Soft start up

Discussion in 'The Projects Forum' started by Mustafatarhan49, Jan 3, 2013.

  1. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    Hi guys,

    Any idea how to make a soft startup circuitary for the buck converter? in the simulation of the buck converter design, i am having some overshoots in the start up due to the LC output resonance? And as i know, i can remove it by using some soft start up. is that possible to make the soft start circuitary? I googled a bit and most of the people use integrated soft start. i wanna know how do we design this soft start up circuits.

    yours,
     
  2. JMac3108

    Active Member

    Aug 16, 2010
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    66
    What buck controller are you using? They often have a note somewhere in the datasheet on how to implement soft-start.
     
  3. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    0
    Actally, i am just trying on the simulation without a controller.
    Please see the attached file
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The comparator is the voltage regulator. Add a resistor and large capacitor on the voltage reference, so on turn on the voltage reference slowly rises to finish at the desired voltage.

    At turn on, the SMPS will regulate the output voltage to match the rising voltage reference.
     
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  5. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    Thank you so much! Now i solved the overshoot problem :)
    I wanna ask what should be challenges if i want to build that circuit on board?
    What do you suggest me to do?

    Thanks
     
  6. JMac3108

    Active Member

    Aug 16, 2010
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    You can get better efficiency if you improve the mosfet gate drive circuit. Right now you actively pull the gate to ground with a transistor, but let it passively return high when the transitor turns off. An active gate driver would improve efficiency.

    Also you have a very large inductor. Did I read it correctly (1mH = 1000uH)? Have you done the calculations? Do you really need one this large?
     
  7. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    Correct! I would try a gate driver for the design circuit. can you advise any good gate driver?

    Actually, i know it is really big inductor but according to the requirements that was the minimum inductor that i can use. Here is the eqution;

    Vin=60;
    Vout= 6V
    fswitch=150000
    D=0.1
    Imax= 50mA
    Imin=15mA
    Delta(iL)= 50-15=35mA


    L = (Vin-Vout) x D / (Delta (iL) xfsw) = (60-6) x 0.1 /( 35mA x 150000) =1.02mA


    Do you think that is wrong ?
     
  8. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    Your equation is correct. And the inductor value is no problem since your current levels are so low.

    Off the top of my head I don't know of a gate driver that will work in your circuit. A little searching and I'm sure you can find one. Or make one.
     
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  9. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    0
    I have one more question. I wanna try synchronous buck converter by replacing the freewheeling diode with the pmos but i could not realized how to model mosfet driving seperately. Because i am only having one output from the comperator (PWM modulator) and i could not realize how to feed two mosfet with only one output that i have from the modulator :) Can you please help me ?
     
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