Digitally Controlled Buck Converter

Discussion in 'The Projects Forum' started by Mustafatarhan49, Oct 11, 2012.

  1. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
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    0
    Hello,

    I want to ask a question regarding to output controlling mechanism for Buck Converter.
    Designers mostly use analog control circuits, which are created from a comparator and error circuit, to stabilize output voltage/current.
    How does this work if I want to do this operation digitally with uC?


    Thanks
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    You will still need to measure the analog output voltage (or other quantity). This is done with an Analog to Digital Converter (ADC). Everything else is done digitally, usually with a micro-controller. Usually, the ADC will be part of the micro-controller chip.

    This allows software control of the converter.
     
  3. RamaD

    Active Member

    Dec 4, 2009
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    33
    Si8250 from Silabs is made for such application. It provides multiphase PWM, an architecture for control loop updation at some fairly high frequency, control loop parameter changes on the fly, etc., !!!
     
  4. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    JDT, Yes, You are right ;) Let's look from other point.
    Imagine if I have 8 bit ADC which has a 255 level and it corresponds to 19.61mV between levels. Let's say output of my converter is 6V. We have to bring down that voltage to the 5V with the zener or voltage divider will cause a resolution problem :confused: My whole concern is because of the resolution of ADC... Since I will have some ripple in the output that will reduce resolution of the ADC. Is not it? How can we fix this problem then?


    I saw the datasheet for this controller. It is expensive! I checked from the digikey and it says 3.7usd (here is the link : http://www.digikey.com/product-detail/en/SI8250-IM/SI8250-IM-ND/964010)

    Assume that we have 8bit controller (really simple and cheap one! :D)
     
  5. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    True, the ADC resolution is important. 8-bit not really good enough. 12-bit would normally be a minimum spec. The ADC does not have to be inside the micro-controller. In fact it might be a serial device (I2C or SPI) isolated from the controller.

    BTW Many micro-controllers have built-in PWM units. This would be used to control the switching device. The resolution of this is also important but not so much as the ADC because the switch is before the output filter.

    Most PIC micro-controllers have (for example) ADC and PWM included.
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    You also must have a compensation network to stabilize the loop against oscillations from the resonant LC output filter. In a digital controller this is done with a PID or Fuzzy Logic program loop.
     
  7. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    Yes, That's true. I missed that part :)

    But, I actually don't know why you said 12bit would be min spec Instead of using 8 bit controllers :confused:

    One more thing that I want to ask,instead of using Opamps and voltage divider, what if i make the operation like below;

    [Buck Module (Bring down voltage 50V=>7.5V), Voltage Divider (6V => 3.3 or 5V), RC filter (avoid ripples to read ADC value), PID controller (tabilize the loop against oscillations from the LC filter) ]

    Do you guys think that will work out too? :confused:

    Please check the attached file. I tried to bring down the voltage (50V to 7.5V) with the buck, I used voltage divider, than Follower and to remove offsets and restore the gain, I used U2B. At the end I have put and low pass filter for ADC. I did not know how to put use POT here in Orcad, that's why I did not finished it completely (I am googling it):D But the the schematic will be roughly like that...
     
  8. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Nothing to stop an 8-bit controller processing data with more than 8 bits!
     
  9. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    That's true, no objection:)
     
  10. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    You don't really need the amplifiers before the ADC input. Just feed the divider voltage directly to the ADC.

    8-bits is fine if you don't mind a 1/255 * 7.5V ≈ 30mV ripple in the regulated output voltage.
     
  11. Mustafatarhan49

    Thread Starter Member

    Aug 16, 2012
    56
    0
    I believe it is the only the question of how sensitive I wanna have my measurements. But as you said before, it is a must to use PID or fuzy logic for compensation.
     
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