mosfet questions

Discussion in 'The Projects Forum' started by caliusoptimu, Mar 6, 2007.

  1. caliusoptimu

    Thread Starter New Member

    Mar 5, 2007
    im kind of new to the concept of using, more specifically, driving mosfets under different conditions. just a couple questions:

    1. ive been playing with a few IRFZ44N's and i noticed on the datasheet it specifies a max drain-source voltage of 55v, and a max gate-source voltage of +/- 20v. i was led to believe you would need around 12v more than your Vds, on the gate, in order for the mosfet to be completely 'on'. obviously i wouldnt be able to go to some 60v on the gate to drive a 48v load. so my question is, would it be fully on at 20v on the gate regardless of drain-source voltage?

    2. ive been thinking of trying logic level mosfets, but the same question popped up. would a logic level mosfet be fully on at 5v TTL or 12v CMOS on gate regardless of Vds?

    3. is there a somewhat simple way to obtain a higher voltage (very low current) from a 12v source other than a dc-dc converter? i have a PWM hooked up to the lights on my gokart, but the IRF540s doing the work are being driven at 12v. even paralleled they are generating a good amount of heat. (dont ask me why i have PWM on my headlights, i was bored, lol)

    thanks guys!
  2. kender

    Senior Member

    Jan 17, 2007
    Charge pumps. They are still considered dc-cd converters, but they are considerably cheaper and smaller than inductive boost converters. In fact, many FET driver ICs have built-in charge pumps.
  3. caliusoptimu

    Thread Starter New Member

    Mar 5, 2007
    hmm. interesting, any suggestions? that reminds me, i saw international rectifier has high voltage drivers for high side and low just not sure what the difference is in gate requirements. again, thanks!
  4. rwmoekoe

    Active Member

    Mar 1, 2007
    the Vgs, as what the initials stand for, is actually measured From Gate To Source. Suppose your source is tied to ground, then Vgs can be said as gate to ground voltage difference.
    20v is the maximum rating. the mosfet is at its lowest resistance with Vgs only 12V or above.

    Logic level mosfets are fully conductive at 5V Vgs.

    You don't need this anymore do you?
    If you use the irfz44 to control the positive line, then you need to have a gate voltage above the postive line.
    In this case, you can use the charge pump. It's low on current, but much more than enough, since driving the gate takes almost no current. It a field effect transistor right?
    The mosfet being hot could be because of the driver voltage not being high enough. It could also be because the Rds-on of irf540 is not low enough (44mOhm, generates 1watts at 10A, if continuous and without heatsink will be hot enough to touch)

    you're welcome!
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    Should be 12v more than source, not 12v more than Vds. The 12v is to achieve maximum switching speed - you can turn it on with only 6 volts or so. Refer to chart of figure three in the datasheet