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UCC28950: OFF time disappear issue from Cycle-by-Cycle ILIM hiccup mode

jafar faroughi
Prodigy 30 points
Part Number: UCC28950

HI DEAR
i have problem such as Yoshwa in this link
http://e2e.ti.com/support/power_management/isolated_controllers/f/188/p/533945/1944195

and guide to this link
e2e.ti.com/.../1813540

i study all document about ucc28950 and can't solve this problem.
when decrease load and increase current the "OFF Time Before Restart" (datasheet page 24 Cycle-by-Cycle Current Limit Current Protection and Hiccup Mode fig18)can seen .but in short circuit condition this time disappear.and soft start capacitor voltage jump to 4.6volt and don't discharge.

the guide link not relative with this problem.please help me to solve it.
best regard

  • 0
    TI__Mastermind 25570 points
    Hello Jafar

    Please send me your schematic so that I can study this problem. I don't understand your comment 'when decrease load and increase current...', it seems to be a contradiction so it would be good if you could explain it further.

    If you don't want to post the schematic here, you can send it to me privately at colingillmor@ti.com

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 30 points

    hi dear colin

    i attached control schematic .some description:
    1- in "over load" condition hiccup mode occur but in near "short circuit" condition hiccup mode not occur and csoft  voltage jump to 5v
    2-i disconnect all component across csoft and no change seen.
    beast regard.
    4426.Schematic1.pdf

  • 0 in reply to jafar faroughi
    TI__Mastermind 25570 points
    Hello Jafar

    Disconnecting unnecessary components from the SS pin is a good place to start - they can always be put back in later once this problem has been solved.

    Your schematic looks ok. One comment - The RC filter on the CS pin has a pole at 72kHz, this is probably a bit low. A more usual filter would be 1kOhm with 1nF. and I think you should try that first. I'm assuming that your PCB layout is not adding significant noise to the CS signal and that the RC filter components are physically close to the controller.

    A short circuit condition may be continually retriggering the Hiccup shutdown - can you check the waveforms at the SS and CS pins and the waveform across the transformer primary during the near short circuit to see what the controller is doing.
    Please post the screenshots and I will take a look at them.

    Regards
    Colin