help in buck converter design!

Discussion in 'The Projects Forum' started by idmond, Dec 22, 2010.

  1. idmond

    Thread Starter Member

    Oct 5, 2010
    i was asked to design a buck converter that takes a 22.4vrms and down them to 9vdc
    for a load that draws 1amps.

    the ripple current= 0.2 A
    Fswitch= 400khz

    i did all the math to choose the right values for the shottky diode, inductor
    and the output capacitor, but i got stuck in choosing the MOSFET switch.

    i don't know exactly whether i choose PMOS or NMOS.
    if i chose NMOS, that will require a gate to source voltage higher that 22.4v to drive it
    and i'm not sure i can do that.

    and if i chose PMOS that will require at least 18v or more to turn the switch OFF
    assuming the Vgs(th)= -5
    (my info about MOSFETs are a little bit rusty, so i might got it all wrong)

    so i am stuck now!
    and i am waiting for your answers and recommendations.
  2. Wendy


    Mar 24, 2008
    How about a schematic?
  3. idmond

    Thread Starter Member

    Oct 5, 2010
    Here it is
    • 000.JPG
      File size:
      8.1 KB
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    If you don't have any voltage higher than 22.4V , that basically decides what you can use in your circuit, right? You can often get a lower voltage a lot easier than a higher one.

    Do you have any schematics of examples you've been taught? Formulas? There are a few more things to consider than when creating a linear power supply.
  5. idmond

    Thread Starter Member

    Oct 5, 2010
    i don't realy quite understand what you mean??

    of course, my design was based on an example of designing a buck converter from Microship Webseminars.
    i calculated every thing right with the output capacitor, inductor
    and the Shottky diode, but in Microship Designing Example, he chose the P- channel MOSFET over the N- channel
    because that will save him the higher gate voltage required to drive the N-channel MOSFET.

    i am just confused. how i am to suppose to drive the MOSFET gate (N or P-channel)
    to pass this input voltage (24.4V):rolleyes:??​

    i already have stepped-down the 24.4V with a linear regulator to get +10V from 7810 and -10v from 7910,
    but i want to design SMPS to compare their efficency with linear regulators.​

    could i somehow just use the 10V output from 7805 to drive the gate,
    or i will need another boost converter to step this 10v up to drive the MOSFET's gate of the Buck.​

    i realy need help with this one.

    Here is the PDF of the whole Designing Example:​
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Is your 22VRMS input 24VDC, or 24VAC?

    Do you understand the formulas given in the PDF?