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Choose MOSFET for PWM signal

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  • [Solved] Choose MOSFET for PWM signal

    Hi all, like said in the title i'm looking for a mosfet that capable to swith from a frequency between 200 KHz to 1 MHz. This Mosfet can be used to synchronise a clock for buck. See under the schematic that i used for the moment :

    Click image for larger version  Name:	2021-05-04_11h11_12.png Views:	47 Size:	129.7 KB ID:	17436

    Vin can vary between 9V to 24V.
    With a 2N7002 Mosfet, if i use a pwm with a clock > 10 KHz i have this :

    Click image for larger version  Name:	2021-04-30_15h48_29.png Views:	42 Size:	313.2 KB ID:	17438
    Can you help me (how to calculate) to find a mosfet that can be ok with this parameter ?
    Attached Files
    Last edited by Modjo; 05-18-2021, 12:37 AM.

  • #2
    What is pulling up the drain? I am amazed that it goes up at this rate.
    For fast signals you need a push-pull design. Either with a totempole or something like SN74LVC1G17.

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    • #3
      I agree with qdrives , maybe using a simple single gate chip instead of the transistor could help.

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      • #4
        Thanks for your reply.
        I'm a rookie on electronic design and i didn't think to use pushPull design for that !
        I made some research on the web about push-pull design with mosfet :
        For example i found this one :
        https://electronics.stackexchange.co...-zxmc4559dn8tc
        but this need to use a anti-shoot-through protection.
        Ohterwise, if i want to use SN74LVC1G17, i need to use a resistor divider to go under 5V for the gate ?

        Click image for larger version

Name:	2021-05-10_11h03_37.png
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        What do you think about this design ?

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        • #5
          - Check datasheet what is the required HIGH signal level for the sync pin - maybe 3V3 is fine.
          - Also, if VIN is +5V, you can use a schmitt gate powered by 5V ...

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          • #6
            robertferanec the enable theresold is 1.2V --> i can use 3.3V for sync.
            In this way i can connect-it directly to the gpio of my uC ? What's the advantage to use a circuit like SN74LV ?

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            • #7
              The shoot through in the reference is high due to the voltage difference between input (5V), output (15V) and gate threshold voltage.
              Modjo the advantage of the 74LVC1G is the high drive then most microcontrollers can handle and can be used as a level converter (take the 74AHCT1G17 for worse case 3.3V to 5V).
              The enable/sync input is a bit different then the others due the AC coupling capability. From the little I read the datasheet I too would say you can connect it directly to the uC. If so, you also do not need the buffer.

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              • #8
                I agree, you may be able to connect it directly to the MCU.

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                • #9
                  qdrives and robertferanec thanks for your help. I will connect the en/sync directly to uC gpio.

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