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UCC28780: ucc28780

robin gangopadhya18
Mastermind 6326 points

Part Number: UCC28780

Ulrich:

Thanks much for review of the results I sent only last night.

Yes: I think we got to close postings. They become lengthy & confusing.

I did not select " reply"  to the last post from you this morning...

Here is what I note with alarm: from the Debug document

I do not even get REF & RUN going hi after VDD reaches 17.5 V as we can see in the results.

I have double-checked all values: these are close to what I sent you in xl rev C. So RTZ & BUR values are as you suggested & as obtained from SLUC664 revC.

What I am struggling with is to hunt down what would cause VDD to start discharging as soon as it hits the peak.

It does not seem to come from outside.

I double checked the soldering at the controller- from outside they look OK.

YET: there MUST be something not right. At this initial startup hardly anything in layout would matter...other than connection to /from controller pins.

BUR & RTZ are all connected  as verified up to the controller pads accessible  at the boundary of the package.

CS, SWS, HVG   appear to be connected.

Any clue you can get from the way SWS & HVG  are exactly the same?...they are not shorted for sure..

 I removed source-gate resistor 1 Meg to explore if HVG indeed coming out of the chip & not from source. At low input voltages, we risk no damage....will report soon what we see during startup.

Any impact of having hi ESR caps at VDD? I have X7R of value 20uF rest are tantalum. If I measure on LCR, of course, you won't get correct value beyond 10 kHz.

appreciate your help.

\robin

  • 0
    TI__Mastermind 40580 points

    Hello Robin,

    From your comments in this posting, it is apparent that you have not seen my reply to a previous post that addresses your concern about REF.

    Run will not go high unless REF is valid, and 3V is not valid.
    VDD will always start falling after reaching 17.5V (or whatever the specific threshold is for any specific device), because of the IC's self-loading and any external loading.  Normally this loading is a few mA and start-up will run off of VDD stored charge until Vout is high enough to reflect back to the AUX winding to keep VDD alive. 

    REF is powered internally by VDD and since REF appears to be externally overloaded, VDD will drop more rapidly than normal.  Also when REF is not valid, the HVG function will not work, so it will not go to 11V as expected.

    Please check your email to see my reply to the other thread this morning. It is the first one, in time, on this date (Sept 09, 2020). This reply, for reference, is my third one today, to a new separate thread with the same previous name.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Mastermind 6326 points

    Ulrich:

    Oh, so kind of you to respond! So promptly.

    Appreciate in these times of struggle...

    I thought I read your response early mo9rning here. - about REF. and others related.

    Then chose a "new question" posting...

    .For sure, like in all controllers, REF output is very important.

    I am digging into it..

    What we are doing  right now is to remove the controller.

    Install a new one differently- making sure we use correct temp to solder. Reflow not possible on this assy.

    BUT solder each solder pad on the chip to the pcb by a well-defined solder col. The part will be off the PCB by 5 mils or so. The GND pad will be connected after all this.

    This way we can see under the microscope every solder connection to the part. 

    Goal  is to get REF value right.

    thnx much