Need advice for PWM driver

Discussion in 'The Projects Forum' started by assassin___0, Nov 21, 2012.

  1. assassin___0

    Thread Starter New Member

    May 10, 2011
    So I will be providing my buck converter with external PWM signal, which I will give through a microcontroller. The microcontroller spits out 3.3V PWM signal but to drive the MOSFET in the converter I need a 5V PWM, so bacially I need to design a circuit that converts 3.3V PWM signal to 5V PWM signal. I have attached by circuit design and the oscilloscope graph for input PWM and output PWM as well.

    My problem is that because of high frequency, the output is not a square wave. I only tested for 50% duty cycle, and the converter worked properly with my circuit, however, I fear that if the duty cycle is anything else, the weird waveform will interfere with correct convert output.

    How do I fix the output so that it is a square wave? Do I need to add capacitor elements or should I replace the optocoupler? What do you think about using ADUM3400 here?

    Last edited by a moderator: Nov 21, 2012
  2. Wendy


    Mar 24, 2008
    You will need a specialized MOSFET called a logic level MOSFET. A conventional MOSFET requires around 10V GS drive. I will redo your post to show the schematic in the post, as opposed to the external PDF file, which tends to make people nervous as virus bait.

    You did not post your MOSFET part number, and as I said, it matters a lot.
    Last edited: Nov 21, 2012
  3. assassin___0

    Thread Starter New Member

    May 10, 2011
    The MOSFET that I am driving is IRF640N

    Like I said though, I managed to get an output, so I must have successfully made switching at the gate. The MOSFET is working, if that's what you're asking.
  4. JDT

    Well-Known Member

    Feb 12, 2009
    You also need a high speed optocoupler. In your diagram you show an optocoupler where the transistor has a un-connected base. This will make it very slow. Best to get a high-speed logic type.
  5. assassin___0

    Thread Starter New Member

    May 10, 2011
    Can any one recommend a high speed optocoupler? I mentioned the ADUM3400, which is a "Digital Isolator, enhanced level-system ESD reliability" but I don't know if it will get the job done.
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
    If that is the mosfet you need to use, there is something called a mosfet driver that does all of the things your optocoupler and other components do, in one IC. Google "mosfet driver" for many, many choices.