• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 250 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28740: UCC28740 Copto selection

Yunjing Wang1
Expert 4300 points
Part Number: UCC28740

Hi team,

My customer is using UCC28740, and they meet issues during low Vin start up.

During low Vin start up test/72Vac Vin, due to the suddenly load change from 0A~1.6A, UCC28740 VCC will be pull down below UVLO making device enter hiccup mode.

Customer reduce Copto in below schematic will improve the issue, but they don't know the usage and selection value of this Copto.

Would you pls kindly help share comment on the purpose of this Copto? As well as the value calculation of Copto?

Why it will impact start up during low Vin/72Vac Vin? Vin=72Vac, Vout=24V, Iout change from 0A~1.6A at start up.

Thank you!

Yunjing

  • 0
    TI__Genius 15566 points

    Hi Yunjing,

    -For a better no-load to full load transient response, I recommend decreasing Rfb4 (see below section 8.2.2.9 of Datasheet). This resistor will help to pull down the emitter of the opto-coupler transistor and allow a faster no load to full load response (High I_FB value to low I_FB value). What is your actual Rfb4 value? Try reducing Rfb4 to a half and see if a better response is gotten.

    -I have never seen a Copto capacitor connected across the opto-transistor. Where did you get that schematic? I can not comment on it as I am not aware of any test with that capacitor.

    Thank you