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UCC28950 / audible noise

K.Yaita
Guru 29690 points
Other Parts Discussed in Thread: UCC28950

Hi Team,

My customer is evaluating UCC28950 and concerns about the following issue.

He confirmed audible noise when current limit is activated(overload condition).
I heard it is occured when UCC28950 is operating exceed maximum duty cycle(the condition of lower input voltage ).
Is it possible case(function) for UCC28950?

If the answer is yes, could you tell me the workaround about the issue?

Best Regards,
Yaita / Japan disty

  • 0
    TI__Mastermind 25570 points

    Hello Kensuke-san

     I'm already dealing with a similar issue to this and I want to check with you whether this is the same customer.

    Please post your email address and I'll take this thread off line 

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Guru 29690 points

    Hello Colin-san,

    Thank you for your support.
    I had a talk about it with TI local FAE in japan(Yokohama), so I think it is the same customer.

    My address is the following.
    yaita-k@clv.macnica.co.jp

    Best Regards,
    Yaita / Japan disty

  • 0 in reply to K.Yaita
    Prodigy 105 points

    I also have audible noise from transformer when no load connected to power supply output.

    In this case UCC28950 works in burst mode.

    There are 5 pulses inside burst at frequency is 250 kHz, Burst interval is about 116uS, it's about 8 kHz.

    How to avoid this audible frequency range for burst mode? Maybe it's possible to move it above 20 kHz?

  • 0 in reply to Jura Urasinov
    TI__Mastermind 25570 points
    Hello Jura

    Unfortunately there is no direct way to control the burst rate in the UCC28950 but there are some indirect methods you should try.

    Increase the bandwidth of the feedback loop - the voltage at the COMP pin determines the entry / exit point for each burst. A faster response here should give shorter bursts at more frequent intervals.

    Reduce the TMIN by reducing the value of the resistance at the TMIN pin. This reduces the amount of energy delivered per burst and should increase the burst rate - be careful when doing this because if the minimum on time is too low you won't have enough energy in the circuit to force ZVS and switch dissipation will increase. Dissipation is proportional to the square of the voltage so partial loss of ZVS may be acceptable - for example a resonant transition to 25% of the initial voltage will save over 90% of the switching losses and may be acceptable. What is your input voltage range - if it's less than 70V or 80V then loss of ZVS is probably not going to be a problem.

    The other side of this equation is the behaviour of the magnetics when exposed to these pulse trains. Your magnetics probably include a shim inductor, the power transformer and an output inductor. Any of these can generate audible noise due to magnetostriction in the core and movement of the current carrying conductors. It is also possible that there is a further mechanical resonance between the magnetic and the PCB. These effects can be reduced by varnishing or impregnating the magnetic - doing this also improves their resistance to moisture ingress and internal breakdown due to the electric field. If the PCB board is part of the resonance you may be able to suppress that by adding extra mechanical supports to it - ideally at the point where the movement is greatest.

    BTW - The pulse train you show is very regular - my experience with the UCC28950 EVM is that the bursts are erratic - not all the same width and a changing interval between them. Does your PSU always behave with a regular burst train like you show here or are there conditions where the bursts become less regular.

    I hope this helps
    Let me know how you get on.
    Regards
    Colin