MOSFET needed!

Discussion in 'The Projects Forum' started by Skfir, Nov 20, 2012.

  1. Skfir

    Thread Starter Active Member

    Aug 19, 2010
    135
    4
    Guyz! Please, do not know which to choose. I badly need a MOSFET which can turn on and off really fast, which can hold about 2a continuous current and about 3-4 or 6 peak current. Please help!!
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    MILLIONS of Mosfets can do what you want but you gave no details.
    A Mosfet has an input voltage requirement to completely turn on which is usually 10V but some Mosfets need only 4.5V. The pins on a TO-220 case have a maximum current of about 75A.
     
  3. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    All MOSFETs switch very fast - providing you drive the gate correctly. This means charging and discharging the gate capacitance with a high current. Special driver ICs are available.

    To specify your MOSFET you need to know:

    • P or N channel?
    • Maximum drain - source voltage.
    • Maximum drain current.
    • Minimum ON-state resistance.
    • Gate drive voltage (threshold voltage).
    • Safe operating area.
    • Package type (TO220, TO92, surface-mount, etc).
    Select a type which is better than you need to allow a safety margin.
     
  4. Skfir

    Thread Starter Active Member

    Aug 19, 2010
    135
    4
    GS voltage about 4v
    N-channel
    DS V 5-6.5v with peaks probably up to 20
    Current about 2a with 4% duty
    What is on state resistance?
    packaging type is not of big importance and I will operate this device at common house temperature, about 20C.

    Thank you guyz!
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    The Rdson is the tiny remaining resistance when the MOSFET is driven fully on. It's typically in the mΩ range, like 0.02Ω. You need it to estimate the heat that will be generated by the MOSFET itself while the load is on.
    The pkg always matters, since it affects the ability to dissipate that heat.

    You need a "logic" level N-channel MOSFET rated to at least 10A, and more would be better. Extra safety factor adds little to the cost.

    You haven't defined what you mean by "really fast". What frequency are you talking about? The gate of a MOSFET has a small capacitance and inductance, like any real device. These become increasingly important at higher frequencies and you have to design around them.
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
  7. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
  8. Externet

    AAC Fanatic!

    Nov 29, 2005
    763
    57
    Disembowel a dead computer power supply forgotten behind the spider webs, it likely has a beefy mosfet as switching element, usually recognized as part number beginning with 'K' , but many others are used too.
    Research its data sheet.

    And comes mounted on a free heat sink...
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Isn't worth the labour, and these heatsinks are really clunky.

    Better invest a few buck in your semiconductive needs!
     
  10. Skfir

    Thread Starter Active Member

    Aug 19, 2010
    135
    4
    Thank you guyz a lot for help.

    I need a mosfet which is able to produce really short squared pulses (0.4uS) with good steep fronts. Thanks for great help.
     
  11. Skfir

    Thread Starter Active Member

    Aug 19, 2010
    135
    4
    Will probably stick to IRF710. Thanks guyz!
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    An IRF710 is terrible for your circuit because it has a very high on-resistance of typically 3.3 ohms when the gate-source voltage is 10V. It is a high voltage Mosfet (400V) which is why its on resistance is so bad.

    You need a "logic level" Mosfet that has an extremely low on-resistance when its gate-source voltage is only 4.5V.
     
  13. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    Try NEXFET series from TI. These have very low rise and fall times with low input capacitance.
     
  14. Skfir

    Thread Starter Active Member

    Aug 19, 2010
    135
    4
    Got it, thank you people!
     
Loading...