• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 65 subscribers
  • Views 1816 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

About UCC28740 Switch Frequency and Audio Noise issue

Kuo Nowal
Expert 2315 points
Other Parts Discussed in Thread: UCC28740

Hi sir,


Currently we have some question about UCC28740 switch frequency.

Schematic

1. May I know phase node have two switch frequency?

32k Hz @VAC110, 5V/3A

46.3k Hz @VAC110, 5V/3A

2. Why change RCS can't change switch frequency?

3. May I know how to improve Transformer audio noise?

  • 0
    TI__Mastermind 31580 points
    Hi Kuo,

    I have asked one of the applications engineers to respond to your question. You should see a reply within a day.

    Regards

    Peter
  • 0
    TI__Expert 7395 points
    Kuo,
    Thanks for your interest in TI. I've contacted an applications engineer to support your questions. Please include information on the Vin range, Vout, and Iout min and Max for your design. My initial thoughts though:
    1) There is frequency dithering to help EMI compliance so cycle to cycle you will see slight changes in frequency.
    2) Frequency is set by many factors including Magnetising inductance, turns ratio, peak current, where you are in load/line, etc. Please refer to the design guide or design calculator here: www.ti.com/.../toolssoftware
    You choose your transformer, Rcs, and other components to set your peak frequency. from there it varies according to the control law internally to optimize efficiency. note also that there are portions of the control law where the peak current changes but the frequency does not(AM mode). Only during FM mode does it maintain a constant peak current and vary the frequency.
    3) In general, to improve audible noise, we recommend designing to operate at the highest frequency possible for peak load. That way, when you enter audible Fsw, the energy is very low. Have you done that?
    Regards,
    John
  • 0
    TI__Expert 4015 points
    It might be a good idea to measure the gain phase response of you circuit at the relevant operating point. If stability is marginal there could be an oscillation at a frequency which is in the audible range.

    If the 5V, 3A opearating point mentioned is the full load point, then it looks like the frequnecy is quite low and as John has suggested I would recommend increasing it by modifying the inductance and Rcs. Is it this operating point where the audible noise is present?

    Changing Rcs will affect the frequency, but there are some demand ranges where the frequency is fixed. Also, in other ranges, the IC regulates Vout by modulating the switching frequency, so the switching frequency is not fixed for a particular load point.

    If you are modifying Rcs for test purposes it is important to make sure that the transformer does not saturate. Among other things, this could give rise to transformer saturation.

    Lastly, ensuring that the cores are glued and the windings are varnished, heatsinks are stably mounted etc can sometimes be the solution to audible noise problems. The audible frequency range is aprroximately between 20Hz and 20kHz so if your switching frequency is outside this range, the issue is probably a beat frequency.
    Adding extra output cap may help as well.

    Thanks
    Billy