.

Also, if your application isn't critical about switching speed, there are no good reasons not to include a gate resistor (other than component count); I would still add a small one (say 100 Ω, as Andy suggested) to prevent or reduce possible problems; it is the safe way of doing things.

At the low switching frequency you mention it will do no harm other than slowing the switching action a bit and very slightly increasing power dissipation in the MOSFET for the very short period during the switching transition.

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    \$\begingroup\$ EMC is kind of universal for all applications, yeah? \$\endgroup\$
    – Lundin
    Commented Oct 13, 2020 at 11:04
  • \$\begingroup\$ True, but caring about it for hobby projects is rather less universal. \$\endgroup\$
    – ocrdu
    Commented Oct 13, 2020 at 11:06

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