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Thread: help finding smd n-mosfet for an application

  1. #1
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    help finding smd n-mosfet for an application

    Hello, I am feeling totally dumb asking help but I have made already a mistake buying wrong component so I would like to ask some advice.

    I am driving some laser modules using a PWM 16-channel driver.
    Laser modules are: output: 5-10mw, working voltage: 4.5-4.8vdc, working current: 40mA.
    I am using the the PWM output from each channel to drive a N-mosfet (2SK4017) with a 10K resistor. Voltage at gate-source is around 3.3V.

    I wanted to miniaturize this circuitry and bought some 2N7002 but emitting light is really weak and mosfet is getting very hot. Maybe I can try another resistor value? However when I put my finger on it and on the laser module pot at same time, light gets stronger. Some impedance I can't understand well right now.

    I am using SOT23 package.

    This is my first option. Any recommendation or advice?
    first option

  2. #2
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    While trying to fix it, I have also came to some (maybe) inductance problem.

    I have removed the 10K resistor from its place (as shown on the picture) so gate is floating (when Teensy is not sending any signal). Any value of resistor I tried until here produced just very pale light, and forward voltage on laser module terminal was about less than 2V.

    Inductance (...) happens when I hold a (conductive metallic) screwdriver on the laser module trim pot (picture), and simultaneously another screwdriver on the mosfet gate, laser module light gets vivid again (I can measure 5V on terminal between Drain and 5V) and circuit seems to work as expect. I am not sure how to start to fix it.

    Click image for larger version. 

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    Last edited by Bontempos; 09-25-2018 at 04:01 AM. Reason: added information

  3. #3
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    Rds at 150 mOhms is pretty high, but current is low, so I don't think you are creating heat due to current. what frequency are you running PWM at? What mosfet driver are you using?

  4. #4
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    I am running frequency at 1600, as I just kept values from the example driving led sequence available on the PWM driver library.
    If I understood the question about the mosfet driver, this is the link for the mosfet I was using earlier (which works, but package is big) (2SK4017).

  5. #5
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    Can you do a quick test and change the PWM frequency to say 450 or so and see if the mosfet still gets hot? I see your link to the mosfet, but how are you driving the gate? Directly from teensy, or is there a driver also? Reason I ask is that I had the same issue, trying to drive a mosfet at 20,000 and it would get burning hot after 2-3 seconds. But driving at 490hz it ran cool. I turns out that higher frequencies on the gate reguire significant current--hence a special mosfet driver circuit. I added one and my mosfet runs cool even up to 60,000 hz.

  6. #6
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    Oh, I am using one Adafruit 16-Channel PWM driver board connecting the board's channel output to the gate.
    I am connecting the 5V from the driver board to the laser Vcc.

    I tried both running at 450 and 60 hz but mosfet is still very hot (comparing to when it was working). And in all cases, light is still very weak.
    Currently, the only way to make it work, I I sad on post #2 is to cause some interference with my hand (through some tool) holding the DIM output of trimpot at laser driver, and the floating gate.

    I have tried to pull up gate to 5V, and pull it down using a 10K resistor, but nothing changed.

    edit* actually now trying pull up the gate at 1K it worked great! and also mosfet temperature looks cool!
    Last edited by Bontempos; 09-25-2018 at 11:30 PM.

  7. #7
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    Quote Originally Posted by Bontempos View Post
    edit* actually now trying pull up the gate at 1K it worked great! and also mosfet temperature looks cool!
    Well, not quite actually.

    I was trying to fix my board in a not-so-complex circuit with another teensy, but when I put it to the test with a more complex circuit having all channels connected to similar custom boards (the green one, on the picture) and I have noticed several other issues. I am not sure if I can still use this current mosfet on this application.

    [By the way, I said 1K resistor to pull gate up, but the strongest light I got when using 220 ohms.]

    The application is basically driving laser modules individually using the previous linked 16 channel PWM board.

    Well, basically I think I can't pull up the gate to 5V having the same pin (gate, wired to a PWM board channel), right? Because it seems I can't turn laser off completely. When pulse is 0, it stills emits a pale light.

    Also, having the gate pulled up means the laser will be on by default, which is a bit dangerous.

    Its hard to admit but I am still bit confused when reading datasheets of mosfets and interpreting if VGS is compatible.
    Last edited by Bontempos; 09-26-2018 at 01:24 PM. Reason: text was not so clear

  8. #8
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    Apparently worked after changing mosfet to BSS128 with a 1K resistor at Source gate.

  9. #9
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    After a few tests, and changing 1K to 10K resistor at DS, I have burn some laser modules and/or mosfets / resistors.

    Suddenly, for some modules, the light becomes very pale, and when I plug it to a power supply directly, the light will continue pale. Some problem on the laser power driver or the laser diode itself. I would like to ask for support on how to debug this issue and what variables I might be missing...

    Thanks!

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