Problem in designing the power supply dc to dc with ucx84x

Discussion in 'Power Electronics' started by qwwe, Apr 28, 2018.

  1. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    Hello
    I want to design an dc to dc converter with input voltage of 9 to 30 volts dc with cuk topology. For this I chose the uc2843 IC.
    But in the design of the feedback part and the capacitor and the resistance of the compensator connected to the comp base and the resistance and the capacitor connected to the base of the vcc (which I specified in color), I have problems because I did not find any good guidance.
    Thanks for the advice.
    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That's a somewhat complex circuit to use.
    Read section 9 in the data sheet.
     
  3. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    An example in Section 9 is Transformer
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
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  5. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    Yes, I have seen these but my problem is in calculating the feedback and compensator that is used in these pdfs for the case that is used with opto-coupler and trans
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Could you post a circuit showing the proposed components, opto etc.?
    Your initial diagram does not represent what you are asking.
    E
     
  7. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    I want to design a circuit like this, but with uc2843:
    [​IMG]
     
  8. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    No one can guide
     
  9. Aplicant

    Member

    Feb 23, 2017
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    4
    Put 12v regulator (7812) to vcc if input voltage is greater than 20 or more
    Put 100k and 100pF in parallel across FB and Comp
    What voltage do you want at output
    Feedback resistors are calculated according to the output voltage.
     
    DickCappels likes this.
  10. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    Thank you for your guidance
    The input voltage varies from 9 to 28 volts.
    Output voltage is 21 volts.
    How do you calculate the resistance and capacitance associated with vfb?
     
  11. ian field

    AAC Fanatic!

    Oct 27, 2012
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    There are 4 devices in that family with overlapping common specifications - 2 types for direct off line applications and 2 types for automotive. there are 2 different UVLO specifications, each with 50% & 100% duty factor output.

    can't remember the UVLO lower thresholds, but you won't be going down to 9V unless you run the chip off an auxiliary booster.
     
  12. Aplicant

    Member

    Feb 23, 2017
    32
    4
    Put Rtop =50k
    Put Rbottom = 6.8k
    And i think your output will almost be 21v
    Compensation resistor and capacitor are well calculated. It is a guess
    They are usually good values (compensation resistor and capacitor)
    Try it and write how it wokrs
    Aplicant
     
  13. Aplicant

    Member

    Feb 23, 2017
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    Sorry i was going to write :
    Compensation resistor and capacitor are not very well calculated instead of compensation resistor ad capacitor are well calculated
     
  14. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Probably easier with an MC34063 - there's tons of examples online.
     
  15. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    thanks for the reply
    But I only have difficulty calculating the capacitor and compensating resistor, but it's an example in a datasheet that is calculated in terms of zero and amplifier poles and etc., which is complicated.
     
  16. crutschow

    Expert

    Mar 14, 2008
    20,035
    5,637
    Yes, designing proper feedback compensation for SMPS is complicated.
    If you want simple, use one that already has the components designed for you.
    Otherwise, the attached may help.
     
  17. ian field

    AAC Fanatic!

    Oct 27, 2012
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    There are some calculator apps, an online one and one you download and install.

    They give you the inductor value if you tell it frequency and how much current. The downside is it only applies to the bare chip - if you use an external transistor for higher current, you have to interpolate which requires more knowing what you're doing.
     
  18. qwwe

    Thread Starter New Member

    Jul 18, 2017
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    Thankful
     
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