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UCC256404: UCC256404 Burst soft ON/FF

user6455458
Prodigy 140 points
Part Number: UCC256404

Tool/software:

Hi team

Burst soft ON doesn't work on UCC256404.
Burst soft OFF works.
This is true whether the load is light or heavy.

The settings are as follows:
BMTH: 1.8V
BMTL/BMTH ratio: Option 1

Lowering the BMTL/BMTH ratio tended to make it work normally.
Both Burst soft ON and OFF worked normally with Option 6.
Is there any way to make Burst soft ON work properly?

UCC256404 Burst soft ON_OFF.pdf

  • 0
    Prodigy 140 points

    Hi team

    There was an error in the file I attached earlier, so I am attaching the corrected version.

    6327.UCC256404 Burst soft ON_OFF.pdf

  • 0 in reply to user6455458
    TI__Genius 16155 points

    Hi,

    Since you select the option 1, what could happen is, when the controller enters into burst mode, the FBreplica is going above BMTH which makes the controller comes out of burst mode. So, burst soft on is not working.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    Thank you for your answer.
    I understand the situation.

    I have an additional question about burst operation.
    When bursting with a light load of output power of 0.4W or less, there may be a period of oscillation suspension as shown in the attached file.
    Could this cause a problem with the operation of the UCC256404?
    This is at a light load and does not affect the output voltage ripple, so we do not believe it will have any impact on our equipment.

    Also, can the waveforms of FBreplica, BMTH, and BMTL be observed?

  • 0 in reply to user6455458
    TI__Intellectual 2480 points

    Hi,

    Let me answer this as Mani is out.

    FBreplica can be measured indirectly.

    FBreplica = (IFB - Iopto_emitter) x RFB.

    From datasheet

    RFB ~= 100kohm typ

    IFB is per the datasheet based on the variant used (for UCC256404 IFB = 82uA typ)

    To measure the current out of FB pin (Iopto_emitter)

    • add a resistor, say - 5kohm in series with opto transistor (between emitter and GND).
    • Measure the voltage across this resistor (V_5k)
    • Calculate the current flowing through emitter (=V_5k / 5k).

    The BMTH and BMTL can't be measured directly. However, the understanding of FBreplica can provide an idea, along with the BMTH and BMTL values that were programmed using section 7.4.3.1 in datasheet.

    Regards
    Hemanth

  • 0 in reply to Hemanth Sreeperumbudur
    Prodigy 140 points

    Thank you for your answer.
    I will try to measure the FBrplica using the method you suggested.

    What about the period when oscillation stops during burst operation at light loads?

    Could this cause problems with the operation of the UCC256404?

  • 0 in reply to user6455458
    TI__Intellectual 2480 points

    If the load is very light, the control voltage can stay below the minimum threshold value to start delivering another pulse. During which, you would not see switching. 

    If you have a related question click on 'Ask a related question', if you have a new question, please use 'Ask a new question' option.

    Thanks for considering TI products.


    Regards
    Hemanth