PWM of Pic 16f877a to buck converter

Discussion in 'The Projects Forum' started by 7hekingofkings, Apr 1, 2013.

  1. 7hekingofkings

    Thread Starter New Member

    Apr 1, 2013
    2
    0
    hello guys i need some helps here. I am implementing a buck converter which will be trigger ON and OFF by pwm for pic 16F877a. Find the attachment of the schematic circuit in the message. note that the heater is replace by a 12 V fan. when testing this circuit, its does not switch the fan at different duty ratio. The two grounds of the microcontroller and the buck converter are already common ground. help plzz

    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You have the BJTs connected as emitter followers thus the high output voltage to the MOSFET is equal to the PIC's output voltage minus about 0.7V, which is not enough voltage to turn on the MOSFET. You need a buffer amp (such as another transistor) that can provide a 12V signal to the BJTs base.

    What is the purpose of the other MOSFET in parallel with the load? Is that used as a free-wheeling diode?
     
  3. 7hekingofkings

    Thread Starter New Member

    Apr 1, 2013
    2
    0
    yeah .. it is a free- wheel diode...what kind of buffer amp and circuit do you recommend me?
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    The BJTs are just wrongly configured.

    You can do it with just one BJT, use a resistor and short to ground.
    Or you can use two + care for dead time.

    This would be inverting if you use N channel MOSFET and NPN BJT, but not a problem for the controller.

    The wasted power is normally neglible, if you don't use high speed, you can also increase the resistors.

    Some PICs even have programable dead time.
     
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