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UCC28780: About audio noise from transformers

Tak
Intellectual 480 points
Part Number: UCC28780

I am using the UCC28780 to develop a compact, high efficiency power supply for 200W peak (average 30W) audio amplifiers. (Using Si-FET)
We are currently evaluating the prototype.
There is one problem that I am worried about, and I would like to consider an effective countermeasure.
The power supply I created produces strong noise (transformers).
I think the cause is that the peak current generated at the timing of switching from LPM to ABM is large.
The circuit tuning is set by referring to the Excel tool and data sheet provided by TI.
Is there a way to suppress this noise?
Transformers are already varnished countermeasures.

By default, Rbur1 was 200kΩ and Rbur2 was 47kΩ, but when Rbur2 was lowered to 24kΩ, the transient peak current was significantly reduced and the noise was also reduced. Does such an extreme setting have side effects?
Please let me know if there is another good way.

Also, I made a big mistake in the layout this time.
The ground plane is placed directly under the Vs, RDM, and RTZ pins.
(The entire IC is mounted on a strong GND plane)
So far, we haven't identified any major issues, but is there a possibility of a fatal issue?

In addition, although it is stated in the application note that the capacitor for LPF to the CS pin can be up to 330pF, currently about 10pF is used. (I referred to the evaluation board and Excel tool)
The feedback relation uses the value based on the tool, but is there a possibility that increasing the capacitance of the capacitor will have a positive effect on this problem?

B.R.Tak

  • 0
    TI__Guru* 85616 points

    Hello,

    By adjust Rbur1 and Rbur2 changes the VCST(BUR) threshold this is where the converter will enter burst mode.  By lowering it you will extend the AAM range of the converter lowering the peak when you enter burst mode and reducing the 1/2Li^2 stored in the transformer so I would think this would be O.K.

    Just to be shore I would evaluate the design with varied load and input voltages; as well as, large signal transients to make sure it is stable.

    I don't think you made a mistake if you ground plane is directly under VS, RDM and RTZ pins.  What you need to make sure of is that RVS1 and RVS2 connections are as close to the VS pin as possible short traces reduce inductance and the trace acting like an antenna.  The data sheet does give more guide to layout that is probably worth reviewing.

    When it comes to CS filtering in the low pass filter you don't want to over filter the pin.  I generally set the pole for at least 10 times the maximum switching frequency.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 480 points

    Dear Mike

    Thanks for your advice.
    By adjusting Rbur1 and Rbur2, I adjust the audible band noise so that it doesn't bother me. I would like to reduce noise by lowering the peak current, even if I sacrifice some power efficiency. Change the setting to expand the AAM range, although it is different from the recommended setting value of the application note.
    Is it okay to think that there are no side effects even if the setting value is changed freely?
    (Of course, we will verify under various load conditions and input voltage conditions)

    I understand that there is no particular problem with the ground plane.
    I was worried because the data sheet stated that I should avoid the ground plane, but I placed the Rvs resistor as close to the IC pin as possible. So far, there is no malfunction. Rather, I think that it is more stable to place it on the ground plane, but is it wrong?

    Thanks also for the advice on setting the lowpass filter.
    It was very helpful.

    I'll try lowering the burst threshold further to see if the noise is even smaller. If this method is not a problem, this question will be closed.

    B.R. Tak

  • 0 in reply to Tak
    TI__Guru* 85616 points

    Hello,

    I am glad that my suggestion help resolve the issues you were having. If you have decided that the new AM range you have selected works better for you than what it is recommended in the data that is O.K.  However,  I would recommend that you verify with production testing, large signal line and load transients and a large enough sample to make sure you don't have any issues.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 480 points

    Thank you for the advice.
    We are currently verifying under various conditions.
    Currently, there is no problem.

    Also, if I have a new question, I will post it again.

    B.R. Tak