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UCC28780: UCC28780 Active Clamp Flyback Circuit doesn't start switching.

irfan yakut
Prodigy 40 points
Part Number: UCC28780
Other Parts Discussed in Thread: UCC27712

Hi,

I am currently working on a Active Clamp DC-DC Flyback Converter based on UCC28780 and UCC27712. If I succeed we will use this topology and components in various designs. But I am struggling. When I power up the circuit, Vdd starts to go up and down between 10V and 18V. Meanwhile PWML sends a single pulse, waits a while (maybe 1 second) then sends another single pulse. What am I missing here? I have calculated all the components according to the equatins which are in the UCC28780 datasheet. Please help me out.

Here are the parameters:

Vin is in between 30V and 60V.

Vaux:15V, Vout1:36V, Vout2:18V, Vout3,Vout4 and Vout5:15V

Best Regards.

  • 0
    TI__Guru 73595 points
    Can you probe CS pin? During start, the very first pulse has to make CS pin voltage reach 0.28V in 4 us. If not able to reach 0.28V within 4us, the IC takes this as a fault and recycle VDD UVLO. Please confirm this as what you see.

    Please provide your transformer primary inductance so we can have a look.
  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi Hong Huang,

    Thanks for quick answer.

    I probed the CS pin. Vcs is going over 0.5V in 25ns, then it is going down to approximately -0.25V in 25ns. After that, in approximately 50ns, it is attenuating and settling zero. Transformer primary inductance is approximately 20uH.

    I couldn't see the problem yet. I will re-calculate everything but I don't think there is a mistake. Looking forward to your answer.

  • 0 in reply to Hong Huang
    Prodigy 40 points
    By the way, PWML pulse width is 2us. I don't know if it makes any difference but I thougth I should mention that.
  • 0 in reply to irfan yakut
    TI__Guru 73595 points
    Please show the waveform of PWML, CS-pin, and switch-node (the low-side MOSFET Vds) so we can know if these waveforms are normal or something need to attention.
  • 0 in reply to Hong Huang
    Prodigy 40 points
    We re-calculated everything at least three times and changed components accordingly but it wouldn't work so we decided not to use this product. Currently working on another one so I am not able to share those information with you because of time limitation we have. It is a slim chance to go back and work on it in the near future but I will let you know if I start to work on UCC28780 again. This product is too complicated to use and way too sensitive.
  • 0 in reply to irfan yakut
    TI__Guru 73595 points
    We ask you provide the below in order to make analysis. If you do not have time to provide these at now, we need to close this thread per TI E2E policy. You can open a new thread when you have time.

    Please show the waveform of PWML, CS-pin, and switch-node (the low-side MOSFET Vds) so we can know if these waveforms are normal or something need to attention.