mosfet

Discussion in 'General Electronics Chat' started by mdsaad, Nov 2, 2005.

  1. mdsaad

    Thread Starter New Member

    Nov 2, 2005
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    would like to ask you about the power mosfet. How can I find the suitable mosfet for the DC motor? Is it depend on the specification of the motor? Example, if I get the 12V,0.3A motor, so the mosfet is 12V, 0.3A too. Is it ok if I use the higher mosfet current than the DC motor (like 12V,4.3A power mosfet)?
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    You _have_ to use a higher rated MOSFET than your application. E.g. To switch 12V 0.3A you might need 20V 0.5A MOSFET. These differences are called safety margins. Of course higher margins are desirable but they would lead to higher cost and might be in different/bigger packaging.
     
  3. mdsaad

    Thread Starter New Member

    Nov 2, 2005
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    how about the power? i mean, if using a 1.62W motor,how about the mosfet?
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    The power dissipated on the MOSFET (Pd) is affected by two factors. The first is the power dissipated during switching. If the MOSFET does not switch on or off frequently, compared to the on period, then this could be ignored safely.

    The second is the power dissipated during the conducting (on) period. This can be calculated from the current through the MOSFET during on period multiplied by the voltage drop across the MOSFET.

    Pd=Vds*Id

    Id is the current needed by the motor. Vds depends on the Id and the Rds(on). Look under Rds on the datasheet it should tell you its value for your given Id and Vgs.

    Vds=Id*Rds

    The MOSFET Pd(max) has to be higher (safety margin) than the Pd required by your specific application. Be careful of the quoted Pd conditions. Some manufacturers quote Pd value under specific conditions, such as the MOSFET is mounted on fibreglass substrate of a certain minimum size, on FR4 PCB of a certain minimum size with full copper, mounted on heatsink, etc. If your circuit can not follow these conditions then just increase your safety margin appropriately (downgrade the quoted Pd).

    Note also the room temperature used, if it is higher than the 25degC most manufacturers quoted their Pd, then downgrade the quoted Pd as well.
     
  5. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    in addition to what "n93" has mentioned, it is always advisable to have a safety margin. as in your case of 12v 0.3A you would need a device that is rated at least twice or thrice your requirement.

    moz
     
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