need help in DC DC Buck Converter

Discussion in 'General Electronics Chat' started by sa7her, Mar 6, 2011.

  1. sa7her

    Thread Starter New Member

    Oct 19, 2010
    8
    0
    hi ,

    Im plannig to build a DC-DC buck converter ( 12-6)V and I have calculated each components on my circuit and when i try to run it oi PSPICE Capture CIS program or Multisim program i get around 12 v at output.

    my output current should be about 0.5A and Fsw = 10KHz. I have use N-MOSFT ( IRF520) im my circuit. You can see my circuit diagram below. I couldnt figure out where is the problem. The pic below is from Multisim program. I use a VPULSE source instead of the PWM circuit for now which it should give me 10Khz.

    So can any when help to find the problem and sorry for my poor english.



    [​IMG]
     
  2. Vahe

    Member

    Mar 3, 2011
    75
    9
    The circuit you have posted is not visible. Can you check again to make sure you have uploaded it?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'll give posting the pic a shot:

    [​IMG]

    [​IMG]
     
  4. ashish19183

    New Member

    Sep 16, 2009
    13
    0
    The circuit u send seems ok

    using 50% duty cycle you can do that

    if you still find issue check ur N-MOSFET ..... other component looks ok.....
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's not true, ashish.

    The basic problem is that there is no feedback from the output (voltage across the load) back to control the PWM percentage.

    The inductor is very large, so response time will be rather poor. There will be a considerable amount of overshoot of the target voltage. The 100k Ohm load resistor is a very high value; if the output is at 6v, only 60uA current will be required to maintain it.
     
    sa7her likes this.
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    To repeat what Sgt has commented, use a 12Ω resistor for load instead of 100K at the output and you will get a more realistic output voltage.
     
    sa7her likes this.
  7. sa7her

    Thread Starter New Member

    Oct 19, 2010
    8
    0
    Thanks for fast replys.
    It finally work with 5.6V. I spend the last month trying to find where is the problem. But I get 27A at the output, it should be abound 0.5.

    About the PWM, Dose any one advise me a good PWM circuit which provid me a frequency of 10KHz.


    and thanks again for evrey one helped me
     
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