MOSFET selection

Discussion in 'The Projects Forum' started by vini_i, Oct 13, 2013.

  1. vini_i

    Thread Starter New Member

    Apr 19, 2010
    14
    1
    I am working on creating a battery pack management system for some lithium polymer batteries. for the project i have selected the texas instrument BQ77PL900 chip.

    chip data sheet.http://www.ti.com/lit/ds/slus844b/slus844b.pdf

    battery data sheet. http://www.batteryspace.com/prod-specs/5072.pdf

    the battery pack will consist of seven cells in series. each cell is 3.7 volt nominal, 10Ah and can provide 80A continuous. the pack will be used to run an amp flow motor that is capable of consuming 150A.

    to protect the pack the TI chip uses two very large p-type mosfets. if an overcurrent condition is detected either during charging or discharging the chip shuts down the pack by blocking the current flow with the said mosfets. selecting the current rating for the mosfets is easy. as long as it exceeds 80A and i properly select a heat sink then everything is fine.

    my question is how to select the proper voltage rating for the mosfets?

    the pack should never exceed 4.2 volts a cell so as a whole the seven cells should never exceed 29.4 volts.

    would a 30 volt mosfet be plenty or do i need to provision a safety margin for instance 60 volts?

    is there any disadvantage in using a mosfet that has double the working voltage?
     
  2. edmundopt

    Member

    May 4, 2011
    60
    0
    I will only mention mosfet selection, not you application values, but you are missing one important factor : temperature!

    ID (continuos drain current) is mentionet with the die at the maximum temperature and the case at 25ºC, because this in practice is impratical a safe value of 1/4 or 1/3 of this value is safe.

    RDS(on) increases with temperature.

    You are also mentioning blocking the current flow, VGS values will determine that, you shoul have the best case scenario and the worst case scenario, so the maximum VGS and VDSS should be calculated.

    I recomend you that read carefully the Advanced power technology APT-0403, it is a good application note
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    For good reliability the working voltage of a transistor should generally be no more that about 50-75% of its maximum rating. Thus for a working voltage of 30V the transistor should be rated at 40V-60V. The disadvantage of using a transistor with a higher voltage rating then that will generally give a transistor having a slighter higher ON resistance.
     
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