How to design the feedback loop for the current-mode buck converter?

Discussion in 'The Projects Forum' started by Zhang Jian, Dec 31, 2014.

  1. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Dear all:

    I am building a buck converter to charge a battery, and the battery and the buck converter have been built, but I need a current mode feedback to charge the battery in a constant current.

    I will upload some files to help you to understand.

    In the first file, you can see the feedback is composed of compensator and modulator (comparator). But this document tells me how to build the feedback in voltage mode not current mode. I know I also need compensator and modulator in my current feedback. and I am not sure the difference of the two modes, because I need keep the current constant. What is more, I think you can understand what I want to build from the first document, because I think it is quite similar.

    And my circuit is like file2, and I will upload the Ltspice file too, if you can download the Ltspice which is free you can check it out.


    Thanks and Regards,
    ZJ
     
  2. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    The whole circuit with voltage mode feedback and battery.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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  4. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hello ronv:

    Thanks, but I need to build the circuit in LTspice, and I cannot find the Ltspice file like ZXTC1009.

    Regards,
    ZJ
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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  6. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hello ronv

    Thanks, but I think this document cannot conclude the feedback loop I need or I cannot understand this document.

    Regards,
    ZJ
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That circuit seems to be rather complex for just charging a battery.
    What is the supply voltage, the battery voltage and charging current?
     
  8. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hello

    The supply voltage is 40V, but the output voltage can be controlled by the duty cycle of PWM, so the output voltage is equal to the input voltage multiply the duty cycle.

    the battery voltage is set to be 3300MAh. And the charging current should be controlled under 1A.

    Thanks.
    ZJ
     
  9. crutschow

    Expert

    Mar 14, 2008
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    3300mAh (I assume you mean milli not Mega Ah) is the battery capacity, not its voltage.
     
  10. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hello

    Yes, it is milli and the voltage is around 5V.

    Thanks
     
  11. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hi

    I think it should be built by compensator and comparator, but I have no idea how to set the value of components.

    And what is the difference between the voltage mode and current mode when I build the feedback?

    Thanks.
    ZJ
     
  12. crutschow

    Expert

    Mar 14, 2008
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    As implied by their name, voltage-mode feeds back the output voltage for regulation, and current-mode feeds back the output current (typically from a small resistor in series with the load) for regulation.
     
    Last edited: Jan 4, 2015
  13. crutschow

    Expert

    Mar 14, 2008
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    Below is the simulation of a simple hysteretic (bang-bang) current feed-back converter. It shows an average battery charging current of 1A with a 11 mA ripple current.
    An advantage of the hysteretic converter is that it requires no compensation in the feedback loop.

    Battery Chg.GIF
     
    Last edited: Jan 4, 2015
  14. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hello crutshow:

    Thanks, and I think the difference between the hysteretic converter and buck converter is the hysteretic need the capacitor, right?
    then you choose another way to build the feedback.

    Regards,
    ZJ
     
  15. crutschow

    Expert

    Mar 14, 2008
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    What capacitor?
    The converter I posted is a (hysteretic) buck converter.
     
  16. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hi crutschow :

    Thanks I understand, and I mean in the buck converter it usually has a capacitor in the output.

    Regards,
    ZJ
     
  17. crutschow

    Expert

    Mar 14, 2008
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    You don't need an output capacitor for a current-mode hysteretic converter.
     
  18. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    My question would be, how accurate do you really need this charge current to be? Normally for charging a battery you dont need a super accurate current level such as you get from a full blown error amplifier/integrator type feedback system. In fact, if it varied from 80 percent to 120 percent of nominal it would probably still be ok. Normally only the voltage needs to be regulated with any pseudo high accuracy. A relaxed specification on the charge current accuracy means a simpler circuit is possible.
     
  19. Zhang Jian

    Thread Starter New Member

    Dec 31, 2014
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    Hi MrAl:

    Thanks. Actually, I do not have a idea about the accuracy, because I want to build this feedback loop to stimulate the charging process as practical as possible, so I want to build the feedback loop, as when you charge a battery, the impedance is always changing, so I should keep the charging current constant which is my purpose to build the current feedback loop.

    Regards,
    ZJ
     
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