how to calculate gate resistor for power n-channel mosfet that uses with IR2111 driver

Discussion in 'General Electronics Chat' started by raneem amer, Jun 1, 2017.

  1. raneem amer

    Thread Starter New Member

    Jan 7, 2017
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    hi.
    please help!
    how to determine and calculate gate resistor for power Mosfet @ 6KHz frequency, VDS: 24V, ID 50A.
    i put 47 ohm and it work very good. but now i want to use irfb4110 @ 6KHz, vds 30V, ID 100A. i uesd 47 ohm but there is a lot of heat dissipation and i not work good.
    please how to calculate suitable resistor gate for irfb4110 with ir2111 driver?
    thank you in advance
     
  2. Sensacell

    Moderator

    Jun 19, 2012
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    Here is an excellent document that covers this in detail.
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Sounds like gate ringing, creating heat, have you got a circuit diagram?
     
  4. raneem amer

    Thread Starter New Member

    Jan 7, 2017
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    yes
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You didn't mention what MOSFETs you were using before the IRFB4110, but I'd be willing to bet that it's total gate charge was quite a bit less.

    If it was working so well, why change it?
     
    Last edited: Jun 4, 2017
  6. raneem amer

    Thread Starter New Member

    Jan 7, 2017
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    sorry, @ 24V system i used irf3205. it work so well. but now i want to use 48V system with irfb4110...
    vds of irf3205 is 55 V. @48V system the vds maybe arrive to 60V at charging batteries. so irf3205 not work at 48V system
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    The gate resistor value selection is a tradeoff between fast switching speed and gate/source ringing, but also sometimes involves the back emf energy at the drain. The whole thing is based on switching speed and where the excess energy is dissipated or returned to the power system.

    Drain emf energy control is probably the last thing you will be concerned with so let us say that you only want to reduce gate/source ringing and you want fast turn on speed.

    Since the resistor also limits the power dissipation in the driver, you have to consider the driver first because you may end up needing a higher current rated driver. I did not look up your driver yet, so you may already have a good enough one, but this is worth noting just in case.

    A design like this would never go unchecked without a scope, so you really have to check what is happening with a scope. That might be the only way to get it right because you will have variables that are hard to nail down theoretically without making assumptions that may not actually apply in your case. Looking at various places in the circuit you can see if your transistor is switching fast enough, and if it gets hot it may not be. The gate resistor controls the switching time both on and off, so you look at that and see if it looks fast enough and the transistor does not overheat.
    If the transistor overheats then you probably have to speed it up, assuming there is nothing else wrong like bad choice of magnetics. Once you speed it up, it may ring, which can also cause overheating, so you have to look for that. If you cant find a happy medium between speed and ringing, then you probably have to get the driver closer to the transistor(s) and start over again. Of course there's also the chance that you just did not select the transistor correctly as to it's rating.
     
    raneem amer likes this.
  9. SgtWookie

    Expert

    Jul 17, 2007
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    The new MOSFETs have a total gate charge that is over 150% of the previous MOSFETS. Try 24 or 27 Ohms for the resistors.

    You may need to double up on your boost caps.

    Your schematic looks a tad "iffy"; there are some components which may not be connected on both ends. Have you run the error checks?
     
    raneem amer likes this.
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Just one more opinion:
    Since your switching frequency is low, I doubt it is slow switching. My guess is that there is still some overlap where the top and bottom FET are on together. You could rule out the switching losses by making your load a high value - say 24 ohms. If the FETs still get hot it would be a good indication of shoot thru.
    The 51 ohm resistors slow down both the turn on and turn off. I think I would remove them and count on the 100 ohm and diode.
     
    raneem amer likes this.
  11. raneem amer

    Thread Starter New Member

    Jan 7, 2017
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    Yes sgtwookie my schematic is right. But that image just for understad.
    I tried 4.7 ohm and I got a little heat dissipation.
    I opened 5kva Indian inverter board and I didn't see any resistors on gate transistors. Maybe I should remove it!!!???
    Is there any equation to calculate it!!??
    Thank u in advance
     
  12. raneem amer

    Thread Starter New Member

    Jan 7, 2017
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    what is your opinion SgtWookie!!
     
  13. ezzipaye

    New Member

    Jun 8, 2017
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