• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 298 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.

UCC256302: About how to prevent overshoot at startup

Masazumi Ishii
Genius 3945 points
Part Number: UCC256302

Hello.

I have a question about UCC256302.

To prevent overshoot at startup, we switch to SS operation → FB operation halfway through the rise before the output voltage reaches 12 V (typ).
However, a burst operation starts at the timing of SS operation → FB operation switching, and a stage occurs when the output rises.

I have a question about this mode of operation.

① It will be confirmed again, is it OK with FB terminal voltage = internal FBReplica thinking?
② If the upper resistor of LL / SS pin is removed, is it OK to think that the burst threshold is 0.7V?
③ In this mode, the burst operation is in operation with the FB pin voltage being about 5.2 V.What are the possible causes?
④ Switching between SS operation and FB operation is considered when the FB voltage falls below the SS voltage.
　However, it seems that the operation is switched when the FB voltage is higher than the SS voltage. Is it a possible cause?
⑤ Is there a recommended method as a method to switch from SS operation to FB operation without burst operation in the middle of output rise? please tell me.

Please see the attached document for details.

Thank youUCC256302-2.xlsx

  • 0
    TI__Mastermind 27135 points

    Hi Masazumi-san,

    Thanks for your interest in UCC256302.

    1. FBreplica is equal to RFB*(IFB-Iopto) so its not exactly the same as the FB pin voltage. Looking at the FB pin voltage is typically a good indication of what the FB signal is doing however.

    2. You can remove the upper LL/SS resistor and this will place the burst mode threshold at its minimum. Please keep in mind that it is still possible to enter burst mode with this configuration.

    3. one thing you could try is reducing the capacitance from VCR pin to ground. this will reduce the primary current threshold equal to the minimum burst threshold.

    4. Switching thresholds are handed over to FBreplica once FBreplica falls below the soft start pin voltage. Please keep in mind that the FB pin voltage is not the same as FBreplica. 

    5. Reducing the bottom VCR capacitor will reduce the primary current level that is equal to the minimum burst mode threshold. You can also look into adding more phase margin to the feedback network to further damp oscillations in the feedback signal which can cause burst-like behavior.

    Best Regards,

    Ben Lough