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UCC256404: Using this part for multiple outputs

Jonathan Geronga
Expert 7930 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC256301

Hi Team,

I am posting this inquiry on the customer's behalf.
The customer would like to use UCC256404 in his new design (changing the UCC256301 to UCC256404 because their power requirements increased) in the image below:


Here's the question from the customer:
The customer plan on having 4x outputs. One 40V output that the voltage feedback will sense. A 400V output, and two housekeeping outputs. There are conditions where the 400V output will need +700W while the 40V output is essentially unloaded.

They are curious about cross regulation performance with multiple outputs for the LLC controller. Specifically, concerned about how the control loop will respond as the loads change on the various outputs.
Also, curious about how the control loops handle transients and steady state operations when supplying power to multiple transformer outputs. If the control loop regulates the voltage based on an unloaded output, what happens to the other outputs when they're at full power?

Please let me know if you need more information.

Thanks!

Jonathan

  • 0
    TI__Guru 74115 points

    What is the 40V output load range?

    In general, the feedback should  be from the highest rated load output, especially when considering the transient.

    The output with feedback loop connection will be regulated and compensated by the loop either in steady state or transient, other outputs are cross-regulated. The cross-regulated outputs may not have good regulation and transient response if the leakage inductance is high.

  • 0 in reply to Hong Huang
    Prodigy 20 points

    The 40V supply load range is 0-10A.

    There's also the possibility that charge will be dumped onto the 40V rail from the load, which would raise the voltage. The 40V rail powers a motor that can do regenerative braking. 

  • 0 in reply to Scott Jensen1
    TI__Guru 74115 points

    Which output needs most accurate regulation and minimum undershoot/overshoot?

    After you decide this, you should put the feedback loop to that output.

    You can also use weighted feedback if both 40V and 400V share the same output ground.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    If both 40V and 400V outputs require very good regulation, you can have a post regulation on 40V.

  • 0 in reply to Hong Huang
    Prodigy 20 points

    Would the SIMPLIS model give me accurate results for what the control loops will do when I have transients on the outputs? I understand the leakage of the various outputs will have a strong effect on the cross regulation performance. I'm more concerned about the IC entering into a skip mode when the 40V output is unloaded (or has charge dumped onto it), while the 400V rail is outputting maximum power.

  • 0 in reply to Scott Jensen1
    TI__Guru 74115 points

    It will depend on how you model the leakage inductance which may not reflect reality of your transformer.

    You can list each output regulation requirement, transient requirement, load range, then decide to use the most required output for your feedback loop. If you have two outputs with similar requirements, you need to consider additional techniques such as weighted feedback or post regulation.