Buck converter problem ...Please Help

Discussion in 'Power Electronics' started by xxnatjadexx, Jul 10, 2016.

  1. xxnatjadexx

    Thread Starter New Member

    Jul 10, 2016

    i need help with my circuit as it is not working as should.

    using the application notes(AN486) from silicon labs i have calculated the required bootstrap capacitor which is anything above 151nF .

    the problem is that when simulating this circuit in multi sims 14 ...the high output of the IR2011PBF stops pulsing the mosfet after 400uS...then the output voltage of the buck decreases til it gets to 6v then starts pulsing again. i have check varies points on the circuit and made sure undervoltage lock out is not activated on any of the pins according to the data sheet. and i have no control loop on this circuit so dont know why it is doing that

    i am totally lost as what is wrong ....

    input voltage 40V
    Freqency is 200Khz
    Duty Cycle is 0.225
    output voltage should be 9V

    [​IMG] [​IMG]


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    Last edited: Today at 6:20 PM
  2. crutschow


    Mar 14, 2008
    What's the voltage across C7?
    Try increase the value of C7 to 1μF or more.
  3. xxnatjadexx

    Thread Starter New Member

    Jul 10, 2016
    its 14V with around 1.3V ripple ..i have tried that and get same problem
  4. ci139


    Jul 11, 2016
    why rise - i gess it accumulates the carge for VS sensing it has to stay off (i never done anything with high-low side drivers - i don't want to go gessing here futher on)
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    A little hard to explain, but I'll try.
    The output voltage overshoots because there is no closed loop. When that happens the inductor current goes to zero so there is no inductive kick to make the left side of L1 go to ground so the boost cap doesn't charge.
    It actually works for me in Ltspice, but I don't have the same components.
    You could add another FET to make it synchronous or if you close the loop it will probably be ok.