Picking the Proper MOSFET?

Discussion in 'The Development Lab' started by chilloutdocdoc, Apr 20, 2011.

  1. chilloutdocdoc

    chilloutdocdoc Full Time Elf

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    So, in the true spirit of DIY, what's the magic way to pick the proper MOSFET to use?

    I could easily say i'm going to be using ##Amps at ## Volts and ask if ###### P/N is suitable, but how do i narrow down my search? Do I just start looking for something that suits my requirements? Or should I include an overhead, say double the current? Triple?

    In going with the trend of putting the controller with the element, I'd like to match the parts to the item, while not going overboard using 600V and 60A things for a 12V 2A project :)

    Thanks!
     
  2. David_AVD

    David_AVD Good news, everyone!

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    Simplified version:

    Volts: You want a rating a reasonable amount higher than the actual supply voltage. Say, a 50V rating for a 24V supply. Remember, these are maximum ratings, not some nominal figure you can push.

    Amps: It would be unwise to use a 3A rated MOSFET for a 2A intended load. You need to think about inrush current (incandescent loads mainly), so for a 2A load, a 6A rating would be more appropriate. Going higher should cause no problems (in general) and it's often a case of price or package size then.

    Rds: This is the resistance of the Drain-Source connection when the MOSFET is turned fully on. Lower values are better, as that means less voltage being dropped across the D-S junction. High values mean high losses, which shows up as heat.

    Vgs: The is the voltage required on the Gate connection to turn the MOSFET on. For most DIY designs, a "Logic Level" type of MOSFET is what you want. This type turns on with only a few Volts on the Gate, hence is suitable for driving from a micro-controller (PIC, AVR, etc). This assumes the micro is running from 5V, as a 3.3V supply may not be high enough to turn a Logic Level MOSFET on fully, resulting in losses, excessive heat and failure.
     
  3. AAH

    AAH I love blinky lights :) Community Project Designer

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    Mosfets are a lot more forgiving of peak currents than bipolars transistors are. The peak current is commonly 10 times the continuous current. the main governing factors are whether
    1) they are N channel or P channel. N channel is more common, are cheaper and have lower on resistance
    2) voltage. It is always good to have some headroom. For low voltage DC stuff 5 to 10 volts is ample. The lower the voltage rating of the MOSFET the lower the ON resistance will be and the less loss/heat
    3) gate voltage. You need to check that the MOSFET will be turned on fully with the voltage you are turning the fet on and off with and you also need to check that the voltage isn't higher than the maximum gate-source voltage
    4) On resistance. This controls how much heat the fet will be dissipating and what portion of the volts won't be getting to those all important lights. Low on resistance is always what you strive for but as the resistance goes down the price goes up
    5) Package. The on resistance is dependant on the package as is the power dissipation. Some of the TO-220 packages you can dissipate 60W whereas DPACK surface mount ones can only dissipate 1W with plenty of copper around them
    6) Price. From maybe 30c to $10 depending on package, specs and quantity

    If you don't know what to look for then about your best option is to leave a few specs of what you want in here and someone can point you in the right direction
     

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