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A really great Mosfet to use with the Prop... but can the Prop drive 20 of them — Parallax Forums

A really great Mosfet to use with the Prop... but can the Prop drive 20 of them

ElectricAyeElectricAye Posts: 4,561
edited 2009-10-20 16:57 in Propeller 1
I've been using the Mosfet IRF3708 with great success (thanks to your generous help) on just a couple of circuits, but now I'm faced with running about 20 mosfets off the same Propeller and I'm wondering if the Prop can handle the required current needed to control so many gates. On the IRF3708 data sheet (attached), page 2 shows something called IGSS to be 200 nA. I presume this is all the current that the gate requires to switch. If so, then 20 mosfets should not be a problem for a single Propeller, I think. But I'm kinda stupid when it comes to reading data sheets, so I was wondering if I'm reading that correctly or if I'm totally off the mark in using IGSS to make this calculation???

thanks,

Mark
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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-10-20 16:42
    It will depend on how you switch them. The steady-state leakage current is insignificant compared to the current transients required to charge and discharge the gate capacitance. If 20 of these were to switch in unison, you could conceivably exceed the Prop's overall current specs for an instant. But with proper timing and/or current-limiting resistors in the gate drives, I'm sure you can make it work.

    -Phil
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-10-20 16:53
    why not just stuck a buffer in between?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
  • ElectricAyeElectricAye Posts: 4,561
    edited 2009-10-20 16:57
    Phil Pilgrim (PhiPi) said...
    It will depend on how you switch them. The steady-state leakage current is insignificant compared to the current transients required to charge and discharge the gate capacitance. If 20 of these were to switch in unison, you could conceivably exceed the Prop's overall current specs for an instant. But with proper timing and/or current-limiting resistors in the gate drives, I'm sure you can make it work.

    -Phil

    Thanks, Phil,

    I suspected it wouldn't be so simple as adding up the IGSS. I suppose what I could do is monitor the voltage drop across the current-limiting resistor that I'm using now and see what the voltage drop is across it, then from that figure out what the current draw is. With a 330 ohm current-limiting resistor going to the gate, the current will have to be something less than 10mA.

    thanks,
    Mark
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