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  1. #1
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    MOSFET is not turning off perfectly at high frequencies

    I am currently trying to build a circuit for load switching with power mosfet. But at high frequencies i am getting irregular waveform. kindly help me to sort this out.

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    Re: MOSFET is not turning off perfectly at high frequencies

    Sure, I'll just use my magic crystal ball. Yes, yes, I can SEE your "irregular" waveforms They look awful!

    And your schematic, why, I can see the problem right THERE! See there, where I'm pointing? No? Oh, too bad.


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    Re: MOSFET is not turning off perfectly at high frequencies

    arun:

    what barry is trying to say, in his inimitable way, is that you need to provide the schematic, including the mosfet, the mosfet drive, and the loads being switched
    you also need to provide the oscilloscope wave traces showing the irregular waveforms
    and if you have good waveforms at lower frequencies, provide those also

    help can be provided, but it requires information



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    Re: MOSFET is not turning off perfectly at high frequencies

    Quote Originally Posted by wwfeldman View Post
    arun:

    what barry is trying to say, in his inimitable way, is that you need to provide the schematic, including the mosfet, the mosfet drive, and the loads being switched
    you also need to provide the oscilloscope wave traces showing the irregular waveforms
    and if you have good waveforms at lower frequencies, provide those also

    help can be provided, but it requires information
    Click image for larger version. 

Name:	Trigger.JPG 
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ID:	152357

    please find the above image/attachment in this reply



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    Re: MOSFET is not turning off perfectly at high frequencies

    Has your drive circuit enough 'push-pull' ooomf to overcome the MOSFET gate's capacitance at that speed, yet not introduce 'ringing' inductance ??



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    Re: MOSFET is not turning off perfectly at high frequencies

    What's exactly shown in the waveform? You say "drain current", how is it probed? Can you show related Vds and Vgs at the MOSFET?



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    Re: MOSFET is not turning off perfectly at high frequencies

    also, is your gate drive loop (pcb layout) nice and tight?



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    Re: MOSFET is not turning off perfectly at high frequencies

    your gate driver is not powerful enough.



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    Re: MOSFET is not turning off perfectly at high frequencies

    Yes i also suspect the gate drive is indeed not powerful enough...
    How have you connected up the ADP3418? Can you show schem?

    ADP3418
    https://www.onsemi.com/pub/Collateral/ADP3418-D.PDF



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    Re: MOSFET is not turning off perfectly at high frequencies

    (1) 180 Ohm gate resistor is not suitable for high frequencies. Reduce gate resistor in-between 10 Ohm to 22 Ohm as operation during high frequency gate requires significant current to turn on/off mosfet due to its internal capacitance.

    (2) According to my experience also put resistor of 4.7k to 10k between source and gate which ensures false switching as well as also increase turn off speed.

    (3) As mentioned in other replies your gate driver may not supply sufficient current to turn off/on mosfet at high frequencies.


    Schematic or more details from your side may be useful for assisting you.

    You can find blog of one of Edabaord member "Thamid” Which explains gate driver circuits for MOSFET in detail.


    Regards



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    Re: MOSFET is not turning off perfectly at high frequencies

    Hi,

    I think the OP is fooling us somehow by not giving useful informations.

    Even after beeing informed about the lack of information he replied a couple more "riddles".
    * No schematic, (especially the MOSFET driver circuit is requested a couple of times now)
    * a useless scope picture without X scale, without Y scale and without information where in the circuit it is connected and whether we see voltage or current....

    I have an idea whatīs happening here, but I donīt want to post another unvalidated assumption.
    (I donīt think the driver is too weak.
    I donīt think the 180 Ohms resistor causes the problem.
    And - in opposite of many of my other posts - I donīt think the PCB layout is the main problem here.
    And I think even Thamidīs blog doesnīt discuss this behaviour.)

    --> We need the requested informations to give good assistance.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: MOSFET is not turning off perfectly at high frequencies

    I donīt think the 180 Ohms resistor causes the problem.
    It depends if the post #4 waveform is 100 ĩs or 1 ĩs scale. It's only been said that the "MOSFET is working perfectly till 1 kHz".



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    Re: MOSFET is not turning off perfectly at high frequencies

    1kHz is not high freq, you have some problems - and possibly dead components you are un-aware of ...



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