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UCC28710: Starting Device at Heavy Output Load Condition

Matt Hoover
Prodigy 50 points
Part Number: UCC28710

Universal Input 85-265Vac to 24Vdc @ 0.4A output.

Everything is working, except when turned on with heavy output load UCC28710 goes into protection mode. If still powered, unplug the load, and plug load back in, output voltage goes back up to 24Vdc, and current back to rated at 0.5 Amps.

So if an end user were to use this, and the power goes out, and comes back on, then they would need to manually go to where ever power supply is installed, and open and close the load. 

What would be the best application recommendation on the output so make sure the UCC28710 is up and running, before it powers the output for the heavy load.

Using primary side auxiliary feedback.

I was thinking maybe a voltage detector, but wanted some advice or direction from an applications engineer that may have ran across this before.

Thanks for any feedback, or pointers to another similar question posted as I couldn't find one like my question.

  • 0
    TI__Expert 6080 points

    Hello Matt,

    Thank you for your interest in UCC28710.

    I need to understand what kind of fault is triggered at heavy load? There are two possible faults, over current and over voltage. I will recommend looking at the VS and CS pin voltage when the fault is triggered.

    What are the line and load condition when the fault is triggered? By heavy load do you mean Full Load?

    It will be useful to see your schematic.

    Regards,

    Sonal

  • 0 in reply to Sonal Singh
    Prodigy 50 points
    Schematic is basically what WEBENCH spits out from UCC28710 page with 85-265Vac input, with 24Vdc @ 0.5A single output. I put pots in for Rfbb and Rfbt to get my voltage output, and experimented with different values as according to the design report that WEBENCH provides. I will take a look at the VS and CS pin when get bench set back up with scope.

    For starters, using 120Vac as line, and yes, by heavy load I mean the full load of 0.5 Amps. I do have a 1.6kohm resistor with an LED also in parallel with the load, and a 120uF Capacitor. Maybe the cap needs to charge first, so maybe doesn't cause a short current surge until the cap is fully charged.

    Again, if I disconnect the "LOAD", (LED and its resistor, 15mAmps) still connected in parallel, after initial startup, all is goes back to working.

    So just LED and its resistor in parallel, then cold start all is good, and can apply load and works.

    Keep load connected ( approximately 48 ohm resistor ), cold start, then no output voltage, and no current draw.

    Quickly disconnect load by unplugging load and plugging back in, all works. Just the initial cold start, and the text from the WEBENCH design report stating that "It is recommended to start this device at light load condition".

    Would like to start up at full load. Maybe a voltage detector with slight delay to make sure chip is actually running could help or until Cap is fully charged? Or change output capacitor value?
  • 0 in reply to Matt Hoover
    TI__Expert 6080 points
    Matt,

    Along with the waveforms please send me your final schematic as well.

    If the controller is not starting up at full load, this suggests that the VDD cap is too large and it is taking longer for the cap to charge up. One possible option will be to choose a lower value of Vdd cap.
    Any additional ciruitry is not required, the controller is capable of a full load startup.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 50 points
    Currently my Cdd is 1.00uF as I guess defaults to in WEBENCH designer. I have a 0.33uF on hand to give that a try. Thanks again for the feedback. Will respond back with any changes in results.
  • 0 in reply to Matt Hoover
    Prodigy 50 points
    I changed Cdd to 0.33uF, and nothing changed. I left the 0.33uF and lowered the Cout Capacitor value to 47uF from suggested 120uF. And that seemed to solve the issue of starting with a 48 ohm load at 24Vdc 0.5 Amps, and went as high as 686mA using a decade box. Seems to have solved the Cold start on heavy max load, but the output voltage on the scope inherited a lot of noise, or is the result of the switcher controller itself. Thanks. Got me on to the next step of playing with the Cout value for a smooth output waveform.
  • 0 in reply to Matt Hoover
    Prodigy 50 points

    Ran it all day at full load. I turned off input voltage. Then powered back up, and then got triangle wave on output? Seems circuit component values are very finicky. I thought may be since heat. Let cool down till next day, then powered back up, then output voltage went to 5.8 Volts, which should be 24Vdc and 120mA instead of 500mADC. Very weird. Sometimes starts up cold at FULL load, and sometimes it does not. When disconnect load, then quickly reconnonect then all is fine. Get my 24Vdc at 500mA output at 48 ohm load.

    Thought had resolved, but is hit or miss. Nothing changed. 

    Cdd changed to 0.33uF, and Cout changed to 100uF worked at one time on cold start with no issues. Seems to be hit or miss. Vs feedback resistors are also used as POTs when fine tuned output voltage to match transformer. Still figuring.