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CSD18535KCS: Switching of MOSFET using optocoupler

Tejas Bhagwat
Prodigy 120 points
Part Number: CSD18535KCS

I'm using an optocoupler for using the CSD18535KCS as a switch. The input pulse to the optocoupler is 3.3V at 50KHz. The output from the optocoupler is a square wave as desired, but after connecting the terminals of the output of optocoupler across the gate and source of the MOSFET, the ouput is not as desired. The charging time seems a lot excessive than desired which I believe is due to the input capacitance Ciss. Any suggestions how to rectify the same? 

  • 0
    TI__Expert 7255 points
    Tejas,

    Thanks for considering our MOSFETs.
    The output stage of optocoupler is not strong enough to drive the FET directly. You will need a MOSFET driver IC after the optocoupler to drive the FET.
    Here is a link to TI selection of gate drive ICs: www.ti.com/.../overview.html
  • 0 in reply to Chris Bull
    TI__Expert 7255 points
    Tejas,

    I see you have clicked the not resolved button but have not indicated the reason or replied any text. Please can you let us know what else you need. If I do not hear from you in a couple of days I will close this out as assumed the question is resolved.
  • 0 in reply to Chris Bull
    TI__Mastermind 31935 points
    Hi Tejas,
    Just following up to see if you have any additional questions. As my colleague pointed out, the output drive capability (10's of mA) of typical opto-coupler is typically not adequate to turn the FET on and off very fast. In order to get faster switching times, a gate driver IC or transistor-based push-pull buffer is required. This particular FET is very low rds(on). To achieve this, the die size is large and it has high gate charge and input capacitance (~5000pF). You will need 100's of mA or A's to drive the FET.