Serious: Switching Mode Power Supply

Discussion in 'The Projects Forum' started by Sherif Amer, Mar 28, 2013.

  1. Sherif Amer

    Thread Starter New Member

    Jul 6, 2010
    8
    0
    Dear all,

    I'm working on some project which requires 100v converted from 9v, so i called the basic idea of Boost converter.

    The circuit is quiet simple, i use IRF540NS switched from PWM (Pulse Value 3.3v, Pulse Width 278us, Period 300us)
    [​IMG]

    I used TI application note http://www.ti.com/lit/an/slva372b/slva372b.pdf to calculate the values of the coil and the output capacitor using the equations 5,6 and 12, 13.
    The inputs are: Vout = 100, Iout(max) = 5mA
    After all, L = 148mH and Cout = 511uF

    The simulation output is 10v not 100v!!!
    [​IMG]

    Any help please?!
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    You might need a "load" to provide a DC path when the switch is open.
     
  3. Sherif Amer

    Thread Starter New Member

    Jul 6, 2010
    8
    0
    First off, thx 4 your reply
    second it doesn't need a load, i figured out the problem, it's in the IRF, it doesn't turn on 3.3v, changed the pulse into 5v, it settled finally at 100v.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    The IRF540NS is a standard MOSFET which requires 10V Vgs to fully turn on (it's only partially on at 5V). If you want one that turns on at a lower voltage you need to use a "logic level" type MOSFET.
     
  5. Sherif Amer

    Thread Starter New Member

    Jul 6, 2010
    8
    0
    The graph in the datasheet VDS vs ID is drawn with VGS varies from 4.5v to 15v. I will need another MOSFET anyway. Thanks a lot guys!
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    What is you peak inductor current? I'll assume 2 Amp for now.

    If the inductor max peak current is 2 amp or less then Vds with a Vgs of 5V is only 0.15 Volts. That is 0.15 X 2 Amp = 0.3 Watts and Rs = 0.15V / 2A = 75mΩ. Also, the temperature has very little effect at Vgs=5 so Rs won't change much either. A 5 V pulse is okay for this situation.

    Regards,
    Wayne
     
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