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UCC256403: Operation when output load fluctuates

Kaji@PAN
Mastermind 7815 points
Part Number: UCC256403


Hello,

I would like to ask you about UCC256403A.
I am evaluating an LLC resonant circuit.
When the output load approaches no load, the switching frequency of the LLC primary FET rises sharply.
Burst operation such as pulse skipping is not performed.

Specifications are as follows.
Input voltage: 390V (PFC output voltage)
Output voltage: 12V
Output power: 150W (load: 12.5A)

The figure below shows the designed output voltage and frequency characteristics.

In the actual measurement, the LLC primary side FET is switching at the following frequency.
Load 12.5A: about 110kHz
Load 6.25A: about 115kHz
Load 0.05A: about 136kHz
Load 0.01A: about 200kHz
No load: about 240kHz

(Question 1)
The switching frequency rises to 240kHz.Is this normal operation?
Also, up to about 1A load, the output voltage and frequency characteristics are generally consistent with the attached diagram. However, when the load becomes lighter than 1A, the switching frequency rises from the design value. Is this normal behavior?

(Question 2)
I think that this device had a function (protection function) to determine the upper limit of the switching frequency.
Which constant should I adjust and how?

(Question 3)
Table 7-2 below is the content described in the data sheet, but it is currently designed with Option 6.
Is Option6 sometimes unable to enter burst mode even with no load?
I expected it to go into burst mode when it was no load.

Best regards,

  • +1
    TI__Genius 16155 points

    Hi Kaji,

    Please see the responses below:

    Kaji@PAN said:
    (Question 1)
    The switching frequency rises to 240kHz.Is this normal operation?
    Also, up to about 1A load, the output voltage and frequency characteristics are generally consistent with the attached diagram. However, when the load becomes lighter than 1A, the switching frequency rises from the design value. Is this normal behavior?

    1. The switching frequency depends on the resonant tank design and input voltage and load. If the tank design is such that switching frequency needs to be 240kHz in order to regulate the output voltage at light load, there is nothing you can do from controller's side except you make the controller to operate in the burst mode.

    Kaji@PAN said:
    (Question 2)
    I think that this device had a function (protection function) to determine the upper limit of the switching frequency.
    Which constant should I adjust and how?

    2. To limit the switching frequency during light load, the controller needs to operate in the burst mode. Please use design calculator for finidng the components for LL/SPin and BW pin  resistors.

    Kaji@PAN said:
    (Question 3)
    Table 7-2 below is the content described in the data sheet, but it is currently designed with Option 6.
    Is Option6 sometimes unable to enter burst mode even with no load?
    I expected it to go into burst mode when it was no load.

    3. You can try changing the BW option to option 2 and option 3. See if that makes the controller operate in burst. Otherwise you can further adjust the BMTH threshold and VCR capacitors values. Please use design calculator.

    Regards

    Manikanta P