The purpose of MOSFET Rg

Discussion in 'General Electronics Chat' started by Flug540, Dec 1, 2013.

  1. Flug540

    Thread Starter Member

    Mar 24, 2012
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    Can someone please explain what is the purpose of Rg? I've found this explanation on another forum, but it still doesn't fully make sense to me:
    "Basically, if you make RG too small then your FET might turn on very quickly which might cause noise in your circuit. If you make RG too large, then the MOSFET takes a long time to turn on -- this will increase the conduction losses in the MOSFET. This is because we really want to use the MOSFET as a switch -- ON (saturation) and OFF (cutoff). If we turn it on slowly, then the MOSFET spends much more time in the LINEAR (ohmic) region and it acts like a resistor and burns up power at each cycle."
    Rg value too high making a slow switch is intuitive, but if we want to use the MOSFET as a switch and the intent is to turn it ON as fast as possible, why do we need a resistor at all? Can it be eliminated and what are the side effects? The author of the quote is suggesting it may cause noise, is it true?
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The resistor is used to damp oscillations caused by stray inductance and the capacitance of the FET. This ringing when severe enough will cause the FET to turn on and off a few times dissipating power and making electrical noise.
     
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  3. Flug540

    Thread Starter Member

    Mar 24, 2012
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    I see, thanks. I've found this on another forum:

    "The answer is that NO resistor should be connected between the gate driver and the gate. If you want to connect a resistor somewhere, then connect a high value (>10K) from gate to source of each FET. This will guarantee that the FET is OFF in case the wire to the gate driver gets disconnected."
    Sounds like this is a better idea, would you agree?
     
  4. crutschow

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    Mar 14, 2008
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    Well, obviously there are different opinions on this. In general for fast switching you want little or no resistance, but if you have ringing in the gate voltage due to parasitic circuit inductance then you will need to add a small resistor in series. The optimum value may need to be determined after you build the circuit and can observe the waveform with an oscilloscope.
     
  5. Alec_t

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    Sep 17, 2013
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    If you do use a series gate resistor it must be as physically close to the FET as possible, to minimise wire/track length (and hence stray inductance). The gate-to-source resistor is a good idea.
     
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  6. shortbus

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    Not trying to start an argument, just asking a real question. While I agree with the keep the driver close to the mosfet part, is the resistor to source part needed, when using a IC type driver? Most gate driver ICs have a totem pole output and connect to both gate and source when off.

    In a driver made from individual discrete components, a resistor between gate and source should be used, because most times the "off" side of the drive isn't even considered.

    But with a dedicated, IC driver, is the gate/source resistor even necessary?
     
  7. wayneh

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    Sep 9, 2010
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    My 2¢. It's a balancing act of risk management against a statistically rare event. The odds of driver failure may be very low, but for the tiny cost of a resistor, you insure against what might happen if the driver system somehow fails to open, allowing the gate to float. If that float could potentially cause a lot of damage, adding that resistor buys some protection for almost no cost.

    It would make some sense for MOSFETs to come with non-floating gates, with an internal pull-down, to begin with.
     
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  8. Flug540

    Thread Starter Member

    Mar 24, 2012
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    Thanks guys for the great replies!
    So if I have a gate to source resistor, I do not need the gate to driver resistor, right?
     
  9. adamclark

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    Oct 4, 2013
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    I would like to thank everyone here for opening my eyes to the many other things to be considered when designing/building electronic projects. Until I found this site Ive never really considered things like risk management and multiple ways to get the same outcome. It has always been full on or full off, It either works or it don't. Ive never considered using completely different methods to achieve the same outcome. It would appear that there are many ways to skin a cat. And Ive never really thought about how long something will work, as long as it works was always fine with me. It would appear that my road in this arena is indeed going to be a long one.. Anyways, Thanks to every member here that posts. Especially the ones that post the questions that some might think are dumb or stupid because those are usually the ones I ask too..
     
  10. wayneh

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    Design goals vary a lot, and sometimes lifespan or reliability trumps all other considerations. A roadside fix to get your car to the shop is a very different design scenario than, say, a life support system on the space station.
     
  11. adamclark

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    Oct 4, 2013
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    sorry for the thread hijack, that was supposed to be a new thread, guess I had to many windows open at once..lol.. oh well.. It definitely has opened my eyes to looking at things from all angles though. thank you
     
  12. shortbus

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    Sep 30, 2009
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    I'm world famous for my dumb questions.:D There are many ways of doing things, especially in electronics. When ever I start to think I'm getting a handle on things, something new pops up. Being in my middle 60's and trying to learn on my own, this site has been a godsend.
     
  13. Flug540

    Thread Starter Member

    Mar 24, 2012
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    Since no one seems to mind stupid questions... :)

    If I choose to have the gate to source resistor, I do not need the gate to driver one, correct?
     
  14. #12

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    Nov 30, 2010
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    I was just going to chase that one down. You'd be amazed at the microhenries that live in circuit board traces. Recently, somebody posted, "The myth of ground" which explains that, well, ground is a myth, especially at high frequencies. You can now buy surface mount capacitors that have their connections on the sides instead of the ends, just to save a millimeter of circuit board trace!

    As always, it depends. How fast does it need to be and how smart are you about circuit board layout. A lot of first designs go in the trash can because of mistakes measured in thousandths of an inch when you get into multi-megahertz.
     
  15. Flug540

    Thread Starter Member

    Mar 24, 2012
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    Very interesting, thanks. In my case, as I mentioned in my other MOSFET thread, I'm only at around 300Hz.
     
  16. #12

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    You're operating at 300 Hz, but you're switching a lot faster. Throw in a Schmidt trigger and a gate driver and suddenly you have rise times in the microsecond range. Ignore that and it will ring like a bell.
     
  17. Flug540

    Thread Starter Member

    Mar 24, 2012
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    I just ordered slightly higher rated MOSFETs for my application. When I get them I'll try playing with different resistor configurations and monitor the reaction with the scope. BTW, looks like I'm going to have to go with two of them for each channel (if you remember what I mean).
     
  18. ronv

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    Nov 12, 2008
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    If you are driving the FET with a FET driver they are usually fast. This is the case where the resistor in series with the gate can be important.
    Here is an example with and without a resistor and with 100nh of inductance.
     
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  19. TheComet

    Member

    Mar 11, 2013
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    I made a recent discovery about this. Basically, you want to be careful with driving a MOSFET with a driver-IC and not having a Gate resistance, because you don't want to turn the MOSFET off too quickly in high frequency applications either.

    See page 7-8: http://www.aosmd.com/res/application_notes/mosfets/Power_MOSFET_Basics.pdf

    During body diode reverse recovery (when the body diode is transitioning from conducting to non-conducting), most MOSFET datasheets specify a maximum dV/dt. If this value is exceeded, i.e. the MOSFET is turned off too quickly, it's possible that enough voltage is transferred to the gate, switching it back on again and causing catastrophic failure.

    The Gate resistor should be chosen wisely as to not exceed the MOSFETs maximum dV/dt, but also not cause the MOSFET to switch too slowly.
     
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  20. Flug540

    Thread Starter Member

    Mar 24, 2012
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    Great input guys, thanks a lot!
    In my case I'm using an LM2902 in comparator configuration to drive the FETs. I measured the on-time to be somewhere around 25-30us.
     
    Last edited: Dec 6, 2013
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