MOSFET Help

Discussion in 'General Electronics Chat' started by eeboy, Sep 27, 2007.

  1. eeboy

    Thread Starter Active Member

    Sep 27, 2007
    90
    1
    I've got a strange problem with a MOSFET (IRLML2502PBF). I've used it in several places within my circuit with no problems but I am having an issue with one particular implementation. It's a logic level N Channel FET with a very low Vgs threshold (.65V). The FET is not sinking a large current (on the order of 20mA) and the load is not inductive. I've switched much larger loads with this particular FET. I have driven these particular FETs directly from a microcontroller in the past.

    The problem is that the FET fails to turn on when driven by a single gate inverter (NC7S04). It's very difficult to troubleshoot because the problem goes away as soon as I attach a scope probe to it. My first thought was a bad solder joint. I found this was not the case. So, I tried adding a large impedance between the gate and ground (10k). This does the trick. It happily works that way... but I would really like to know why!

    The connection between the inverter and the gate is direct (no series resistance) and there is no pullup.

    Any suggestions?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I took a look at datasheet for the MOSFET you are using. What you have described sounds like an oscillation condition. The additional loading by the scope probe or the resistor was enough to squelch the oscillation. Another possibility is a poor ground connection.

    Can you sketch a diagram of your circuit and post it just so that we may see what you are working with?

    hgmjr
     
  3. eeboy

    Thread Starter Active Member

    Sep 27, 2007
    90
    1
    Thanks for the look... here is a screenshot of the relevant portion of the schematic.
     
  4. eeboy

    Thread Starter Active Member

    Sep 27, 2007
    90
    1
    By the way... a poor ground connection was my initial thought as well. This is the first rev of the board and so I manually soldered all components in place. The ground plane has a large thermal mass and so it's difficult to make all the GND and VCC connections without applying so much heat that you damage the component. I ran a few tests in which I forced the GND tab of the FET to the pad hoping this would solve any mechanical issues. Made no difference.

    I even went as far as placing the board on a hot plate to elevate the temp of the ground plane so that I could reflow the GND tab. Still... nothing.

    So, I think I can rule out the poor connection.
     
Loading...