MOSFET transients and latch-on

Discussion in 'The Projects Forum' started by ryanlloydmiller, Sep 7, 2012.

  1. ryanlloydmiller

    Thread Starter New Member

    Oct 31, 2011
    15
    0
    I've constructed the following circuit...(I think everything is drawn correctly)

    [​IMG]

    Using the following components...

    IR Receiver

    P-chan MOSFET

    LED

    Resistor is 0.1 Ohm.

    The circuit works quite well in the sense that I can turn it on with 56 kHz pulsed IR and it draws around 750 mA -- just what I want. My problem is when plugging in the battery. When doing so, the LED flashes on and off multiple times (same thing when unplugging). This is not good for my application, but far worse is that sometimes it goes on and stays on just from plugging it in.

    After doing some research, I think this may be referred to as MOSFET latch-on. I'm not entirely clear on how to fix it though. I've seen a couple circuits that add a resistor from gate to ground, but those are n-type and I'm not sure if it will solve my issues. Alternatively, a series resistor from IR out to gate to keep Vgate lower than Vsource. Or a capacitor somewhere to limit transients. Or a diode somewhere...

    I've been fiddling with this circuit for a long time, trying different parts and configurations to get this thing to switch ~750 mA for an LED and in a package that's ~ 7mm x 8mm. Now that I've finally got that, I just need some help with this one (hopefully simple) issue to perfect it.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    What is the voltage between the MOSFET gate and its source (+4.2V) when the unit latches?

    It is unlikely you are experiencing latch-up with the MOSFET since it generally requires a high voltage and rapid current change to cause that.
     
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  3. vegeta

    New Member

    Sep 7, 2012
    6
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    What does your gate voltage look like when you are plugging in or removing the battery. The IR detector may be producing some false triggers as its starting up or shutting down.
     
  4. ryanlloydmiller

    Thread Starter New Member

    Oct 31, 2011
    15
    0
    Thanks for the feedback

    Its really hard to measure voltages under these conditions. Everything is SMD and extremely close together. Transients are extremely quick so I don't think I could really measure much and latch-up is hard to make happen when you want it to so, again, hard to measure. Also, when pulling 750 mA for longer than a second or so, the thing will start smoking (its only intended to run about 100 ms at 50% duty cycle, 1000Hz) so that also makes it quite hard to measure.

    I did put a 1uF cap across the battery. That seemed to make the transients longer. Maybe 250 ms when plugging in and same when unplugging. I also tried a 3k pull-up resistor between gate and VCC. LED wouldn't turn on at all and after removing the resistor, I think I fried this detector. Now the LED is always on at a very low intensity.

    I can make a bigger circuit and solder in some leads that I can check with a volt meter though I'm not sure what it will tell us.
     
  5. vegeta

    New Member

    Sep 7, 2012
    6
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    I am curious as to why you chose a P-FET for this application. You can try adding a pull up resistor to the gate of the MOSFET to ensure that it stays off till the detector actually pulls it low.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If you have the Oscilloscope, maybe you can measure the Out of IR Receiver Modules, does it occur any pulse when you plug and unplug the battery?

    If the pulse really occured, then you can plus a resistor and a capacitor, between the battery and Vs(IR Receiver Modules), the position shows on the bottom of the right side of the page 1 from the IR Receiver Modules datasheet, R1=100 Ω,C1=0.1uf.

    IR Receiver Modules datasheet, page 1.
    http://www.vishay.com/docs/81997/tsop753w.pdf
     
  7. ryanlloydmiller

    Thread Starter New Member

    Oct 31, 2011
    15
    0
    I've tried various configurations of BJTs but finally landed on FETs because they seem to be better for switching applications. Mainly, they can handle serious amperes in small packages and don't have major voltage drops. Do you have a better suggestion? Size is priority here. It needs to be SOT-23 or similar in size.

    As I mentioned above, I tried a 3k pull up resistor between gate and positive supply and I think it broke the thing. Not sure why, should have limited it to < 2 mA, maybe soldering heat messed it up?
     
  8. ryanlloydmiller

    Thread Starter New Member

    Oct 31, 2011
    15
    0
    I guess I'll try to look at the output on a scope tomorrow. The transient that occurs when applying/removing power is only an annoyance, however. Far worse is when the LED gets stuck on. Nobody has any ideas for that?
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    One work-around might be to add a start-up delay to the MOSFET gate voltage. This can be done with an RC delay circuit connected from the power supply to to one input of a 2-input AND gate (preferred would be a Schmidt Trigger NAND gate, such as the CD4093 with one gate used as an inverter at the first NAND output to get the AND function). Don't know if that will solve you latch-up problem but perhaps allowing things to settle down before applying the gate voltage will prevent whatever is causing the latch.
     
  10. ryanlloydmiller

    Thread Starter New Member

    Oct 31, 2011
    15
    0
    Remember, this is a p-chan MOSFET so, if anything, the problem with transients may be that the gate voltage isn't going high fast enough, no? The gate going low is what causes the LED to turn on.
     
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