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Original question:

UCC28781: Does not start up

UCC28781: Light load harsh audible noise

Valentinas Pancerovas
Intellectual 410 points
Part Number: UCC28781
Other Parts Discussed in Thread: ATL431

Hello TI,

Under 5k load on both outputs, there is a painful squeaking, sometimes hissing sound, seems to depend on Vin. Until around 52Vin, its a pleasant audible noise but as higher Vin it starts to appear more and more, sometimes it drops and goes into a quiet mode. The harsher the sound is, the lower the Vout becomes.

It does not seem to improve until i load with lower than 80R on the regulated output. Does not seem to matter which output is loaded. At full load its completely silent.

How can this be troubleshooted? Rdm should be 150k but i have 100k because it helps to extend Vin. Otherwise the Vin starts to drop early, but i need around 80Vin support.

In the use case, the un-regulated (0.18A) output will be constantly loaded with 100+mA while the regulated (0.85A) output will have changing load from 5kR to full, Would the audible harsh noise be reduced if the regulated output was the 0.18A one instead, since its actively loaded? 

All the ferrite bead placeholders are jumpers instead.

Thank you

  • 0
    TI__Mastermind 33310 points

    Hello Valentinas, 

    Sorry about the audible noise at light load.  Given that it is audible and only ~5mA on each output, it indicates that the operating mode is SBP(1 or 2).  (LPM operates are ~25kHz.)  But the dynamic nature of the noise suggests that the FB current is not stable; rather varying around for some unknown reason.

    Concerning the RDM value, I don't understand this description "Otherwise the Vin starts to drop early, but i need around 80Vin support."  As far as I know, Vin is under external control, not subject to UCC28781 operation.  But I don't think it applies to the audible noise issue, anyway. 

    Getting back to the issue: I don't know if changing regulation from the 850mA output to the 180mA output will solve the noise, but you can try it and see what happens. 
    Be sure to change from GND2 to GND1 at the ATL431 device. 

    While looking at that section of the schematic, I see that the ATL431 is biased by VOUT_A, but I can't find that signal name at either of the two outputs. 
    I see VOUT,  VOUT_F, and VOUT+.  How does VOUT_A connect to the 850mA output? 
    (By the way, I assume that the symbol for L5 is a schematic placeholder and a higher-rated part is actually in use.) 
      
    I suggest to find out which component(s) is generating the noise.  Transformer?  Inductor?  Ceramic cap?  More than one of these?  Often sample prototype magnetics are unvarnished and can vibrate easily, whereas varnished magnetics are quiet(er) because the varnish dampens vibration.  MLCC caps can make noise, too, due to piezoelectric effect.  It is harder to make those quiet. 

    A stable pure tone is one thing, and harsh squeaking, buzzing, hissing, etc is another.  A stable tone suggests that the operation is also stable, and fSW (of the burst) is merely exciting a mechanical resonance.  Harsh noise suggests that fSW is moving around and not stable, which implies that iFB is also not stable. 
    Please check the signals around the ATL431 section under the noisy conditions.  Monitor how they change as load and line are varied.  
    Maybe some of the feedback-compensation component values need adjustment. 

    Regards,
    Ulrich