Connect with us

PWM LED THROUGH MOSFET

Discussion in 'LEDs and Optoelectronics' started by collatz, Apr 11, 2016.

Scroll to continue with content
  1. collatz

    collatz

    7
    0
    Apr 11, 2016
    I have a boost converter circuit which takes an input voltage from as low as 1.8 to as much as 5.5 and outputs 6 volts at 10 amps.

    The output of the boost converter goes to a P channel mosfet, part number AON7423. What I want to be able to do is PWM this 6v, 10 amp max output by 0.1% duty cycle increments.

    For the output capacitors on the boost converter I'm currently using rather expensive ceramics, about 330uF, with a recent additional aluminum capacitor of 1000uF. Circuit has to fit within a 0.8 square inch area. The majority of my troubles so far have been size related.

    So in these three pictures I'm running a 5.0% duty cycle at around 1000hz. The turn on time of the mosfet appears to be almost instantaneous. The turn off time seems pretty fast but then kind of decays more gradually near the end. I don't understand why??

    When the boost converter is enabled... The turn on time still is fast enough for my purposes (looks instantaneous!) but after turn off it goes super into some kind of crazy oscillation (*** this oscillatory behavior fades away before the next turn on event of the mosfet. Any ideas??

    My MOSFET is driven by a bipolar transistor which brings the gate to ground. (So I think I understand why there is a fast turn on), I am not using a push pull type driver, could this be the cause?

    Thank you so much for any suggestion, help, criticism, etc! I'll really appreciate it.
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Hello and welcome to E.P. Can I see the circuit diagram please.
    Thanks
    Adam
     
  3. collatz

    collatz

    7
    0
    Apr 11, 2016
    Hi Adam, hope this helps
     

    Attached Files:

  4. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Ok one thing you could try is a Baker clamp, this prevents the BJT transistor from saturating fully and will be faster to turn off. This will only work if it's the BJT causing the issue, but it's worth a try.
    Adam
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    I don't think it is the BJT that is causing the problem. It is the 100Ω resistance through which the gate must discharge. You need a two transistor driver (high, low side) so that there is less resistance for turn on and turn off.

    Bob
     
    hevans1944 likes this.
  6. Alec_t

    Alec_t

    2,940
    799
    Jul 7, 2015
    Very likely. I agree with BobK.
     
  7. collatz

    collatz

    7
    0
    Apr 11, 2016
    Thanks everyone for the help! I've spent the last couple days considering these things. Today I built a push pull driver with a 12 volt input voltage to slam the gate capacitance. The mosfet now opens and closes very fastand clean on the oscilloscope. I measured my efficiency at 75%, which is pretty good. I was hoping for 90%... Maybe my board size constraints make this impossible? When I look for lower rdson or lower inductor resistance etc... The parts start growing in size fast. My test today was at 3.5 volts in,,, 6 volts - 8 amp out , best I've seen so far on this circuit. temperature rose and stabilized to 190 degrees Fahrenheit. =)
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-