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UCC256404: Improving efficiency at light loads

user6455458
Prodigy 140 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC256301, UCC28064A

Tool/software:

Considering changing from UCC256301 to UCC256404.

Cannot achieve the same efficiency at light loads.

UCC256301: Output power 0.105W, efficiency 45%

UCC256404: Output power 0.105W, efficiency 39%

The settings are as follows.

Css: 0.068uF

BMTH: 2V

BMTL/BMTH Ratio: Option 1 or 2 (Option 6 was worse.)

Only IC peripheral parts can be changed, transformers and inductors cannot be changed.

The PFC is UCC28064A.

I would like some advice on improving efficiency at light loads.

Also, I have a question. On page 63 of the Data sheet SLUSD90E, do "VLL/SS precharge" and "VSSinit" mean the same thing?

  • 0
    TI__Genius 16155 points

    Hi,

    Could you share both the schematics with 301 and 404. Also, Could you let me know what's the burst frequency.

    In 301, the RVCC is 12V whereas in case of 404, the RVCC is 13V. So, there would be an increase in the gate drive losses.

    Can you try increasing the BMTH voltage further by changing resistor values at the LL/ss pin to see whether it can make any improvement in the efficiency at the lighter loads. Increasing BMTH should further reduce the operating switching frequency at the lighter loads. Keep the option 1 for BMTL/BMTH ratio.

    user6455458 said:
    Also, I have a question. On page 63 of the Data sheet SLUSD90E, do "VLL/SS precharge" and "VSSinit" mean the same thing?

    Thats correct.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    Sorry for the late reply.

    I would like to share the schematics, but how can I do it?

    The burst frequency is as follows.

    UCC256404: 3 to 5 Hz

    UCC256404A: 8 to 10 Hz

    I will try to increase the BMTH voltage.

    The data sheet recommends BMTH: 1 to 2 V, but is there any disadvantage to making it 2 V or more?

  • 0 in reply to user6455458
    TI__Expert 8580 points

    Hello User,

    You can share the schematic by clicking Insert --> Image/video/file to upload a file or zip file. Please expect a delay in our response until January due to the holidays.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 140 points

    Thank you for your reply.

    I will share the schematics.

    4130.schematics.pdf

  • 0 in reply to user6455458
    TI__Genius 16155 points
    user6455458 said:
    The data sheet recommends BMTH: 1 to 2 V, but is there any disadvantage to making it 2 V or more?

    As you increase the BMTH, controller will operate in burst mode at higher loads. That would increase the output voltage ripple. 

    What's the burst frequency when you were using 301?  Do you see any efficiency improvement while using 404 over 404A?

    Could you also share the design calculators?

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    There was a mistake in the circuit diagram, so I'm sharing the corrected version.
    I'm also sharing the design calculator.
    The burst period when using the 301 fluctuated from 9 to 50Hz.
    The efficiency of the 404 and 404A is better with the 404A.
    UCC256404: Output power 0.105W, efficiency 39%
    UCC256404A: Output power 0.105W, efficiency 41%
    The one with the increased BMTH is still not working well.

    correct schematics.pdf2514.UCC25640x Design Calculator Rev4.0.xlsx

  • 0 in reply to user6455458
    TI__Genius 16155 points

    Hi,

    Thanks for sharing the design calculator. 

    Could you keep try to increase the BMTH value with the 404A? Could you let me know the efficiency data vs BMTH value vs Burst frequency at 0.105W?

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    I increased the BMTH.

    The results are as follows (at 0.105W):

    BMTH: approx. 2.5V, efficiency: 41.7%, burst frequency: 7-10Hz

    BMTH: approx. 3.0V, efficiency: 42.2%, burst frequency: 5-8Hz

    Even if the output power exceeds 100W, it remains in burst mode, so it is difficult to adopt this specification.

    I am thinking of giving up on using UCC256404 and considering UCC256604.

    Can I expect improved efficiency at light loads with UCC256604 compared to UCC256404?

  • 0 in reply to user6455458
    TI__Genius 16155 points

    Hi,

    Could you try reducing the PFC output voltage from 390V to 385 or below. That should reduce the LLC switching frequency at the light load.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    Sorry for the late reply.

    Changing the PFC output voltage is not possible as it would affect other evaluations.
    Currently, efficiency at light loads is improving, and it looks like we will soon achieve the desired efficiency.
    However, I have some concerns about burst operation at light loads, so I will ask about that separately.