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UCC28750: UCC287502DBVR

Nguyen Duc Khanh
Prodigy 20 points
Part Number: UCC28750

Tool/software:

I am designing a 150W Flyback converter with 85VAC-265VAV input and 24VDC-6A output using IC UCC287502.

When I supply 15V DC voltage to the VDD pin of the IC, the converter works stably, 15V I get from an external voltage source.

When I run the converter with the IC, without 15VDC for VDD, I use the Aux coil for the VDD pin of the IC and start it with a 3MOhm start up resistor. However, when starting up, the output voltage does not reach 24V and the auxiliary winding voltage is not supplied, making the system inoperable. The waveform I give in the attached Figure below + Schematic. I still have not found out where the system error is, I hope to receive help.

Regarding the waveform image, CH2 is VDD, CH3 is VDRV of the IC. It can be seen that when starting to supply VDRV pulse, VDD voltage drops and is not fed by VDD voltage from the auxiliary coil, because at this time the output voltage only fluctuates from 0 V -0.34 V. I have verified and confirmed that the converter operates normally when applying a fixed voltage of 15V to the VDD pin of the IC.

Fig. EXP. waveform

Schematic: Schematic

  • 0
    TI__Expert 6265 points

    We cannot access google doc. Please upload the files here directly.

  • 0 in reply to Ning Tan
    Prodigy 20 points


    I have reattached the data. Thanks for the help! 

  • 0 in reply to Nguyen Duc Khanh
    TI__Guru* 78306 points

    Hello,

    I reviewed the schematic and functionaly it looks correct.

    You had mentioned the design work when biased sepeartaly but does not work when trying to power off the aux winding.

    You might want to check the turns ratio Na/Ns to make sure it can support your 15 V output?  

    VDD/Vout = Na/Np

    Regards,

  • 0 in reply to Mike O'
    Prodigy 20 points

    I am sure Vaux is 16V. I have solved my problem. But now I have a new problem I run the system at rated load, my system makes a buzzing sound from the IC. When the load is < 50% rated, my system is stable. The controller I designed works stable on simulation, I have a problem when I verify it on the circuit.

  • 0 in reply to Nguyen Duc Khanh
    TI__Guru* 78306 points

    Hello,

    Thankyou for letting us know that we had answered your original question.  I will close out this issue.

    To have this new issue looked into.  Could you repost this new question in the e2e under a new thread title with the part number and audible noise in the title, so we can have your issue looked into an addressed.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards and thanks,