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UCC256404: VCC current higher than expected

Jason Wong
Prodigy 10 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC28070, UCC27517, PFCLLCSREVM034

Hello,

I am using UCC256404 in my design. I noticed that the chip is running a little hot. Thermal imaging shows that the case temperature of the chip is about 60C at ambient room temperature. I measured the VCC supply current and I am getting about 90mA - 100mA. The datasheet indicates that the ICC run current is about 2.5mA or so. So I would like to check whether is this expected. 

I also have another question with regards to the RVCC regulated 13V supply. From the block diagram in the datasheet, it is shown that there is an internal LDO that regulate the RVCC from the HV input pin. HV input is coming from a rectified AC line. If the AC line is 240VAC, this would also mean that there is big delta voltage between the LDO input and output. If RVCC current is high (~100mA), wouldn't this cause a big power dissipation within the chip itself?

Lastly, is there any relationship between the RVCC current to the VCC current? Example, if RVCC is pulling 100mA, VCC current would also increase accordingly. Does not seem so in the datasheet but would like to confirm this.

Thank you in advance.

Regards,

Jason

  • 0
    TI__Guru**** 189401 points

    Happy New Year! One of our experts will be back in office after the holidays and answer your questions. Thank you! 

  • 0
    TI__Genius 16155 points

    Hi Jason,

    Could you share your schematics. That should help me to debug why the IC current is too high.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 10 points

    Hi Manikanta,

    I cannot share the schematics here due to company policy but the use case design is very similar to the evaluation board, PFCLLCSREVM034. The UCC256404 is used in conjunction with the PFC controller, UCC28070. However, the gate driver output of UCC256404, HO and LO does not drive the LLC FET directly. Instead, it is fed into 2 external gate driver, UCC27517. The bias for the gate driver is from RVCC.

    Hope this helps.

    Regards,

    Jason

  • 0 in reply to Jason Wong
    TI__Genius 16155 points

    Hi Jason,

    You can share the schematics to me through e2e direct message so that no one else will have access to it. It would help me to locate the issue.

    Jason Wong said:
    I also have another question with regards to the RVCC regulated 13V supply. From the block diagram in the datasheet, it is shown that there is an internal LDO that regulate the RVCC from the HV input pin. HV input is coming from a rectified AC line. If the AC line is 240VAC, this would also mean that there is big delta voltage between the LDO input and output. If RVCC current is high (~100mA), wouldn't this cause a big power dissipation within the chip itself?

    During HV startup, the voltage drop will be across HV mosfet and the series resistors with the HV pin. Not across the LDO.

    The LDO will only be enabled after VCC reaches VCCstartswitching threshold (This can be seen in above picture). So the max voltage drop across the LDO during start up is VCCstartswitching voltage (26V). Once the LLC starts switching, VCC is supplied by the aux winding which will be very close to RVCC voltage. So, during steady state operation, LDO voltage drop will be very minimal. 

    Could you try reducing the loading on the RVCC pin and see if lowers the temperature of the IC.

    Regards

    Manikanta P