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UCC28704: UCC28704 IVCC waveform

Lenna Yan
Intellectual 1960 points
Part Number: UCC28704

Hi Expert,

Customer is using UCC28704 on their design. When they test the load transient performance. They found some strange phenomenon on the VDD current. 

when adding 0.1ms and >0.8A load, the I_VDD current becomes higher even though the switching frequency is pretty low. While when adding 0.1ms and <0.8A load, there is no such current can be seen. Could you share your comment on why there's current difference here?

Test condition:

0 to high current load transient @264Vac

  • 0
    TI__Expert 7080 points

    Hi Lenna,

    If transient load on , the switching frequency will be higher ,then driver loss is increase , that is why I_VDD become higher . what I can't understand is why customer said the switching frequency is pretty low . I can see the VDD voltage is boost to higher , it indicates that much energy was transfer to AUX and output .

    And could you please ask customer to take a picture to show where the I_VDD current they captured . also I am little curious about why customer car about the VDD current , what is their concern about it ?

    Thanks.

  • 0 in reply to Jaden Ning
    TI__Intellectual 1960 points

    Hi Jaden,

    You could see the I_VDD in the figures I attached. The green waveform is the I_VDD waveform. The load only exits for 0.1ms, after that, it will go into no load condition. Description in the below figure will be more clear.

    on their previous design, the VDD capacitor is only 680nF, the VDD will trigger the UVLO when did load transient test. Increasing the capacitor can solve the UVLO issue. But during their debug, they tested the current on the VDD pin to see what happened here and found the current difference. 

  • 0 in reply to Lenna Yan
    TI__Intellectual 1960 points

    Hi Jaden,

    Thanks a lot for your offline discussion!