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TPS92692: Audible noise Problem when using External PWM Mode

Mathiyazhagan Anbuchezhiyan
Prodigy 40 points

Part Number: TPS92692

Hello
I use TPS92692 in external PWM mode

My input is 24V,9A Max and Output is 50.8V,2.6A Max.

I have selected Switching frequency as 120 kHz and inductor value is 22uH
When i run in 100% duty cycle of PWM
Less noise is generated at the inductor but I am trying to reduce duty cycle inductor noise will be  high.
How to reduce this noise at the inductor?

  • 0
    TI__Mastermind 18890 points

    Hello,

    What frequency are you doing the PWM dimming at?  Can we have your schematic along with part list?  We would also like to get scope capture of the following signals on your board when you are doing PWM dimming.

    CH1:   DIM/PWM         CH2:  Inductor current         CH3:  Output LED current             CH4:  IMON

    Inductor current is the main power inductor.  You can lift one end of the inductor and add a wire as a wire look for the current probe to measure the current.  I want to rule out any other instability first.

    In general, if you are PWM dimming in the audible range (20Hz-20KHz) you can get this audible noise.  The two things that causes this noise are the power inductor and output caps.   For the inductor, it’s the mechanical vibration in the inductor that causes this audible noise then try different inductor or molded inductor might have less audible noise.  You might will need to find an inductor that has less mechanical vibration.

    The ceramic caps if used is also the source of this.  There is piezo effects in the ceramic caps that in turn vibrates and mechanically couple to the board and amplify the sound.  This is a known phenomenon with ceramic caps.  If it’s capacitor then they can use different type of caps but that usually cost more. Layout of these caps can help reduce this cap to board coupling and noise.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 40 points

    Hi Tuan,
    PWM dimming frequency is 200Hz.
    I am consider the schematic as per the reference design in Boost converter.
    I am didn't able to capture Inductor current and Output LED current.
    I have attached DIM/PWM and IMON waveforms

    Inductor
    Manufacturer: BOURNS INC,Manufacturer Part no: SRP1770TA-220M





    This is in the audible range but 100% duty cycle less noise will be created.
    when I am trying to reduce the duty cycle of PWM signal Noise will be more
    How to reduce this audible noise?

  • 0 in reply to Mathiyazhagan Anbuchezhiyan
    TI__Mastermind 18890 points

    Hello,

    We will still need the current waveform that we've requested above to verify stability of the system first.  The reason you do not hear it when it's running continuous (100% duty cycle) is that you are not PWM'ing in the audible range.

    Again from above reply, you might want to try different inductor such as a molded one to see if you can reduce this noise.  The other is the ceramic capacitors...You might want to try different type of caps.  Beyond that there is not much that can be done since you are PWM'ing in the audible range. 

    Thanks Tuan