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UC3823a

Prodigy 100 points

Replies: 17

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Hello. I'm building a buck dc/dc power supply with the UC3823a pwm controller. How do I control the Pwm signal? I don't understand how i should calculate the value of my resistors and capacitors. Can anyone help me understanding the pwm controller.
     

  • Morton,

    I'm not certain where your inputs and output are in your schematic so I will start by pointing out some obvious errors.

    On the extreme left. The NPN transistor needs to be inverted. Swap the emitter and collector.

    Same with the FET. The way it is configured the body diode will be conducting all the time.

    You have both outputs connected with Shottky diodes which are then shorted. Save yourself the components or put them from ground to the outputs to prevent the outputs from going below ground in operation.

    Your voltage sensing is being done off the collector of a PNP transistor. Not a good idea. Go to http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua125&fileType=pdf and look at fig 17 for an idea on how to do the feedback.

     

    John Bottrill

  • In reply to John Bottrill:

    Hello 

    Thank you for your answer.

    I still don't understand how to chose the resistor values to get the voltage that I want. I want an output voltage of 14 V (I'm going to charge a battery). The input voltage varies a lot. It is the rectified voltage from an alternator on a motorcycle. I think the voltage will vary from 20-180 V

    About the sensing. Do you think i should drop the PNP transistor? This design is copied from a buck power supply for motorcycle, but i don't understand  how its working. Why is such a bad idea? 

    This is the schematic so far. I have corrected the errors.

     

     

     

  • In reply to Morten Paulsen:

    Morten,

    The circuit is configured as a shunt regulator. Far from the most efficient configuration. You  might want to review designs for flyback regulators.

    If you want to measure and control the Vout voltage with respect to ground, connect the R8/R10 junction to the C1/L1 junction and remove R5, R6, D2, T1 and R2.

    The voltage at the R10-R11 junction when Vout is in regulation should equal the voltage on the error amp non-inverting terminal (5.1V) inside theUCC2823A.

    Regards.

    John

    John Bottrill

  • In reply to John Bottrill:

     Thank you again.

     

    I have now removed all of the components like you said. So if i use R10 = 17,5K and R11 = 10k, I will get 5,1 V on the INV pin, when it is 14 V. 
    But what about C3, R8? And C4, R9, C6 between pin 1 and 3.

    Isn't this a buck regulator? That's what I taught I designed. 

     

  • In reply to Morten Paulsen:

    Morten,

    The regulation is done by the current through what you have listed as Rload. What voltage are you regulating with respect to what other voltage?

    The resistors and caps you questioned are part of the feedback loop poles and zeros.

    Take a look at the attached paper.

    Are you in university or industry?

    John

    John Bottrill

    SEM300 1984 switching power Supply topology review.pdf
  • In reply to John Bottrill:

    I'm in university. And I'm not having so much experience with electronics. (clearly). But I'm trying to learn this.  

  • In reply to Morten Paulsen:

    Is this better?   I will take a look in the attached paper. Again Thank you. 

     

  • In reply to Morten Paulsen:

    Close but the FET is in backwards and has to be transformer driven.

    John Bottrill

  • In reply to John Bottrill:

    This mignt help:

    How to Design a Discontinuous Flyback Converter”

    Pages 74-79: http://www.bodospower.com/restricted/downloads/bp_2012_05.pdf

    John Bottrill

  • In reply to John Bottrill:

    Why do you need frequency compensation, regarding c3,r8,c4,c6 and r9? Is it necessary? I don't think i understand the meaning of it? Could I just skip c3, r8 c4 and c6. 

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