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Original question:

UCC28C45: the unit of total power dissipation

UCC28C45: Different batch of ucc28c45 under different output current conditions, the output voltage will drop

Gabriel Wang1
Genius 4945 points

Part Number: UCC28C45

Tool/software:

Dear Expert

We found different batch UCC28C45 could have under different output current conditions, the output voltage will drop. 

Below is our sch ,could u give us some suggstion to find what the reason.

ucc28C45 Sch.pdf

  • 0
    TI__Guru 74075 points

    Hi,

    Your design is based on non data sheet specs. Basically you need modify R95 to be smaller value to about to 0.25k from 1k, and also short across R107 to 0.

    These changes are due to  COMP pin source current range which is not a data sheet specs so your design made on non data sheet specs for this parameter then need to make such adjustment

  • 0 in reply to Hong Huang
    Genius 4945 points

    Dear Hong

    We use R107 to 0 and the output votage still drop , the R95 is input votage is 12V so the source crrent is crrect.

    And I test FB pin found that when the input condition is 5A output, the FB pin voltage is 2.6V through C68. Measure the FB pin voltage with a multimeter and it will return to 4.65V. At the same time, the C68 voltage is also 4.65V and the output is normal. I noticed that the customer's FB pin is not used, but connected to the COMP pin through C68. Is there any risk in using it this way.

    Update the lastest verison of sch:

    储能-UCC28C45DR相关原理图-2024-8-6.pdf

  • 0 in reply to Gabriel Wang1
    TI__Guru 74075 points

    Hi,

    R95 needs to change to 0.25k or even lower so allow U20 diode current over 5.5mA when U21B output close to 0. The 5.5mA assumed with CTR = 1 of the opto coupler. COMP pin needs to reach below 1V to provide duty cycle close to 0 which helps the Vout regulation you currently see not ok.

  • 0 in reply to Hong Huang
    Genius 4945 points

    Dear Hong

    Thanks for your suggestion .But we change the R95 to 0.25k  is still same could not solve the output voltage is drop.

    But we let FB connected the GND and the output voltage is not drop since customer do not use the FB pin .

    Now could u tell cusotmer what difference between these two differnt CSO wafer from PCN ,customer just used these two different CSO and there application could cuase two different output.

  • 0 in reply to Gabriel Wang1
    TI__Guru 74075 points

    Hi,

    I am not understanding the results. Can you test the COMP pin voltage difference between new and old devices?

    The COMP pin voltage range for duty cycle is the same between new and old. The difference is the new needs wider source current up to 5.5mA which is roughly 2x the old. That is why suggesting to reduce the resistor R95 to increase the current range.

    So I need to know the COMP pin voltage difference to see if any clues. 

  • 0 in reply to Hong Huang
    Genius 4945 points

    Dear Hong

    Yello is output voltage ,blue is COMP pin voltage:

    \

    COMP脚DC波形图.rar

  • 0 in reply to Gabriel Wang1
    TI__Guru 74075 points

    Hi,

    I noticed you provided two schematics. U20 shows different connections.The latest schematics show 12V_iso is used for feedback but 5V_iso is cross regulated. Is your 12V_iso ok with both IC devices?

  • 0 in reply to Hong Huang
    TI__Guru 74075 points

    Hi,

    2A, New, 5V_iso, 4.64V to 5.04V

    2A, Old, 5V_iso, 4.56, to 5.12V

    Where do you think "New" has "Unormal" compared to "Old"? It looks New is even better than Old, right?

     

  • 0 in reply to Hong Huang
    Genius 4945 points

    Dear Hong

    Sorry for my chinese english ,new means new batch ,and old is old batch .

    Yes new batch seems COMP pin is better than old ,and we take the FB pin to GND and both new batch old batch output voltage is normal.

  • 0 in reply to Gabriel Wang1
    TI__Guru 74075 points

    Hi,

    So actually no problem.