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UCC256303: Ho pin no output

PYTsai
Genius 10665 points
Part Number: UCC256303
Other Parts Discussed in Thread: UCC28063,

Hi, 

We are using UCC28063 + UCC256303 for a 500 W AC/DC project. We are now working on bring up the UCC256303 and see a symptom that Ho pin of UCC256303 no output waveform shown below. Any suggestion where should we check on UCC256303 circuit ? thanks!

  • 0
    TI__Mastermind 26735 points

    Hi PYTsai,

    Thanks for your interest in UCC256303. I would suggest making sure the boot cap is getting sufficient charge to supply HO. Also check the VCR waveform and make sure the common mode voltage is close to ~3V. Are you testing under any load? If you can share your schematic, I can take a look to see if everything looks ok.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 55 points

    Hi Benjamin,

    I'm using 1EVM-291 to modify and test.

    Vin=398Vdc, 12Vout, no load then turn on the external bias(+15V)

  • 0 in reply to Henry Liang
    TI__Mastermind 26735 points
    Hi Henry,

    Thank you for the additional information. During the LO on time, the boot capacitor should be charged by RVCC. From the oscilloscope capture you shared, it does not appear the boot cap is getting any charge. Could you share what modifications were made to the EVM?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 55 points

    Hi Benjamin,

     

    As attached file.

    MX-M362N_20180320_120205.pdf

  • 0 in reply to Henry Liang
    TI__Mastermind 26735 points
    Hi Henry,

    Thank you. Could you double check R16 and D6 on the EVM and make sure they have not been damaged? Based on the oscilloscope capture you shared, it looks like the boot capacitor (C18) is not getting charged at all. Could you confirm this? Do you see RVCC equal to 12V?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 55 points
    Hi Benjamin,

    R16 was damaged. EVM can work now.
    What reasons could happen?
  • 0 in reply to Henry Liang
    TI__Mastermind 26735 points
    Hi Henry,

    R16 could have been damaged from exceeding the power rating of the resistor but the likelihood of this happening is extremely low. R16 is connected to a regulated 12V rail and is used to limit the current going into the boot cap when the low side is on. Its not connected to anything that would possibly exceed the resistor power rating.I think the resistor was most likely damaged during modification of the EVM. I would not expect R16 to become damaged again during your testing of the EVM.

    Best Regards,
    Ben Lough