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UCC28740: FET switching noise issue

JH Shin
Guru 10940 points
Part Number: UCC28740

Tool/software:

Hello,

My customer designed a flyback circuit using UCC28740 and is testing its operation. The following issue is encountered:

    - Test Conditions : Input voltage: 750Vdc, Output: 12V/6A (PWM FAN load)

  

Issue

    - Output voltage noise (C1) is 474mVpp → FET switching noise occurs

    - Noise of the same frequency occurs at the input voltage (C2).

    - Stabilization of FET switching frequency is needed (varies between 50 and 70 kHz)

Please advise on what is causing this issue and how to resolve it.

Thank you.

JH

  • 0
    TI__Guru* 85806 points

    Hello,

    Audible noise is generally around 20 kHz you should not be able to here 50 to 70 kHz.  More than likely something else is happening in your design that is occuring at much lower switching frequency.  You might want to probe the aux winding, VDD, Vout and CS and look for behaivors that occur at 20 kHz or less.

    I wonder if some how you are triggering a fault condition. The followling link will bring you to an application note that discusses primary fault sensing that the UCC28740 uses and the common issue with it and how to resolve them.  In most cases aux winding noise or CS noise will trip a fault.  This generally can be resolved with filtering and clamping the switch node.

    https://www.ti.com/lit/pdf/sluaac5

    In regards to ringing on the output.  You might want to double check this with tip and barallel measurements to make sure this is real.  You can also use a differential probe for this purpuse as well.

    If the rining is real on the ouput.  It is tied to the switch node ringing.  This can be reduced by putting a snubber on the output rectifier.  The application note mention above also gives guideance on how to snub ringing as well.

    Regards,