how to calculate inductor value of buck boost converter for range of dutycycle

Discussion in 'The Projects Forum' started by gobi615, Dec 5, 2012.

  1. gobi615

    Thread Starter Member

    Jul 26, 2011
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    Hi i am designing an inverting buck-boost converter . I have to calculate inductor value for range of duty cycle (0.436 to 0.73) i searched in book it only explained calculating inductor value for single duty cycle . Is there any way to calculate inductance for range of duty cycle .
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Use the formula to calculate the inductor value for both your max. and min. duty cycle. Use the largest inductance value of the two for your design. An inductor larger then you need will always work but a smaller one may not.
     
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  3. slapshot136

    New Member

    Oct 31, 2012
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    correct me if I am wrong, but a larger inductor will make the converter less sensitive and thus have noisier output, while a smaller inductor won't be enough to protect it from voltage spikes?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    "Less sensitive" to what? The inductor stores the energy for the voltage conversion during the off time of the transistor. The inductor has nothing to do with protecting the transistor from voltage spikes. A larger inductor will reduce the ripple current in the inductor and thus the output voltage ripple, which is normally a good thing.
     
  5. slapshot136

    New Member

    Oct 31, 2012
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    less sensitive to load changes - in that case, is there any down-side to using a larger than necessary inductor?
     
  6. #12

    Expert

    Nov 30, 2010
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    There is always a limit, somewhere. Too much inductance changes the switching frequency or the duty cycle, depending on which way the converter works. That will change the required output filter capacitor size. That's why you're always calculating for "optimum". Good design costs less money than bad design.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    To a first order an increase in the inductance above the needed design value does not have a large effect on switching frequency or duty-cycle. But, as you say, for a good design, from the standpoint of cost and inductor physical size, you don't want to go much above the minimum value.
     
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  8. gobi615

    Thread Starter Member

    Jul 26, 2011
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    Thanks for reply . What about capacitor whether it should me minimum or maximum ?
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    "You can't put too much capacitance in a power supply."
     
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