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PWM controller circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Archit, Mar 4, 2018.

  1. Archit

    Archit Member

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    I am using Texas Instrument SG3524 PWM controller for 400Khz but I am getting constant output and not pulse (100% duty). Someone please tell where I am doing wrong or can share the circuit.
     

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  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Do you have Vcc=10V ? Should have a capacitor from Vcc to GND.
    Do you have "Vref on the left side connected to Vref on the right side?
    Is the switch on pin 10 open or closed?
    1k pot should be turned so pin 4 and pin 5 have almost the same voltage.
    upload_2018-3-4_6-34-20.jpeg
     
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  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Are you using component values exactly as shown in that datasheet test circuit?
    What are your Ct, Rt values?
     
  4. dave miyares

    Dave New Member

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  5. Archit

    Archit Member

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    I am using 5K pot for Rt and 1nF for Ct.
     
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Trouble shooting from 1/2 way around the world is hard. You must give much more information.
    Do you have a oscilloscope?
    What is happening on every pin? Voltage, wave form, etc
    Example: CT should have a saw-tooth wave form. What about OSC OUT? Vref?
    upload_2018-3-5_7-31-10.jpeg
     
  7. Archit

    Archit Member

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    I have every component as shown. I have connected as shown but I am not able to get full range of duty cycle but only 75% to 95%.
     
    Last edited: Mar 5, 2018
  8. dave miyares

    Dave New Member

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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    That is how it works. That is what it should do when connected as shown.
     
  10. Archit

    Archit Member

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    But how to get full range?
     
  11. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I think not. See picture below. The block "T" will keep duty cycle below 50%. (only one output can be on at a time) If you connect COL 1 and COL 2 together than yes you can get 90%.

    Sawtooth ramp on CT is 0V to 3.5V (about) The voltage at COMP is compared to the sawtooth ramp. This is where duty cycle is made.

    Inputs(-) and Input (+) only work right is they are from 1.8 to 3.4 volts. In your test circuit there is a 2k/10k Pot/2k. It limits the voltage on Input(+).
    Because INPUT(-) is connected to COMP the gain of this amplifier is one. So the voltage on COMP will be the same as the arm of the pot.
    To get less duty cycle the voltage on COMP needs to be lower. There are two ways.
    1) pull down on COMP to force it lower.
    2) increase the gain on the error amplifier.

    upload_2018-3-5_12-23-39.png
    Back to your circuit. I added two 10k resistors to make a 2.5V.
    I cut open pins 1 and 9 and replaced with 100k resistor. Now the error amp has gain!
    Now when you move the 10k pot; low voltage = 0%, high voltage = max duty cycle. When the pot output voltage is near 2.5V you will see the duty cycle change. Now the COMP voltage can change over a larger range.
    upload_2018-3-5_12-36-39.png
     
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  12. Archit

    Archit Member

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    Can you please send me the link to SG3524 LT spice model? I can't find it in the yahoo group.
     
  13. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Here's an SG3524 model I created :
     

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  14. Archit

    Archit Member

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    You are a genius. Thanks a ton.
     
  15. Archit

    Archit Member

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    I think it is not working or I may be doing some mistake. Please rectify.
     

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  16. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I don't know what your circuit is supposed to do, but the IC pins aren't being used in the normal way and the pins don't line up with connections to other components in the sim.
    If it helps, here is a test file for the model.
     

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  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    What sub-directory do I put these files in?
     
  18. Ian Rogers

    Ian Rogers User Extraordinaire Forum Supporter Most Helpful Member

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    I put the .sub in the ltspice/lib/sub directory and just double clicked the .asy and ltspice installed the symbol...
     
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