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UCC25661: Burst mode

Dhananjay Yadav
Prodigy 75 points
Part Number: UCC25661
Other Parts Discussed in Thread: UCC5350

Tool/software:

hello, 

I am using ucc256612 and want to disable only LF burst mode. Is it possible? How to do it? 

thanks

regards 

  • 0
    TI__Guru 50190 points

    Hi Dhananjay,

    The best way to do this is to use the Excel Calculator tool which you can download from TI.com. 

    By setting VLLA-VLLB>2,41V, burst mode will be disable.

    I hope this answers your question. 

  • 0 in reply to JCHK
    Prodigy 75 points

    Hello,

    Thanks for the response. By setting (VLLA-VLLAB) >2.41V, will disable both HF burst and LF burst? 

    If both burst modes are disabled, how light load and standby mode operation is performed by IC?

    Thanks

    Best Regards

  • 0 in reply to Dhananjay Yadav
    TI__Guru 50190 points

    HI Dhananjay,

    With burst mode disabled, the light load efficiency will be worse. Additionally, without burst mode, at light to no-load Vout will continue to rise, but the OVP threshold you set, will cause the controller to stop switching to prevent Vout from rising beyond this. 

    Does this make sense?

  • 0 in reply to JCHK
    Prodigy 75 points

    Hello,

    Thanks for the response. Agree! Light load efficiency will be poor. 

    Let me explain my concern related to LF burst mode. we are using UCC256612 for 800V-24V/17A DCDC. Since HS pin of IC is rated for ~600V, we are using additional UCC5350SB gate driver and capacitive divider to interface switch node to IC HS pin.  You must be aware as this was suggested in this forum by you. If we consider Q-current of primary (2.4mA) and secondary side (1.8mA) of gate driver and LF burst off time, required bootstrap capacitance value (17V gate drive voltage and UVLO is 8V) is coming in the range of >=30uF, which is very high. Need your guidance in this case. 

    Thanks

    Best Regards

    Dhananjay 

  • 0 in reply to Dhananjay Yadav
    Prodigy 75 points

    Hello,

    One additional query, is there possibility to reduce LF burst off time? What is the minimum value that we can set? Please let me know how to set LF burst off to minimum value.

    Thanks

    Best Regards

    Dhananjay 

  • 0 in reply to Dhananjay Yadav
    TI__Guru 50190 points

    HI Dhananjay,

    Can you share your schematic? I agree, 30uF for a bootstrap capacitor is way too high. This should not be required with the gate driver.

    As for reducing the time between LF packets, this can be done by adjusting the burst entry point with the LL pin. Actually, the datasheet does not have a practical minimum recommendation for this value. I will have to get back to you on this. 

  • 0 in reply to JCHK
    Prodigy 75 points

    Hello,

    Thanks for the response. I can not share the complete schematic. But please find attached below image for the concept. 

    Please let me know your comment on this? Also as discussed, please let me know how to set LF burst off time minimum value and how much will be the min. off time value?

    Thanks

    Best Regards

    Dhananjay

  • 0 in reply to Dhananjay Yadav
    TI__Guru 50190 points

    HI Dhananjay,

    For the UCC256612, you don't need a large bootstrap capacitor. 1uF should be more than enough as we are only driving a logic level input. 

    I am still checking on how low you can set the LL pin, but the lower this goes, the lower the chance to actually enter into LF burst mode. I should have a clearer answer for you on Monday. 

  • 0 in reply to JCHK
    TI__Guru 50190 points

    HI Dhananjay,

    The lowest you should set the LL pin is 0.8V. Depending on your design, below this you may not reliably enter into LF burst mode.

    I hope this answers your question.

  • 0 in reply to JCHK
    Prodigy 75 points

    Hello,

    Agree if only UCC256612 is used, HB-HS pin q current is ~100uA and 1uF is more than enough. As we are using additional gate driver UCC5350SB, basically there two bootstrap loops. One is on primary side of gate driver and other at secondary side of gate driver. Primary side Q current (UCC256612 HB-HS pin + UCC5350 primary) is (~2.5mA) and secondary side Q current (UCC5350 secondary side is ~1.8mA. With LF burst off time of 150msec, required both bootstrap capacitor value is >30uF. 

    1. If LL is grounded, is it possible to operate in HF burst mode only. In this way, is it possible to avoid entering in LF burst mode?

    2. If LL is set to 0.8V, what will be value of LF burst off time?

    Just to remind, we are not using any auxiliary flyback to supply PFC and LLC ICs and solely relying on supply generated on VCCP pin of LLC IC from HV pin. 

    Thanks

    Best Regards

    Dhananjay

  • 0 in reply to Dhananjay Yadav
    TI__Guru 50190 points

    HI Dhananjay,

    I am checking to see if there is any impact to simply grounding the LL pin and will have to get back to you on this one to see if there are any potential side-effects. 

    As for your second question, the burst off time depends on the load it depends in the actual load according to:

    Regards,

    John

  • 0 in reply to JCHK
    TI__Guru 50190 points

    Hi Dhananjay,

    If the LL pin is grounded, the switching frequency can run really high at startup. To avoid this, you might need to add a preload. 

  • 0 in reply to JCHK
    Prodigy 75 points

    Hello,

    OK, then we will set LL pin to 0.8V.

    Thanks

    Best Regards

    Dhananjay

  • 0 in reply to Dhananjay Yadav
    TI__Guru 50190 points

    OK. Curious to hear how it works for you.

    Regards,

    John