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WEBENCH® Tools/UC3843: 1000V 10W flyback SMPS design issue

Varad Ghate
Prodigy 150 points
Part Number: UC3843

Tool/software: WEBENCH® Design Tools

I have designed a 1000V,10W output flyback-SMPS with 12VDC input and frequency 1KHz using IC3843 and TI's Webench tool. I have calculated the transformer parameters using the IC datasheet and added the softstart functionality too. When I wired the components, the output voltage did not go beyond 200-300 V. However, when I remove the transistor connected to the ISENSE pin I get 1000V. I have added the DSO screenshot of the waveform observed at the ISENSE pin when the transistor is connected. How can I achieve 1000V with the transistor connected in the circuit?

ISENSE DSO snapshot.docx

  • 0
    TI__Genius 13660 points
    Hi Varad,

    In order to facilitate analysis, could you post your circuit schematic below? Thank you.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Prodigy 150 points

    Hello Teng, I have attached the pdf of the schematic.

    T10W-1K-3-TI - Schematic.pdf

  • 0 in reply to Teng Feng
    Prodigy 150 points
    Teng, are you there?
  • 0 in reply to Varad Ghate
    TI__Genius 13660 points
    Hi Varad,

    if the Duty cycle is designed over 50%, a slope compensation is required. However I think the slope compensation value to Isense pin too large in your circuit. You can try to increase R7 and R27. There is also a detailed introduction for slope compensation calculation in datasheet section 9.2.2.10.2.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    TI__Genius 13660 points
    Hi Varad,

    Did you solve your issue? If not, just post a reply below (or create a new thread if the thread has locked due to time-out), Since I am going to close this post soon.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Prodigy 150 points
    Hi Teng, thanks for the suggestion, I'll try it out but before that, I decided to go by the datasheet for calculation of value for each component since half of my components are from the datasheet and the other half from the WeBench. I decided not to rely on Webench but go by the purists' route and tried calculating the values of Rrt and Cct by the graphs and equations mentioned in the datasheet but they seem to violate some or the other rule.

    To calculate Cct, tdeadtime = 2.83us, by graph in Fig.9 (page 10), Cct is coming out to be 10nF
    and from equation fosc = 1.72/(Rrt*Cct) Rrt is coming out to be 1720 Ohm. But Rrt should be >= 5K. So how do I calculate the correct value of Rrt?
  • 0 in reply to Varad Ghate
    TI__Genius 13660 points
    Hi Varad,

    First, you calculated incorrectly.
    In the equation fosc = 1.72/(Rrt*Cct), fosc is in Hertz, Rrt is in Ohms and Cct is in Farads.
    In your requirement, fosc is 1000Hz, Cct is 10^(-8)F, So Rct should be 172kΩ.
    In the datasheet, the recommended range of timingresistor values is between 5 kΩ and 100 kΩ and never use a timing resistor less than 5 kΩ; the recommended range of timing capacitor values is between 1 nF and 100 nF.
    Though a timing resistor valued 172kΩ can be used, I suggest that you could amend operating frequency and dead time. You can select the reasonable value of Rct and Cct by combining picture 9 and picture 10.
    This thread is going to be dropped, please crate a new post if you need more help. Thanks.

    Regards,
    Teng