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UCC28704: UCC28704 VS sensing pin time window min limit

Zhenyu
Prodigy 140 points
Part Number: UCC28704

Hello, 

I am simulating a flyback converter using UCC28704 in PSpiceA/D. The problem is that even though the voltage pulse on VS pin reaches 4.06V, the control chip doesn't reduce switching frequency or the primary current amplitude. So the output voltage on the secondary side continues to increase. I suspect the reason is the voltage pulse of the aux winding has a width less than 0.5us and ucc28704 missed the timing to sense the voltage. What do you think? 

Attached is a screen capture of the waveform. The top plot shows primary and secondary currents. The bottom plot shows the voltage of VS pin. 

Thanks.

Zhenyu

  • 0
    TI__Genius 9130 points
    Zhenyu

    Yes you are correct, the Flyback reset interval is far too short, so the VS pin cannot be sampled.

    As shown on page 13 of the UCC28704 datasheet, at the end of the primary on-time, there is a blanking time tlk_reset to give time for the leakage inductance energy to be absorbed by the primary clamp. After this blanking time, the leakage spike and ringing should already have decayed, so that after this, the VS pin follows the scaled secondary winding waveform.

    The blanking time tlk_reset is not fixed, it varies automatically with the commanded peak current, since the leakage energy and reset time will increase with higher peak current. The blanking time is 0.75 us at Vcsmin (0.25 V) and increases linearly to 3.0 us at Vcsmax (0.75 V).


    Clearly, in your plot above, the flyback interval is already only 0.5 us, which is below the minimum blanking time even for minimum peak current.

    I would recommend that you should double check your calculations and power stage design (choice of Rcs, transformer turns ratio, and magnetising inductance) using the datasheet step by step design example guide (pages 25-30 of the datasheet), or else use the Excel design calculator that is available on the "tools & software" tab of the UCC28704 product folder:
    www.ti.com/.../sluc603


    I hope this helps answer your question, if so please click the "verify answer" button.

    Thanks,
    Bernard