Mosfet current ratings

Discussion in 'General Electronics Chat' started by sdrawkcab, Jun 2, 2007.

  1. sdrawkcab

    Thread Starter New Member

    May 14, 2007
    I've been browsing through some datasheets for power mosfets, and a lot of the high-current ones will have a footnote for the Idmax rating that says "current limited by package." Here's one example:

    Is it a thermal limit? Would you be able to get the rated current with a proper heatsink? Or is it the current limit of the actual package leads? If it is, would the through-hole packages have a higher limit than the surface mount packages?

    And how could you figure out how much the package current limit differs from the published limit?
  2. John Luciani

    Active Member

    Apr 3, 2007
    The current is limited by the amount of power that the package can dissipate.
    In most applications the current will be limited the Rds(on) of the FET which increases with temperature. There should be a graph in the datasheet that plots Rds(on) versus
    temperature. Determine the temperature rise of the FET and you can determine
    the Rds(on).

    Getting the rated current almost always requires proper heatsinking. With the
    SMD packages you need to provide a large copper area under the drain pad
    (which can consume a fair amount of PCB area). For a through-hole device
    you can get the heatsink off of your board. The best choice depends on your

    You can parallel the drains of multiple FETs to increase the current. Depending
    on your application you may need a control loop for each FET.

    (* jcl *)