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I need to drive 2 logic level MOSFETs from digital outputs of a microcontroller (3.3V). These will be used to drive a low-power push-pull converter at 500 kHz. I have identified these "double" MOSFETs:

I have a doubt about the gate resistor.

  • In ROHM and Infineon datasheets, many characteristics are given for a certain Rgs (usually in the range of 10 Ω) and I guess that is a reasonable value to be used for the gate resistor in my final design, in order to avoid ringing and to keep good switching performances.
  • Vishay datasheet indicates an explicit value of Rg (test condition: 1MHz ????), and it sounds like a resistor embedded into the component. I'm quite confused with this specification.

Is there anyone who has experience with this kind of logic level MOSFETs and can tell me if an external gate resistor is actually needed and if Rg ≈ 10 Ω is a reasonable value?

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    \$\begingroup\$ @MissMulan The impact on the switching speed when using such a low value resistor of for example 10 Ohms, is negligible. Such a gate resistor is used to improve the transient response and it can actually improve the switching behavior of the MOSFET. If you make the resistor value too high, then yes it will impact the switching speed. But if you do that, you're missing the point why the gate resistor is present. \$\endgroup\$ Nov 5, 2021 at 14:26
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    \$\begingroup\$ Yes, 10 Ohms sounds like a reasonable value. however, if you drive your NMOS from a driver that has an output impedance of 100 Ohms, adding an extra 10 Ohms is pointless. So: 1) show us what you use to drive the MOSFETs 2) look in the datasheet of that driver to see what is recommended there. \$\endgroup\$ Nov 5, 2021 at 14:28
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    \$\begingroup\$ "from digital outputs of a microcontroller" Directly? Please provide a link to the datasheet. \$\endgroup\$
    – devnull
    Nov 5, 2021 at 14:29
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    \$\begingroup\$ @MissMulan Putting a 10 Ohm resistor decreases the switching speed like 10k time OK, prove that to me. I don't get your logic so prove your point. Let's assume you are correct, then explain to my why in many gate driver + MOSFET designs (application diagrams in datasheets), a 10 Ohm gate resistor is recommended. That would slow down the switching dramatically according to you. So that makes no sense? \$\endgroup\$ Nov 5, 2021 at 14:31
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    \$\begingroup\$ ...But if we sum up the Gate-Source junction Hmm, last time I looked MOSFETs never had a Gate-Source junction. @MissMulan I suggest that you build a simple example circuit of a MOSFET driver and Power MOSFET in a simulator and see what happens when you have 0 Ohms in series with the gate of the Power MOSFET and when you change that 0 Ohms to 10 Ohms. [Edited by a moderator.] \$\endgroup\$ Nov 5, 2021 at 14:42

2 Answers 2

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If you're driving a gate directly with an MCU output you don't really need to worry about adding external resistance to the gate in most cases because the outputs are so weak they'll be the equivalent of adding 25 to 100 ohms of resistance.

The bad news is that you'll not be able to switch power MOSFETs at 500kHz directly with MCU outputs, you'll need an external gate driver of some kind, one that can switch ampere level currents fast. Then you might add a few ohms in series with the gate. Since you need a gate driver anyway, you might consider using non-logic-level MOSFETs where appropriate.

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  • \$\begingroup\$ This seems very reasonable. The logic already have enough dynamic resistance to make the gate resistor not really needed. Interesting, I will investigate about that in my circuit. \$\endgroup\$ Nov 7, 2021 at 21:49
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Is there anyone who has experience with this kind of logic level MOSFETs and can tell me if an external gate resistor is actually needed and if Rg ≈ 10 Ω is a reasonable value?

Each data sheet is giving you a value of external gate resistor based on meeting a specific rise and fall-time characteristic: -

enter image description here

This is so that you can choose a MOSFET driver that can deliver the goods. MOSFET drivers are fairly explicit in telling you how much their internal drive resistance is and so you can choose the "right one" based on what the dual MOSFET data sheets are telling you.

However, given that you are using an MCU to drive the gates, then you don't have the obvious luxury of knowing how much the IO pin's dynamic output impedance is going to be. This may or may not be a problem but appears to be worse for the Vishay part.

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  • \$\begingroup\$ Hi, To comply with this site rule, please edit your answer to include links to the resources (i.e. datasheets) which you used for the image you included. Thanks. \$\endgroup\$
    – SamGibson
    Nov 5, 2021 at 17:13
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    \$\begingroup\$ @SamGibson the data sheet links are contained in the question. \$\endgroup\$
    – Andy aka
    Nov 5, 2021 at 18:24
  • \$\begingroup\$ Thanks, I missed that. It would still be a good idea for you to include the links in your answer, as you only control the answer. If link(s) are removed from the question in future by anyone for any reason, then it seems you would have a problem, as there would then be no links for what you had included. That is the risk you take by relying on links only present in someone else's post. As a minimum, it would seem helpful for readers to point out that you have used the links in the question, don't you think? Thanks. \$\endgroup\$
    – SamGibson
    Nov 5, 2021 at 18:44
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    \$\begingroup\$ @SamGibson I wonder.... The pictures in my answer are from data sheets and, not one of the manufacturer's of said devices is going to balk at me advertising their data sheets so, I guess, there's no infringement of anything tender or secret here; there is no hint that I'm disguising the information because, all the pictures contain sufficient information to track down the sources very easily. However, if there is something (or potentially) illegal I've done, then I'm pleased to hear what that is and make amends. Clearly the great unwashed want an answer with no picture so more fool me! \$\endgroup\$
    – Andy aka
    Nov 5, 2021 at 22:28
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    \$\begingroup\$ And I agree with you - the Vishay data sheet is not very clear but, I believe that the 5.5 to 22.2 ohm limit is true internal resistance. The 1 ohm external I'm sure refers to external gate resistance hence, despite keeping it so low, the overall rise and fall time for this Vishay part is a bit crappy compared to the other two @AlessioCaligiuri \$\endgroup\$
    – Andy aka
    Nov 8, 2021 at 9:54

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