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UCC28711: UCC28711 operation

Punyashree Pattanayak
Prodigy 60 points
Part Number: UCC28711
Other Parts Discussed in Thread: UCC28C43

Dear Sir,

I am designing a DCM flyback converter, in which my transformer is designed for 100kHz frequency.

From UCC28711 datasheet, I am not able to understand, how frequency (100khz) is being maintained or controlled. 

Can I define the frequency in the IC? or Do I need to consider any fixed frequency for UCC28711 and accordingly design my transformer?

Again, by calculation, my max duty cycle is 0.468. Can there be any instant where duty cycle will exceed my calculated max value? If yes, will there be any adverse effect of that?

  • 0
    TI__Expert 7395 points
    Punyashree,
    Thanks for your interest in TI and the UCC28711 here.
    It's a good question. This part and the family of dedicated PSR flybacks are not like the UCC28C43 or other classic controllers used by flybacks where you would set a fixed frequency with a resistor or Resistor+Capacitor combination.
    Note that UCC28711 is a variable frequency controller. As you lower the load current for example the frequency decreases to keep the overall efficiency higher. You design for setting the maximum frequency based on maximum power and minimum input voltage. The frequency is set by the transformer inductance, turns ratio and the setting of the peak current. This can be set and designed using the excel design calculator for the UCC2871x family here: www.ti.com/.../toolssoftware
    If you demand higher duty than what can be provided, this controller will not enter CCM, you just won't delivery the power needed. As such, proper transformer and peak current setting design is critical.
    Regards,
    John
  • 0 in reply to John Stevens
    Prodigy 60 points
    Dear Sir,

    Thank you for your reply. This was definitely helpful.

    I couldn't find/open the excel sheet that you are recommending, but I did manage to find the formulas in the datasheet for UCC28711 and used those to calculate turns ratio, primary inductance etc. for a max. operating frequency of 100kHz. I hope that is okay.

    Once I got these values, I am choosing numbers for my design that might be close enough, but not always exact as sometimes that does not seem to be possible. One question I had - is there a way or a formula to find out the maximum frequency of operation given that I have values like primary inductance, turns ratio etc. This would help me verify and validate my design through reverse calculation. I just want to ensure my max. operating frequency is still close to 100kHz.
  • 0 in reply to John Stevens
    Prodigy 60 points

    Hello John,

    Thank you for your reply. This was definitely helpful. 

    I couldn't find/open the excel sheet that you are recommending, but I did manage to find the formulas in the datasheet for UCC28711 and used those to calculate turns ratio, primary inductance etc. for a max. operating frequency of 100kHz. I hope that is okay. 

    Once I got these values, I am choosing numbers for my design that might be close enough, but not always exact as sometimes that does not seem to be possible. One question I had - is there a way or a formula to find out the maximum frequency of operation given that I have values like primary inductance, turns ratio etc. This would help me verify and validate my design through reverse calculation. I just want to ensure my max. operating frequency is still close to 100kHz.

    Awaiting your reply... 

  • 0 in reply to Punyashree Pattanayak
    TI__Intellectual 880 points

    Hey Punyashree,

    Sorry that you had a difficult time locating the calculator, I'll link it here: www.ti.com/.../sluc590

    For future reference, the calculators of the devices are at the bottom of the tools/software page of the product.

    As for your frequency question:

    At the bottom of section 8.4.1, Figure 16, of the datasheet shows how the devices frequency changes dependent on the control voltage. The maximum operating frequency of the UCC28711 is 100kHz, so depending on your output voltage, you will be working close to that frequency.

    Here is the datasheet section 8.4.1:www.ti.com/.../detailed-description

    Regards,

      Davit