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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.
     

  • In reply to Morten Paulsen:

    Morten,

    The attached should give you some idea about feedback loop. I think you should be discussing these issues with your Prof. That's what he is being paid to do.

    John

    John Bottrill

    SEM800 1991 Control Loop Design.pdf
  • In reply to John Bottrill:

     

    Hey again. 

    I having some problem switching my mosfet on and off. This result in a warm mosfet. How can i get it work better?

    This is the pwm signal, and the voltage on the gate of the mosfet.

    Schematic:

      

     

     

  • In reply to Morten Paulsen:

    Morten,

    Your gate voltage has to be higher than the source to have the FET "ON" and the drive impedance (100 ohms) you are using looks to be to much.

    John

    John Bottrill

  • In reply to John Bottrill:

     

    The gate voltage is going from 0 to 30 V for switching the mosfet on and off. The schematic is working, until the mosfet get to hot. I Tried up to 150 VAc in today, and the problem is that the mosfet gets superhot and breaks. There is obliviously too much heat loss in the mosfet. And i dont under stand why. The gate signal was better than the picture from my last post. Would this design work at all? Could it be an idea to put the mosfet between ground and regulate diffident ground planes?  

    Morten

     

  • In reply to John Bottrill:

    Hello again. 

    The data sheet says that  there should be an 1 mF on Vref. Isn't 1 mF a lot? Why is it so high?

     

  • In reply to Morten Paulsen:

    Morten,

    The Terminal Function Table on page 3 says the pin should be bypassed by 0.1uF ceramic LOW ESR LOW ESL  cap with minimal trace length.

    Figure 14 page 11 shows it as 0.1uF.

    I suspect that the 1 - mF on page 10 is the difference in the type font where the "m" of the "mF" is not "symbol" font.

    I would expect that a 1 uF cap would be acceptable even with the other two places specifying 0.1 uF

    Regards,

    John

    John Bottrill

  • In reply to John Bottrill:

    Hello

    Now when I'm testing, I gets some problems. The the duty cycle gets low very fast. It goes from about 60% to about 5% when the input voltage goes from 22-30V. When the duty cycle get to low, the regulator stops. It seems that the mosfet does not get enough time to switch on and off, when the duty cylce get very low. I'm running the duty cycle at 40KHz, and tried down to 10 kHz, but I'm still having the same problem. 

     Also the source voltage when the mosfet is off, gets a lot of ringing at a ~50 % duty cylce. Why? Do I need to change something in my buck circuit to improve this? See the picture. I'm also attaching the circuit as it is know

    Ringing:. BLUE (Source voltage on mosfet.)

     

     

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