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UCC28C42: UCC28C42 VDD and GND getting short after turning ON

Mohammed Nazeer Ahmad
Prodigy 20 points
Part Number: UCC28C42

Hi,

We are using UCC28C42 floating buck converter to generate 15V output. The input is 3phase but i used single phase only to power this IC and circuit. We are facing problem that once we power ON the circuit the VDD voltage goes on increasing through R4 and R5 and once IC gets input voltage close to 15V which is turn ON voltage of this IC, it immediately gets failed and pin 7 & 5 is showing short or less impedance close to 150 ohm. Even after placing zener of 15V across the pin 7 & 5 also no improvement and IC still failing. we dont observe reference voltage as well. we have removed AC input and used DC input after rectifier of 32V but still similar failure observed, we dont see any output and IC gets failed (Pin 7 & 5 less impedance). Please help to debug and suggest any changes are needed. we have done below changes and tested but no positive result.

We have total 400uF capacitor at the input side, in schematic only 2 is visible.

Also i have tested the demo board of UCC28C42 which has 15V/1A output and after that prepared this schematic.

1- C9, Q2, R23 removed. Same failure

2- R20,D6 removed. same failure

3-C1 changed from 3300pF to 2200pF. same faiure.

  • 0
    TI__Expert 3240 points

    Hello Mohammed,

    In your schematic, I see that you have an additional 15V rail that is powering on the VDD. Is this biasing the UCC28C42 in addition to the AC input? The VDD max of the UCC28C42 is only 20V so if you are testing with a DC voltage of 32V then the chip will not work. Do you have any startup waveforms that show  the behavior you are describing where the voltage is increasing across R4 and R5 until it reaches 15V and breaks down?

    The below snippet is from page 36 of the UCC28C42 datasheet.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 20 points

    Hi Jonathan,

    you are right, the startup sequence is like IC will initially get power from input source and once it generate 15V, this will bias the IC and will keep the voltage at the VDD to 15V. Now the scenario is IC is getting power from input and once it reaches to 15V and again starts decreasing and goes to 9V then again starts increasing. Like triangular waveform. i will capture some waveform and will share with you.

  • 0 in reply to Mohammed Nazeer Ahmad
    TI__Expert 3240 points

    Hello Mohammed,

    I look forward to seeing the waveforms to help support your problem.

    You said that you tested VDD at 15V but in the original post you said that you tested with 32V. Is the 32V coming from the VDC_High rail? Also, you mentioned that the power reaches 15V, decreases to 9V, and then increases again. Does this increasing voltage eventually return to a stable 15V?

    Regards,

    Jonathan Wong